Review ARticle Journal of Agricultural and Marine Sciences 2022, 27(1): 16–37 DOI: 10.53541/jams.vol27iss1pp16-37 Received 3 Jan 2021 Accepted 27 June 2021 Biotic and Abiotic Stresses of Major Fruit Crops in Oman: A Review Rashid A. Al-Yahyai1*, Latifa M. Al-Kharusi2, Mumtaz M. Khan1, Ali O. Al-Adawi3, Ali M. Al-Subhi1, Basim S. AL-Kalbani1,3, Abdullah M. Al-Sadi1 Rashid A. Al-Yahyai1*( ) alyahyai@squ.edu.om,Department of Plant Sciences, College of Agricultural and Marine Sciences, Sultan Qaboos University, Al-Khod, Sultanate of Oman. Introduction The Sultanate of Oman is situated on the south-eastern coast of the Arabian Peninsu-la, having a semi-arid to an arid climate. It’s a unique country that lies within two geographic regions, the south-central part of the country is desert, having its resemblance with Africa (16-26° N and 51-59° E), while the northern part is mountainous which resembles con- tinental Asia (20-40° N and 40-50° E), in its climatic con- ditions, while Dhofar Governorate in the South of Oman has tropical monsoon climate (Al-Khafaji et al., 2017). Oman receives less than 100 mm of annual average rainfall which is 10 times less than the average evapo- transpiration rate. About 75% of its total area is desert while the remaining is highly diverse within its topo- graphic and climatic conditions, which allowed the cul- tivation of various tropical to subtropical fruits (Al-Ya- hyai et al., 2014). Among the agricultural commodities, الضغوط احليوية والالحيوية حملاصيل الفاكهة الرئيسية يف عمان: مراجعة راشد اليحيائي، لطيفة اخلروصي، ممتاز حممد خان، علي العدوي، علي الصبحي، ابسم الكلباين، عبد هللا السعدي Abstract: Oman is located in an arid region of the world that is characterized by adverse climatic conditions, in- cluding heat and drought. In recent years, it has also been affected by climate turbulence and the occurrence of severe weather, such as cyclones and heat/cold waves affecting large agricultural areas of the country. Fruit cultivation area represents 31% of the total cultivated area (97,239.58 ha) in the country. However, the production share is only 17% of the total crop production in the country (2.6 million tons). About 90% of the fruit cultivation area is dominated by date palm, banana, lime, and mango. In addition to the abiotic stresses, such as drought, heat, and salinity, major fruit crops have declined in recent years due to various biotic stressors, primarily insect pests, and diseases. For several decades, the date palm has suffered from the Dubas bug and in recent years from Red Palm Weevil. Lime has been infected with Witch’s Broom Disease of Lime (WBDL) caused by ‘Candidatus Phytoplasma aurantifolia’ that has led to the decline of production to 25% from its peak in the nineties. Banana is Oman`s second-largest fruit crop in production and export. It has also been the subject of studies due to losses incurred by farmers during pre-and post-harvest stages, in addition to several pests and diseases that affect bananas in Oman. Mango is another major fruit crop that is primarily cultivated in northern Oman. Severe infection with mango decline has led to the eradication of mango orchards from many regions of Oman, particularly in Batinah Coast, where increased salinity has led to a decline in mango yield. Research conducted in Oman has investigated several aspects of these challenges. This review paper summarizes the outcome from studies conducted in the country and proposes directions towards resolving current and future challenges to the fruit industry. Keywords: Date Palm, Lime, Mango, Banana, Fruit Trees, Climate Change, Abiotic Stress امللخص:تقــع ســلطنة عمــان يف منطقــة قاحلــة مــن العــامل تتميــز بظــروف مناخيــة قاســية تشــمل شــدة احلــرارة واجلفــاف، ويف الســنوات األخــرة أتثــرت أيًضــا ابالضطــراابت املناخيــة مثــل األعاصــر وموجــات احلــرارة والــرودة الــي أثــرت علــى مناطــق زراعيــة واســعة مــن البــاد، ومتثــل مســاحة زراعــة الفاكهــة مــا نســبته 31٪ مــن إمجــايل املســاحة املزروعــة )97،239.58 هكتــار( يف الســلطنة ومــع ذلــك ، فــإن حصــة اإلنتــاج ال تتجــاوز 17٪ مــن إمجــايل إنتــاج احملاصيــل الزراعيــة يف الســلطنة )2.6 مليــون طــن(، ويهيمــن النخيــل واملــوز والليمــون العمــاين واملاجنــو علــى 90٪ مــن مســاحة زراعــة الفاكهــة، ابإلضافــة إىل الضغــوط الاأحيائيــة ، مثــل اجلفــاف واحلــرارة وامللوحــة ، تراجعــت حماصيــل الفاكهــة الرئيســية يف الســنوات األخــرة بســبب الضغوطــات احليويــة املختلفــة وعلــى رأســها اآلفــات واألمــراض. ولعــدة عقــود، عانــت خنيــل التمــر مــن حشــرة الــدوابس ويف الســنوات األخــرة مــن سوســة النخيــل احلمــراء. كمــا أصيــب الليمون مبرض مكنسة الساحرة والنامجة عن )Candidatus Phytoplasma aurantifolia( مما أدى إىل اخنفاض اإلنتاج إىل 25٪ من ذروة االنتــاج يف التســعينيات. ويعتــر املــوز اثين أكــر حمصــول فاكهــة يف ســلطنة عمــان انتاجــا وتصديــرا وابإلضافــة اىل الدراســات املختلفــة عــن اإلنتــاج، أقيمــت دراســات عــن اخلســائر الــي يتكبدهــا املزارعــون خــال مرحلــي مــا قبــل وبعــد احلصــاد، ابإلضافــة إىل العديــد مــن اآلفــات واألمــراض الــي تصيــب املــوز يف عمــان. وحيــث أن املاجنــو هــو حمصــول رئيســي آخــر يــزرع بشــكل أساســي يف مشــال عمــان فقــد أدت اإلصابــة الشــديدة مبــرض تدهــور املاجنــو إىل فقــدان العديــد مــن بســاتني املاجنــو يف الســلطنة وخاصــة يف ســاحل الباطنــة، حيــث ســامهت زايدة امللوحــة كذلــك إىل اخنفــاض انتــاج حمصــول املاجنــو. وتلخــص ورقــة املراجعــة هــذه نتائــج الدراســات الــي أجريــت يف الســلطنة واملتعلقــة إبنتــاج الفاكهــة وتقــرح حلــول توجيهيــة للتحــدايت احلاليــة واملســتقبلية. الكلمــات املفتاحيــة: خنيــل التمــر، الليمــون العمــاين، املاجنــو ، املــوز ، أشــجار الفاكهــة ، تغــر املنــاخ ، اإلجهــاد الاأحيائــي. 17Review Article Al-Yahyai, Al-Kharusi, M. Khan, Al-Adawi, Al-Subhi, AL-Kalbanim, Al-Sadi fruit crops are the main agricultural products that have been traditionally cultivated for thousands of years. While this climatic and geographic diversity helped cultivate a variety of fruit crops in a subsistence man- ner, there have been many challenges that increased over time as the agricultural sector adopted more modern ap- proaches to cultivation. These challenges include biotic, largely caused by the emergence of pests and diseases, while others are abiotic, including heat, drought, and sa- linity. Future challenges include the shift to the modern- ization of agriculture, climate change, and declining wa- ter and land resources. These challenges were the main research focus in Oman. This review article was aimed to present the status of research and major outcomes that can develop future research directions towards re- solving some of these challenges. Current Status of Fruit Production in Oman A large percentage of the cultivated area in Oman is used to produce a variety of tropical, subtropical, and decid- uous fruit crops. Fruit cultivation area represents 31% of the total cultivated area (97,239.58 ha) in the country. However, the production share is only 17% of the total crop production in the country (2.6 million tons) (Fig- ure 1). This indicates that there are several challenges in fruit production at the pre-harvest and postharvest levels. However, despite the great potential of fruit crop production, the sector remains an unattractive alterna- tive to fodder crops that use plenty of water resources. The topographical and climatic variations in the coun- try provided fruit growers. The high-altitude mountains in northern Oman, Al-Jabal Al-Akhdar at 2000 m above sea level, is characterized by low temperatures that are adequate to meet the chilling requirements of several deciduous fruit species, the most common being pome- granates, pome and stone fruits and nut crops such as walnuts (Figure 2). The southern plains of Salalah are characterized by summer rainfall due to annual drifts from the Indian Ocean monsoon, are characterized by the cultivation of several tropical fruit crops, most prom- inently coconut, bananas and papayas. Whereas, the rest of the country largely features hot summers, suitable for the cultivation of the most dominant fruit crop, the date palm. Other important fruit crops that are suitable for many of Oman’s regional climatic zones include mango, lime, guavas and Annona. According to the latest national agricultural cen- sus that was conducted in 2012/2013, the number of date palms declined slightly by 3.8% totaling 7,563,279 palms compared to 7,859,443 in the 2004/2005 cen- sus. This decline may be attributed to the continued loss of agricultural land to salinity and desertification, in addition to extreme weather events caused by cli- mate change, in date palm cultivation areas. However, other fruit crops have increased in numbers by larg- er percentages, which may also indicate a replace- ment of date palms with a more profitable crop. Lime has increased by 11% (390,218 trees), mango by 8.5% (434788 trees), and coconut by 11.5% (159,527 palms) in 2012/2013 census, compared to 2004/2005 census (MAF, 2017). It is noteworthy, that Oman has initiat- ed a project to plant one million date palms in 2009, of which 600, 000 palms have already been planted to date. The major fruit crops in Oman are date palm, banan- as, lime and mangoes, constituting 83%, 6%, 3%, 3% of fruit production areas in the country, respectively (Table 1). Over the past few years, there has been an increase in fruit production in Oman, despite a slight decline in the cultivated area (Figure 3), which may be attributed to improved cultural practices and the introduction of high-yielding cultivars, such as the case for mangoes. The increase in fruit production is driven by increasing Figure 1. Production and cultivated area of crops in Oman in 2017 (MAF, 2017). 18 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 Biotic and Abiotic Stresses of Major Fruit Crops in Oman: A Review demand, as the country’s import of fruits continues to increase, driven by health consciousness and an increase in economic prosperity. Nonetheless, the increase in fruit production over the past few years varied from year to year, due to several factors, including climate anoma- lies, pests and diseases, and availability of resources, ad- verse abiotic conditions such as salinity and drought, and human factors related to labor, energy and the economy. Challenges in Fruit Production Cultural Practices One of the major challenges to developing fruit produc- tion as an economic sector is the traditional means of cultivation and cultural practices followed by the less cost-effective resource and time-consuming methods. These traditional methods have been reported for vari- ous fruit crops, including date palm (Al-Yahyai and Khan, 2015), lime (Al-Yahyai et al., 2012), mangoes (Al-Jabri and Al-Yahyai, 2017; Al-Yahyai et al., 2013) and pome- granates and deciduous crops (Al-Yahyai et al., 2009; Al- Said et al., 2013; Al-Yahyai et al., 2014). This has resulted in decreased yield and selling price of locally produced fruits. Experiments have shown that several techniques can be followed to enhance the quality of fruit produc- tion, including management of irrigation water by utiliz- ing deficit irrigation for date palm (Al-Yahyai and Khan, 2015) and bananas (Al-Harthi and Al-Yahyai, 2009), and improved storage (Al-Yahyai and Al-Kharusi, 2012) and marketing strategies (Al-Yahyai, 2007). Utilization of post-harvest technology can lead to enhanced market- ability of major fruit crops, such as date palm (Al-Yahyai and Manickavasagan, 2012 a,b) and bananas (Opara et al., 2013, Opara et al., 2012). Abiotic Stresses affecting Fruit Production in Oman Drought, heat, salinity and climate change are major abi- otic factors worldwide that limit fruit crop productivity and sustainability by disturbing their growth and pro- ductivity (Zörb et al., 2019). Generally, plants are highly susceptible to abiotic stresses due to their sessile nature, as these stresses limit their capability. It is reported that 90% of arable lands are suffering from one or more of the mentioned abiotic stresses and are causing about 70% yield losses (Reis et al., 2012; Mantri et al., 2012). Oman Figure 2. Distribution of deciduous fruit crops cultivated at various altitudes in Oman. (Source: Basim Al-Kalbani, Unpublished). 19Review Article Al-Yahyai, Al-Kharusi, M. Khan, Al-Adawi, Al-Subhi, AL-Kalbanim, Al-Sadi fruit industry suffers from drought, aridity, salinity and water scarcity problems (Ahmed and Choudri, 2012). Also, more changes in environmental conditions may cause huge economic losses. Due to climate change, the semi-arid regions are pre- dominantly affected by abiotic stresses, resultant drought and soil salinization have been increased on agricultural lands. In arid and semi-arid regions, there is a natural phenomenon of high evapotranspiration and sea water entry in coastal areas, which causes salt accumulation around the plant rhizosphere (Sobhanian et al., 2020). Drought and salinity stresses have been increased due to global warming and anthropogenic activities such as improper irrigation and drainage. Besides, global warm- ing, the changing rainfall pattern has also increased the evapotranspiration rate, which has ultimately increased the drought severity (Dai, 2011). Drought Due to environmental changes, the agriculture sector is greatly affected by drought stress, which seriously influ- ences plant vegetative and reproductive stages (Chaves and Oliveira, 2004). Water is essential for all physiologi- cal functions occurring in plants. In drought, plants un- dergo complex anatomical mechanisms to regulate wa- ter in their body and inappropriate conditions stimulate fundamental changes in plant morphology, physiology and biochemistry of water-deficient organs (Anjum et al., 2011; Jarvis and Jarvis, 1963). To minimize water loss by transpiration, plants modify their anatomical charac- teristics like the closing of stomata, production of cuticle wax, reducing leaf area, besides it includes certain chang- es in plant physiological mechanism including a reduc- tion in photosynthetic rate, changes in gene expression and increase in osmotic stress (Hadley and Smith, 2011; Munné-Bosch and Alegre, 2004; Chaves et al., 2003). In drought, the creation of drought-resistant varieties is a strategy to ensure proper food supply. The response of drought-tolerant species is different from the sus- ceptible species. Plants adapt to drought stress in dif- ferent ways, such as by changing their growth patterns, morphology and defense mechanism (Zandalinas et al., 2018). During drought, one of the most common mech- anisms adopted by plants is the remobilization of pre- stored carbohydrates from stems to overcome drought (Schnyder, 1993). The other cellular and subcellular mechanism induced by plants during a drought include photosynthesis, antioxidative response, energy metabo- lism, osmotic adjustment, lipid metabolism, ion homeo- stasis and gene expression regulation (Sobhanian et al., 2020). Therefore, evaluation of drought-resistant culti- vars, throughput analysis of plant genomics, proteomics and transcriptomics is required (Peng et al., 2009). Effects of Drought on Fruits Trees Horticultural crops are perishable and require a large amount of water for sustained growth and yield, where- as drought or water-deficient conditions adversely affect crop yields and quality. In fruit crops, the drought timing, duration, intensity, and species determine its hazardous effects. Date palm (Phoenix dactylifera L.) is the top fruit crop of Oman, which contributes about 80% to total fruit crops grown in the country (Al-Yahyai and Khan, 2015). It is considered tolerant to drought and salt stress and provides a significant contribution to the food security of the arid regions. However, recent threatened its pro- ductivity (Hazzouri et al., 2020). The date palm has var- ious anatomical characteristics that enable it to tolerate drought stress, including thick pinnate compound leaves having waxy cuticles and spines on them. Besides, it has a deep root system, which enables it to maximize wa- ter uptake, while all other leaf traits help in minimizing Figure 3. Fruit Area and Production in Oman (2000-2018) (Source: FAOSTAT, 2020). 20 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 Biotic and Abiotic Stresses of Major Fruit Crops in Oman: A Review evapotranspiration rate (Sané et al., 2005; Ramoliya and Pandey, 2003). Despite having drought-tolerant proper- ties, it requires a lot of water for commercial produc- tivity. Dates suffering from drought have reduced pro- ductivity and fruit quality (Hussain et al., 2012). Similar findings were observed by other researchers who stated that long-term drought negatively affects date palms by reducing their growth, productivity and quality (Elshibli et al., 2016; Alhammadi and Kurup, 2012). Arab et al. (2016) reported that 2-year-old date palm seedlings sub- jected to drought and heat showed a reduced concen- tration of antioxidants, e.g. ascorbate and glutathione in leaves. The fatty acid composition under drought was changed but it remained unchanged under heat environ- ment, which shows that date palms may have indepen- dent metabolic systems to deal with drought and heat stresses. Proline accumulation occurs in plants due to multiple stresses; high proline contents were observed in date palms in response to drought and salinity, pro- line can be used as a possible marker in date palm to improve drought and salinity tolerance (Yaish, 2015). The date palm (cv. Khalas) fruit chemical quali- ty changes as a result of decreased water regimes, and overall the drought effects on fruit quality were signifi- cant (Al-Yahyai and Al-Kharusi, 2012). The Ministry of Agriculture and Fisheries reported that climate change and environmental constraints are major causes of date palm production instability in Oman. However, Kheiry (2017) used the stochastic model to analyze and identify the sustainable Date Palm Farming Systems in Oman. He argued that Al-Batinah is the most risk efficient re- gion and recommended awarding economic incentives to growers for sustainable date palm varieties plantation and diversification which can minimize the sustainabili- ty risks of date palm production in Oman. Al-Yahyai (2018) reported multifaceted challenges that growers are confronting in date palm production in Oman. He argued that farmer participation, train- ing and extension agent’s role can help in monitoring the crop limiting factors and mitigating such anomalies for the improvement of date palm cultivation and pro- duction of export-quality dates in Oman. It has been claimed that the date palm distribution in Oman will be changed due to ever-changing climate conditions. They used the basic CLIMEX software package to see the climate responses and later this was further integrat- ed to demonstrate date palm possible distribution with two global climate models (GCMs) including CSIRO- Mk3.0 and MIROC-H. The earlier results showed that some regions may become unsuitable for date palm cultivation in Oman by 2100 (Shabani et al., 2015). Historically, banana has been one of the most do- mesticated crops of the Arabian Peninsula and the gulf ports of Oman have played a major role in the exchange of banana genetic resources between Africa, China, the Indian subcontinent and the Arabian Peninsula, despite being arid and unfavorable climatic conditions for its cul- tivation (Al-Busaidi, 2013). Bananas are highly sensitive to drought, as they have large leaves with large air pock- ets inside; the annual evapotranspiration rate of bananas varies from 1200 to 2690 mm (Robinson and Alberts, 1986). Musa species show differences in stomatal con- ductance based on leaf age, climatic conditions and soil water relation (Ravi and Vaganan, 2016). In water stress, the flowering stage is highly affected as it is sensitive to drought, the lowest yield was observed in Banana geno- type “Elakki” when drought was given at the flower dif- ferentiation stage (Murali et al., 2005). Similarly, the low- est yield was observed in genotype “Robusta” when water stress was applied after the flower set (Hegde and Srini- vas, 1989). Ravi and Vaganan (2016) stated that the gen- otypes, “Rasthali”, “Robusta” and “Karpuravalli” showed a reduction in bunch weights when stress was applied. Mango (Mangifera indica L.) is the third-largest fruit crop of Oman after date palm and banana (Al-Yahyai et al., 2013). Due to the huge diversity of this crop, it has limited the outbreak of mango decline disease in Oman (Al-Adawi et al., 2006). In Oman drought con- ditions, mangoes showed a very little response in leaf visual parameters and leaf potential by adjusting their physiological functions according to drought. Mango growth comes in flushes, in drought, the mango veg- etative flushes growth is reduced significantly, and it also lessens the leaf water contents, leaves in flush and flush length (Laxman and Bhatt, 2017). Water stress has a significant effect on mango reproductive growth, and it has been observed that drought stress for a lon- ger time delayed flowering time (Tahir et al., 2003). However, drought for a lesser time enhanced flow- ering in mango (Scholefield et al., 1986). Similar find- ings were observed by Schaffer et al. (1994) who stat- ed that drought has an inhibitory effect on vegetative flushes, but it promotes reproductive flushes. Drought can lead mangoes towards a decline in terms of yield and quality as well (Wei et al., 2017). Drought reduc- es fruit size by decreasing cell size and number and it also enhances fruit drop in mangoes (Singh, 2005). The variation in water requirement not only affects man- go fruit size and yield but also affects fruit quality at- tributes like titratable acid, total soluble solids (TSS), starch and ascorbic acid contents (Wei et al., 2017). Lime (Citrus aurantifolia) is one of the main citrus species cultivated in Oman, which represents about 3.44% of all fruit crops grown in Oman. However, in recent decades, the production of lime and its cultivat- ed area is greatly reduced in Oman. This decline has been largely attributed to a combination of biotic and abiotic factors that negatively affected lime tree growth and yield (Al-Yahyai et al., 2012). In lime, abiotic stress- es are of great concern as its key production countries such as India, Mexico and the Middle East are highly suffering from water scarcity and salinity (Donkersley et al., 2018). In citrus, irrigation and soil conditions are the important factors of its quality enhancement (Levy 21Review Article Al-Yahyai, Al-Kharusi, M. Khan, Al-Adawi, Al-Subhi, AL-Kalbanim, Al-Sadi and Syvertsen, 2004). Well-managed and precision irri- gation increases citrus economic production and profit (Ruiz-Sanchez et al., 2010). However, drought or wa- ter stress in citrus resulted in poor vegetative growth, yield and fruit quality, besides increasing pathogens susceptibility (Levy and Syvertsen, 2004). Similar find- ings were observed by other researchers who stated that water stress increased the incidence of disease de- velopment and has greatly influenced lime growth and yield (Syvertsen and Levy, 2005; Blodgett et al., 1997). Grape (Vitis vinifera L.) is one of the most cultivat- ed crops in the world. In 2005, drought severely affected grape yield and processing in South Africa, which re- sulted in an agricultural income loss of about 3.7 million dollars (Johnston, 2009). The family of Muscat grapes consists of more than 200 varieties; its name is associ- ated with Muscat the governorate of Oman and these are planted at Jabal Akhdhar where the temperature is highly suitable for their production. Muscat grapes are known globally for their peculiar aroma (Ebuen, 2019). Grape is drought tolerant fruit as compared to other fruit crops and can endure stress conditions; however, it goes through certain morphological, anatomical and physio- logical changes during water stress conditions. Grapes have large xylem vessels as compared to other fruits, which enables them to tolerate water stress or drought (Serra et al., 2014). During adequate water supply its roots remain on the upper topsoil while during drought, roots move deep in the soil (Bauerle et al., 2008), leading to its increased root length but decreased shoot length (Hardie and Martin, 2000). A higher number of new roots were observed in grapes during drought for water uptake (Serra et al., 2014). Water stress causes stoma- ta closure which limits photosynthesis and reduces dry mass production and yield (Berdeja et al., 2014). Howev- er, little drought stress at maturity increases sugar and phenolic contents in grapes (Van-Leeuwen et al., 2009). Papaya (Carica papaya L.) is a drought-tolerant crop, however, to achieve its productive yield, suffi- cient water is required (Campostrini and Glenn, 2007). Drought drastically reduces papaya plant growth, nutri- ent uptake, transpiration, photosynthesis, chlorophyll contents and leaf abscission in papaya (Mahouachi et al., 2006; Slattery et al., 2017). Water stress conditions limit the physiological performance of papaya (Ma- houachi et al., 2007), besides there is the highest ac- cumulation of proline, abscisic acid and jasmonic acid during drought (Mahouachi et al., 2012). Masri et al. (1990) reported a 50% reduction in leaves, 86% in flowers and 58% in fruit, the growth and development of papaya fruit was highly reduced during drought. Guava (Psidium guajava L.) is a dicotyledonous, evergreen plant that has tolerance against drought and saline conditions (Akram et al., 2017), however, various biotic and abiotic factors affect its growth. In guava, several morphological and physiological char- acteristics were affected by salinity (Bezerra, 2018). Salinity was also found to increase catalase, polyphe- nol oxidase, carotenoids, proline, but sodium chloride treatment decreased peroxidase and chlorophyll (a, b, and total) (Ghalati et al., 2020). Besides photosynthesis, drought severely affects certain biochemical and phys- iological functions (Estrada-Luna et al., 2000). Simi- larly, in coconut (Cocos nucifera) water deficit great- ly affects the growth, productivity, and physiology of plants (Gomes and Prado, 2007; Passos et al., 2005). Pomegranate (Punica granatum L.) is being grown in several parts of Oman, however, the green mountains (Al Jabal Al Akhdar) are the top areas where quality pome- granates are produced due to suitable climatic condi- tions, and is also a good source of income for growers (Al-Said et al., 2013; Al-Harthi, 2011). In Oman, harvest- ing of pomegranate local cultivars (‘Hamedh’, ‘Malasi’, ‘Helow’, ‘Qusum’) is done based on fruit size shape and color due to prevailing climatic conditions (Al-Yahyai et al., 2009). Pomegranate is popular due to the presence of antioxidants and phenolic compounds, while these con- tents vary with maturation, ripening and abiotic stresses (Labbé et al., 2016; Zarei et al., 2011). In pomegranate, Khattab et al. (2011) observed a decrease in chlorophyll contents with a decrease in irrigation. Similar findings were observed by Tavousi et al. (2015) who stated that it is highly susceptible to dehydration while it has re- sistance against salinity. Pourghayoumi et al. (2017) examined pomegranate cultivars under drought condi- tions and found increased proline contents in all culti- vars except ‘Ghojagh’, while the cultivar ‘Rabab’ showed high contents reactive oxygen species (ROS) scavenging mechanism in drought conditions, besides these geno- types, showed good tolerance against drought as well. Salinity Salinity is one of the major problems in arid and semi-ar- id regions (Foster et al., 2018). More than 800 million hectares of land or over 6% of world soil surfaces are af- fected by this problem (Munns and Tester, 2008). Global annual losses in crops due to salinity are more than US $12 billion (Shabala, 2013). The first signs of salinity and its effects are osmotic effects, whereby the presence of high sodium chloride (NaCl) levels in soil inhibits wa- ter uptake leading to osmotic stress, which causes slow growth. Moreover, when the salt stress becomes severe, oxidative stress is induced and eventually causes death particularly in sensitive plants (Munns and Tester, 2008; Munns, 2002; Shabala, 2017). In the Sultanate of Oman, salinity has affected a large area of agricultural lands (Al-Yahyai, 2006). However, some fruit trees withstand harsh salinity conditions and thrive very well under sa- linity, such as date palms. In the Sultanate, date palm ac- counts for more than 200 cultivars (Al-Yahyai and Khan, 2015; Al-Yahyai, 2011). It is a dioeciously sustained fruit tree that can withstand salinity, drought, and tempera- ture conditions (Hazzouri et al., 2020; Alhammadi and Kurup, 2012) it is a salinity tolerant fruit crop (Dowson, 22 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 Biotic and Abiotic Stresses of Major Fruit Crops in Oman: A Review 1982; Yaish and Kumar, 2015; Sperling et al., 2014; Levy and Syvertsen, 2004; Furr and Armstrong, 1975). Date palm is a pivotal agriculture crop in Oman (Al-Yahyai and Khan, 2015; Al-Yahyai and Al-Khanjari, 2008), and the largest date palm yield is produced from the Al-Bati- nah region, the northern part of Oman (Al-Yahyai, 2011). According to FAO, the total date production in Oman is 8.5 Mt in 2018 (FAO, 2019). Date palm growth and development decrease under environmental stress conditions. Reduction in date palm under salinity is well documented in different countries (Tripler et al., 2007; Tripler et al., 2012; Chao and Krueger 2007; Serret et al., 2020; Al-Qurainy et al., 2020; Al Man- soori et al., 2006) and in Oman (Jana et al., 2019; Yaish et al., 2015; Al-Yahyai, 2006; Al-Harrasi et al., 2020; Al- rasbi et al., 2010; Haplogypsids et al., 2006; Erskine et al., 2004; Al-Yahyai and Manickavasagan, 2012a,b; Al Kha- rusi et al., 2017); however, the degree of reduction varies within the cultivars. Recently, several researchers in the country reported that salinity has affected the growth of some susceptible cultivars of date palm (Al Kharu- si et al., 2017; Patankar et al., 2019, Al-Rahbi, Al-Mul- la and Jayasuriya, 2020; Al-Harrasi et al., 2020). These cultivars can withstand low-moderate salinity, however, more energy is required to perform normal metabolism. Additionally, in date palm, there are some inappropriate irrigation practices such as using saline water, which is pumped from wells (Yaish et al., 2017) and has changed the groundwater recharge rates resulting in sea-water intrusion into the groundwater leading to soil saliniza- tion (Yaish et al., 2015) . Therefore, a significant cause of soil salinity may be owing to the use of poor-quality water for crop irrigation (Munns and Tester, 2008). Be- sides, other factors may have contributed to the reduc- tion of date yields such as high evapotranspiration rates and low rainfall amount. On the other hand, some toler- ant cultivars, when exposed to high salinity metabolize normally without showing any damage symptoms (Al Kharusi et al., 2017b). In general, salt stress can reduce date palm growth and development through different aspects such as osmotic effects, ion toxicity, inadequate nutrients (Al-Qurainy et al., 2020; Hazzouri et al., 2020). Aljuburi (1992) studied the effects of salt stress on the growth of four date palm cultivars, Lulu, Khalas, Boman and Barhee. He found that the cultivar Lulu was more susceptible to salinity than other cultivars. Salinity tolerance, which involves several complex mechanisms, protects plants against the harmful effects of salinity. Date palm cultivars differ in their salinity tol- erance capacities (Marashi et al., 2017; Al-Khateeb et al., 2019; Abdulwahid 2017; Djibril et al., 2005; Ibraheem et al., 2011; El-Khawaga, 2013; Yaish et al., 2015). In the Sultanate of Oman, some date palm cultivars (i.e., Manoma, Umsila, Fard Nagal and Barni) can thrive in saline conditions (Al Kharusi et al., 2017; Alrasbi et al., 2010). Despite some of these cultivars are considered not good quality and used as animal feed (Al-Yahyai and Khan 2015; Al-Yahyai, 2006), are capable to adapt and mitigating the environmental impacts in their growing zones such as salinity stress. These cultivars possess var- ious mechanisms that allow their roots, leaves and trunk tissues to cope with high salt availability where they are grown (Satisha et al., 2020; Patankar et al., 2019; Haz- zouri et al., 2020; Al Kharusi et al., 2019; Youssef and Awad 2008; Kurup et al., 2009; Al Mansoori et al., 2006). Salinity Adaptation Mechanisms in Omani Date Palm Osmotic Adjustment Mechanism The osmotic adjustment mechanism is a pivotal adap- tation in date palm surviving under salinity because it helps to maintain turgor and cell volume amide salin- ity soil (Shabala and Shabala, 2011; Munns and Tester, 2008). Recently, an osmotic adjustment mechanism was reported in some Omani date palm cultivars (Al Kha- rusi et al., 2017) via an efficient photosynthetic system as indicated by the higher quantum yield, stomatal con- ductance, membrane stability and the excessive photo- chemical energy diverted to various pathways. Also, an osmotic adjustment mechanism was reported in anoth- er Omani cultivar (Al-Harrasi et al., 2020), where salt ions are then regulated and transported across the cell membrane via the proton motive force (pmf ) (Wegner and Shabala, 2020) and pumped by the date palm gene (Pd6NHX) into the vacuole of transgenic Arabidopsis plants (Al-Harrasi et al., 2020), followed by electro- chemical gradients. Then the gradients are balanced by the synthesis of high hydration organic metabolites and compatibles. These metabolites were indicated in recent research in Omani date palm cultivar as an adaptation mechanism under salt stress (Al Kharusi et al., 2019b), including sugars, nitrogenous compounds (proline and glycine-betaine) in the cytoplasm. These solutes are ca- pable to stabilize the cytoplasmic enzymes on the cellu- lar membranes, thereby protecting them against inacti- vation by salt ions (Smirnoff and Stewart, 1985), where they play a pivotal role in mitigating NaCl-induced po- tassium K+ efflux (Shabala et al., 2012), and these have a direct protective role for membrane integrity as osmo- protectants under salt stress and other environmental stresses (Zhang et al., 2010; He et al. 2010; Yaish, 2015). They also have an indirect protective role through par- ticipating in signal transduction pathways (Kumar et al., 2018). Similarly, effects of high salinity irrigation on growth, gas exchange, and photoprotection on young date palms (cv. Medjool) were reported by Sperling et al. (2014), where photosynthetic efficiency and potentially were preserved through osmotically driven stomatal clo- sure. Furthermore, another Omani date palm (cv. Um- sila) was reported as a saline tolerant cultivar and can adapt to salinity by better maintenance of plant-water relations (Al Kharusi et al., 2019). However, the synthesis of sufficient osmoprotectant is metabolically expensive 23Review Article Al-Yahyai, Al-Kharusi, M. Khan, Al-Adawi, Al-Subhi, AL-Kalbanim, Al-Sadi and energy exhaustive in plant tissues (Chen and Jiang, 2010). However, date palms have proven to accumulate high energy primary and secondary metabolites such as antioxidants, vitamins, growth regulators, sugars and nitrogenous compounds (Al-Kharusi et al., 2009; Ab- dulwahid, 2012; Al Kharusi, Al Yahyai and Yaish, 2019a, Jana et al., 2019, Daoud et al., 2019). Ion Homeostasis Mechanism Salt Inclusion vs. Exclusion Some of the physiological and molecular basis of salt tolerance in Omani date palm cultivars were reported (Al Kharusi et al., 2019b; Al-Harrasi et al., 2020; Alrasbi, Hussain and Schmeisky, 2010; Jana et al., 2019; Yaish et al., 2015; Al-Yahyai and Manickavasagan 2012a,b; Pa- tankar et al., 2016). Besides the production of organic Osmo-regulators, date palm tissues also possess an al- ternative mechanism to accumulate high levels of ions, such as K+ and calcium Ca2+ and more other ions from the external soil medium (Al Kharusi et al., 2019b). Date palms can take up low amounts of Na+ in the leaves tis- sues and accumulate high amounts of K+ (Al-Rahbi et al., 2020; Al Kharusi et al., 2017a,b; Patankar et al., 2019a,b), with low energy demand. In a recent study on the salinity tolerance mechanism on Omani date palm (cv. Khalas), it was proven that overexpression of one of the date palm aquaporin genes (PdPIP1;2) confer salinity tolerance and implicated in Na+ and K+ transportation (Patankar et al., 2019a). In addition, it was found that accumulation of this gene in the date palm leaves un- der different stress conditions (e.g. drought) could also have a role in photosynthesis efficiency (Patankar et al., 2019a). Another study on Omani established young date palm seedlings (cv. Umsila) reported that significant ac- cumulation of enzymatic and non-enzymatic antioxi- dants are pivotal and have caused a significant increase in the xylem loading of K+, which then is translocated to the leaves (Assaha et al., 2017) as a result of higher K+/ Na+ ratio balance. Additionally, this study revealed that both leaves and roots tissues maintained higher chloro- phyll content indicating better photosynthesis capacity and high proline content and subsequently decreased reactive oxygen species (ROS), which confer salinity tol- erance (Al Kharusi et al., 2019). Another recent study on Omani date palm revealed that a cloned date palm gene (PdMT2A) improved drought and salinity stress tolerance in transgenic Arabidopsis plants and showed a higher K+/Na+ ratio (Patankar et al., 2019b). Salt tolerance in date palm is associated with lignin accumulation (Al Kharusi et al., 2019b) and the forma- tion of Casparian strips (Jana et al., 2019). Increased deposition of lignin in the steel was reported in Omani date palm salt-tolerant cultivar (Al Kharusi et al., 2019), as lignification was observed in the root endodermal vas- cular tissues in response to salinity. On the other hand, increased deposition of Casparian band was reported in two Omani date palm cultivars (Al Kharusi et al., 2019b; Jana et al., 2019). Casparian strip enhances selective ion and water movements across the root cell membranes and maintains a balanced Na+/K+ ratio and antioxidant defense as well (García-Caparrós et al., 2018). Also, Casparian strips enhance the apoplastic barriers and regulate hydraulic conductivity in roots and therefore, prevent the non-selective apoplastic bypass of Na+ into the stele tissues, which helps increase salinity tolerance (Zimmermann et al., 2000; Karahara et al., 2004). It was also proposed that maize, cotton and beans tend to ac- celerate the growth of the exo- and endoderm cells in response to salinity (Karahara et al., 2004; Reinhardt and Rost, 1995; Schreiber et al., 2005). Also, the effects of high salinity irrigation on the growth of young date palms (cv. Medjool) was reported by Sperling et al. (2014), where toxic sodium ions were excluded from the leaves of the young seedlings through the exclusion method. Accumulation of Global Metabolomics and Differential Metabolites Mechanism Salt tolerance mechanisms are yet to be puzzled out in date palm, since specific metabolites may be associated with salinity tolerance. A study aimed to decipher the salinity tolerance mechanism in date palm based on the information encoded by the metabolomics profiles of the Omani date palm salt-tolerant cultivar revealed the presence of natural metabolites, potent antioxi- dants, growth regulators and essential vitamins under salt conditions. These metabolites may help to improve the membrane capacities to control water and ion flow, as well as modification of the lignin and Casparian strip constituents, which may assist in better salt exclusion. In addition, the enhancement of the accumulation of various antioxidants, which act as reactive oxygen spe- cies (ROS) scavengers, may help to reduce the oxidative damage caused by salinity. Therefore, these metabolites could represent a key mechanism for salt tolerance in date palms. Also, this study showed another adaptation mechanism through the accumulation of some mem- brane and cell-wall constituents that were altered in re- sponse to salinity in date palms such as phospholipids, antioxidants, and vitamins. The phosphatidylethanol- amine (LysoPE), a plant growth regulator, is among the phospholipid metabolites and was found exclusively ac- cumulated in the leaves of the salt-tolerant cultivar (cv. Umsila) when exposed to salinity. This metabolite is in- volved in the growth-promoting activity (Cowan, 2009). Another accumulation of glycolipids was noted from this study; monogalactosyldiacylglycerol (MGDG) increased by 5.8-fold and digalactosyldiacylglycerol (DGDG) were exclusively detected in the leaf tissues of ‘Umsila’ when exposed to salinity. The abundance of these two glycolipids in the membrane determines the stability and physical properties of the thylakoid membrane of the chloroplast, which may influence salinity tolerance (Quartacci et al., 2000). This study also reported various 24 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 Biotic and Abiotic Stresses of Major Fruit Crops in Oman: A Review antioxidants, which were identified using the LC-MS technology and were mapped to the flavonoid synthesis pathways that usually result in the production of antiox- idants metabolites and potential free radical scavengers (Figure 4). These metabolites included catechin, epicat- echin, and eriodictyol. Epicatechin and catechin, which are flavonoids that perform antioxidant and iron-che- lating functions (Morel et al., 1993, Koch et al., 2017). These compounds accumulated in salinity-treated ‘Um- sila’ seedlings. This study also revealed the importance of vitamin B9 in date palm under salinity stress, as it was found that metabolites such as 5, 10-methenyltetrahy- drofolic acid, L-glutamic acid, methionine, cysteine, and other antioxidants were involved in vitamin B9 synthe- sis. These metabolites were differentially accumulated in cv. ‘Umsila’ upon exposure to salinity. Previous studies have also shown that vitamin B9 is involved in abiotic stress tolerance in plants, with susceptible plants unable to maintain a reasonable level of this vitamin under abi- otic stresses (Hanson et al., 2016). Biotic Stresses affecting Fruit Production in Oman Several biological challenges affect fruit production in Oman. Biotic stresses are important due to the econom- ic losses that cause to fruit growers. Several studies have identified, major issues with fruit crops, most notably Dubas bug on date palm, phytoplasma in lime, viral & fungal diseases in citrus, and mango sudden wealth in mangoes, are the most common causing major chal- lenge to the main fruit crops in the country. Dubas bug (Ommatissus lybicus de Bergevin) is a ma- jor pest that causes major damage to date palm and re- duces its growth and productivity (Al-Kindi et al., 2018) and requires large chemical input to control this pest. Studies have been conducted to investigate the poten- tial of natural enemies such as Pseudoligosita babylonica Viggiani, Aprostocetusnr beatus, and Bocchus hyalinus to control the Dubas bug (Al-Kindi et al., 2018). Fur- ther research on the impact of Dubas bug distribution throughout Oman and the environmental and climat- ic factors that affect the distribution of Dubas. Studies have predicted that the north of Oman is presently at great risk of Dubas bug invasions and will remain high in 2050 and 2070 (Shabani et al., 2018). Mango Wilt Disease in Oman Mango is an important fruit crop in Oman with a culti- vation area of 1,269 ha, representing 3.39% of the total Figure 4. The involvement of differentially accumulated metabolites in response to salinity in the flavonoid pathway and the production of antioxidants. The pink boxes indicate the differentially accumulated metabolites in date palm as a result of salinity. 25Review Article Al-Yahyai, Al-Kharusi, M. Khan, Al-Adawi, Al-Subhi, AL-Kalbanim, Al-Sadi fruit cultivated area with a production of 10.9 thousand tons (Table 1). Mango wilts disease, caused by Cerato- cystis manginecans (Al Adawi et al., 2006; Van Wyk et al., 2007), is the most serious threat to the production of mango in the country. The disease killed thousands of mango trees in Oman since its first report in 1998 (Al Ad- awi et al., 2006). The bark beetle Hypocryphalus mangif- erae vectors the pathogen from diseased to healthy hosts, which helps spread the disease (Al Adawi et al., 2013a). Data from the agriculture census in 2004/2005 es- timated the death of over 210,000 mango trees during 2000–2005 in the Al Batinah region alone (MAF, 2002; 2003; 2004; 2006). The disease is more severe in trees propagated from local seed sources and on exotic cul- tivars grafted on the rootstock of Omani cultivars (Al Adawi et al., 2006). Mango wilt disease management includes chemical treatments that are expensive and effective only for short periods of time. The selection of resistant culti- vars is a long-term effective control of this disease. C. manginecans inoculation trials in Oman showed that the cultivar ‘Pairi’ and local mango cultivars were high- ly susceptible. On the other hand, the cultivars ‘Hindi Besennara’, ‘Sherokerzam’, ‘Mulgoa’, ‘Baneshan’, ‘Rose’, and ‘Alumpur Baneshan’ were relatively resistant (Al Adawi et al., 2013b). Witches’ Broom Disease of Lime (WBDL) Acid lime has been a major commercial crop in Oman for decades. However, acid lime production has been seriously hampered over the last forty years due to Witches’ Broom Disease of Lime (WBDL). In Oman, lime production has declined by 75% due to WBDL and now it stands at an average 6.68 thousand tons repre- senting 1.79% of fruit production in the country (Table 1). About 80% of the mature acid lime crops are infected with WBDL and most of them either have been killed or removed from the orchards (Khan, 2000; Al-Yahyai et al., 2012). Affected acid lime trees show witches’ broom, small leaves, production of several branches and re- duction in flower and fruit production from symptom- atic branches of infected trees (Al-Yahyai et al., 2015). WBDL has been worsened by biotic stresses that in- clude Citrus tristeza virus and citrus viroids (Al-Sadi et al., 2017) and abiotic conditions caused by drought and salinity. The disease is caused by ‘Candidatus Phytoplas- ma aurantifolia’ (16SrII-B) (Zreik et al., 1995; Al-Subhi, 2018). WBDL has also been reported in the UAE, Iran, Saudi Arabia, India and Brazil (Bove et al., 2000; Alhu- daib et al., 2009, Queiroz et al., 2016; Ghosh et al., 1999). Infected acid lime trees from southern Oman, Saudi Arabia and Brazil do not show typical witches’ broom symptoms (Al-Subhi, 2018). Environmental conditions may influence symptom expression (Al-Ghaithi, 2017). Other citrus species like citron, lemon, C. macrophylla and rough lemon have been reported as hosts of the ‘Ca. P. aurantifolia’ in Oman (Al-Subhi, 2018, Al-Subhi et al., 2019, Al-Yahyai et al., 2012). WBDL phytoplasmas can be transmitted by differ- ent means including insect vectors, grafting and seeds. There is evidence that WBDL phytoplasma can be trans- mitted via seeds (Al-Shanfari, 2000; Al-Amri, 2006). The main means of spread of phytoplasma diseases in the fields is by phloem-feeding insect vectors in the fami- lies Cicadellidae (leafhoppers), Fulgoridea (planthop- pers) and Psylloidae (psyllids). Two hemipteran spe- cies, leafhoppers and psyllids were recorded as vectors of WBDL phytoplasma in Oman (Queiroz et al., 2016). Citrus Tristeza Virus (CTV) Citrus tristeza virus (CTV) resulted in extreme losses in citrus in different parts of the world. The first report of CTV epidemics was in the early 20th century in South Africa where millions of citrus trees grafted on sour or- ange rootstocks collapsed (EPPO, 2004). Subsequently, millions of citrus trees were lost in Argentina and Brazil in the 1930s (Bar-Joseph et al., 1989). Also, over 40 mil- lion trees declined progressively in Spain in 1957 (Cam- bra et al., 2000). CTV causes different symptoms, the most severe of which are rapid decline (tristeza) and stem pitting. The first causes the decline of trees grafted on sour orange, where grafted trees start collapsing in a few weeks. Trees affected with CTV stem pitting strain can show variously sized pits or grooves on twigs or trunks (Brlansky, 2002). Stem pitting symptoms are common in lime, pummelo, grapefruit and sweet orange (Lee and Bar-Joseph, 2000). Other symptoms include leaf chlorosis, stunting and a reduced root system. CTV dispersal can be either by the propagation of virus-infected buds that can cause an introduction of CTV to a new area (Bar-Joseph and Lee, 1989) or by vector transmission. In Oman, the first detection of CTV was reported in 1986 in grapefruit, mandarin and lemon seedlings shipped from India (Bove, 1995). Aphis gossypii, which is one of the important vectors of CTV, was report- ed in Oman in 1984 (Moneem, 2005). The Ministry of Agriculture (MA) surveyed the Sultanate in 2007 and 2008. They found that CTV is associated with acid lime and is distributed in Muscat, Dhofar and Al Batinah regions (MA, 2009). A subsequent study by Al-Sadi et al. (2012) in 2009 and 2010 showed that CTV is pres- ent in acid lime farms in Barka, Alrustaq, Shinas, Sohar, Alsuwaiq, Boushar, Alseeb, Quarayat, Dhank, Yanqul, Ibri, Mahadha, Bahla, Nizwa, Samael, Dibba, Madha, Ibra, Almudhaibi, and Salalah. The incidence ranged from 3 to 63%. In addition, CTV was detected in citrus seedlings/budlings imported from different countries. Management options of the disease should include strict quarantine measures and eradication programs (Navarro et al., 1984; Frison and Taher, 1991). All cit- rus-growing counties should have certification pro- grams to ensure that CTV is not spread with budwood 26 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 Biotic and Abiotic Stresses of Major Fruit Crops in Oman: A Review or seedlings (Navarro et al., 1988). It is also important to avoid grafting trees on sour orange rootstocks as they make trees more vulnerable to the decline disease. Fusarium Wilt Disease of Banana Banana is the second most important fruit crop Oman. Over 2.3 thousand hectares of agricultural land in the Sultanate are grown with the banana crop of various va- rieties producing an annual average of 30.74 tons, which represents 8.25% of all fruits produced in Oman (Table 1). The North and South Al Batinah governorates come first in terms of cultivated area, with an area of 4,197 faddan (1762.74 ha), followed by Dhofar Governorate with 960 faddan (403.2 ha), then Sharqyia governates with an area of 81 faddan (34.02 ha) (MAF, 2014). Ba- nana varieties grown in Oman include dwarf cavendish (local known as Malindi bananas), Al-Barshi, Somali, Al-Fard, and Williams (Viswanath, et al., 2000, de Lang- he, 2002, Al-Hosni et al., 2010; Al-Saady et al., 2010). Many diseases limit banana production in the world like Fusarium wilt (Panama disease), Moco disease, black segatoka, bunchy top virus disease (Ploetz, et al., 2003). Banana Fusarium wilt disease caused by Fusarium oxysporium f.sp cubense (FOC) is a soil borne disease and pathogen persist in the infected area for decades. The pathogen initially infects banana plants through roots then moves through the plant into the vascular system within the pseudo-stem and blocks vascular tissue and causes plant death. Infected banana plants exhibit yellowing of older leaves and discoloration of vascular tissues. The disease became most destructive in many banana-growing countries. The disease was first- ly reported during 1874 in Australia and later race1 of the pathogen FOC was reported in Central American countries (Ecuador and Panama) wiping Gross Michel banana cultivar from these areas and South America. Consequently, the Gross Michel cultivar was replaced with the cultivation of Cavendish cultivars which was resistant to FOC race 1, and Cavendish cultivars dom- inated the market (Ploetz, 2005). During the early 1990s, a new race of FOC called Tropical race 4 (TR4) was reported Southeast Asia causing a serious problem for Cavendish cultivars plan- tations in this region (Ploetz, 2006a &b). The new race TR4 spread around the world and reported threatening banana cultivation in many countries in Asia, Africa and recently in South America (Damodaran et al., 2019, O’Neill et al., 2016, Ordóñez et al., 2016, Ploetz, 2015, Ploetz, et al., 2015). In Oman, symptoms resembling that of Fusarium wilt disease of banana were first ob- served in northern governates. Samples from diseased banana plants were collected from infected farms and isolated pathogen identified as FOC race TR4 using mo- lecular diagnostic marker (Dita et al., 2010). Integrated disease management options of Fusarium wilt of banana should be considered as until now there is no known ef- fective chemical or biological control measures for this serious disease. Strict quarantine measures are import- ant to prevent the spread of the disease from infected area to new healthy banana plantations and Oman has Table 1. Area and production of major fruit crops in Oman (2000-2018) (Source: FAOSTAT, 2020). Area (ha) Production (tons) Year All Fruits Dates Bananas Limes Mangoes All Fruits Dates Bananas Limes Mangoes 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 42,485.00 40,758.00 40,672.00 40,198.00 42,463.00 37,173.00 36,580.00 38,521.00 37,239.00 36,947.00 38,468.00 39,136.00 35,526.00 39,434.00 41,218.00 29,832.00 29,576.00 32,126.00 32,805.00 35,508.00 33,919.00 33,869.00 33,848.00 35,532.00 31,353.00 31,353.00 32,746.00 31,353.00 31,353.00 31,353.00 31,348.00 30,615.00 34,195.00 36,255.00 24,120.00 24,120.00 24,617.00 25,125.00 2,633.00 2,716.00 2,610.00 2,520.00 2,856.00 2,225.00 2,225.00 2,435.00 2,435.00 2,436.00 3,720.00 4,374.00 2,500.00 1,400.00 1,421.00 1,421.00 1,421.00 1,564.00 1,560.00 1,678.00 1,620.00 1,690.00 1,440.00 1,210.00 1,242.00 1,242.00 1,231.00 1,231.00 1,233.00 1,233.00 1,236.00 710.00 1,200.00 1,208.00 1,208.00 1,207.00 1,302.00 1,399.00 1,507.00 1,510.00 1,510.00 1,500.00 1,470.00 1,070.00 1,071.00 1,071.00 1,071.00 1,071.00 1,071.00 1,071.00 610.00 1,490.00 1,019.00 1,496.00 1,496.00 1,505.00 1,506.00 359,353.00 383,465.00 320,298.00 295,395.00 320,275.00 324,012.00 315,090.00 325,544.00 345,560.00 327,379.00 382,421.00 376,587.00 382,491.00 391,497.00 386,583.00 416,832.00 427,567.00 495,765.00 505,114.00 280,030.00 298,006.00 238,611.00 219,770.00 231,000.00 247,331.00 258,738.00 255,871.00 267,000.00 258,572.00 276,405.00 268,011.00 281,000.00 308,400.00 317,400.00 344,690.00 355,332.00 360,917.00 368,808.00 32,150.00 33,680.00 32,915.00 28,750.00 34,000.00 26,720.00 25,955.00 28,892.00 29,000.00 28,890.00 56,686.00 61,584.00 56,790.00 19,970.00 18,184.00 16,578.00 16,578.00 18,397.00 18,265.00 8,210.00 8,560.00 8,385.00 6,830.00 5,875.00 6,159.00 5,916.00 5,983.00 7,440.00 6,354.00 6,354.00 6,503.00 6,340.00 6,250.00 5,943.00 6,199.00 6,199.00 6,340.00 7,112.00 10,874.00 10,945.00 10,910.00 10,910.00 8,700.00 7,743.00 6,882.00 6,373.00 10,000.00 10,199.00 10,199.00 8,949.00 8,600.00 12,969.00 9,496.00 15,673.00 15,673.00 15,924.00 15,847.00 Aver- age 37,429.32 31,188.53 2,340.63 1,290.53 1,269.21 372,696.21 286,099.58 30,736.00 6,681.68 10,887.68 % of total 83.32 6.25 3.44 3.39 76.76 8.25 1.79 2.92 27Review Article Al-Yahyai, Al-Kharusi, M. Khan, Al-Adawi, Al-Subhi, AL-Kalbanim, Al-Sadi established strict quarantine measures in banana pro- duction regions to prevent the spread of diseases. Fur- thermore, tissue culture banana plant materials should be the sole source for planting banana in an area free of FOC TR4 and avoid using sucker planting material as it could be a source of new inoculum of the pathogen to an area free of the disease. Several banana cultivars were found to be moderately resistant to FOC TR4 including Giant Cavendish Tissue Culture Variants (GCTCV), which was developed in Taiwan and two of these vari- ants including GCTCV 218 & GCTCV 219 were used in several countries to reduce the impact of the dis- ease in the infected area. Furthermore, several hybrids developed through a breeding program at Fundacion Hondurena de Investigacion Agricola (FHIA) in Hon- duras such as FHIA 1, FHIA 2, FHIA 3, FHIA 18 and FHIA 25 were found slightly susceptible to FOC TR4. However, all the above-mentioned bananas were not acceptable in the market, compared with world-known Cavendish cultivars such as William and Grand Nain (Ploetz, 2015a &b). Many of these developed cultivars of banana are being evaluated in the banana gene banks at research centers of Al Batinah and Dhofar in Oman. Climate Change Globally, climate change is of major concern (Thom- as et al., 2004). Due to this climate change, Oman has been ranked at 40th position by German Watch, based on data available at Global Climate Risk Index (Harmel- ing, 2010). According to IPCC (Intergovernmental Panel on Climate Change) findings, Oman has been declared at higher risk due to climate change and the increasing temperature, sea level, erratic rainfall pattern and their combination is leading Oman towards desertification by affecting its coastal regions and ecosystem (IPCC, 2007). The climate of a particular area affects its various environ- mental aspects and climate change greatly reduces agri- cultural productivity and stability, by changing cropping patterns and loss of agricultural land (Waqas et al., 2019). Climate change is one of the major concerns of 21st century, anthropogenic activities have increased the car- bon dioxide level in the atmosphere which has resulted in very high or low temperatures and irregular rainfall patterns. Due to these climatic changes, stresses occur in plants which severely affect agriculture production and are key constraints to the food supply, as climate change disturbed the cropping season, pattern, dura- tion, and production intensity (Harmeling, 2010). Oman is at high risk of these vulnerable climate changes, for example, there is an increase in average temperature, sea level, desertification, and irregular rainfalls. Besides in 2007 and 2010, Oman suffered from two cyclonic storms that caused adverse effects on agriculture production and plant genetic resources (Al-Khafaji et al., 2017). It is also speculated that recent harmful algal bloom out- breaks along the Oman coast are due to climate change (Al-Azri et al., 2010). Cold stress and heat stress, both are related to tem- perature, any of these stresses during plant develop- ment stages severely affect crop production (Zhou et al., 2018). The temperature stress in plants reduces their biochemical and physiological functions by tempering their molecular mechanism (Takahashi and Shinozaki, 2019). High-temperature stress increases plant growth and reduces crop duration (Lobell et al., 2011). It greatly affects plant reproductive growth by reducing pollen vi- ability, pollen fertility, flower abortion, and pistil tissues (Djanaguiraman et al., 2018; Talwar et al., 1999). It has also been reported that heat stress or high temperature causes premature another development, restricted cell proliferation and induces male sterility in plants (Oshi- no et al., 2007; Abiko et al., 2005). Likewise, cold tem- perature stress affects the cropping season and yield as well, according to geographical distribution one-degree drop on average can reduce the yield of summer field crop up to 40% (Larcher, 2003). Cold temperature af- fects plants at various stages including the early stage, acclimatization phase, developmental stage, and fruit maturity stage. Cold stress at any stage of plants high- ly disturbs its biochemical and physiological functions. Potential Impact of Climate Change Climatic factors such as temperature and precipitation affect the quality and production of fruits (Akram et al., 2020). In Oman, high temperature is of great con- cern as it disrupts various plant physiological processes. During the photosynthesis process, high temperature causes carbon fixation in dry matter, disturbs plant parts during its various stages of development (Laxman and Bhatt, 2017). Generally, in fruit plants, it causes sunburn of various plant parts like leaves, branches, twigs and stems. Besides, it retards shoot and root growth, leaf se- nescence and fruit discoloration (Wahid et al., 2007). It also disrupts plant normal cell functioning and repro- ductive stages. The date palm can tolerate a wide range of climatic conditions, it can tolerate frost and is well adapted to heat as well. Temperature widely affects date growth, flowering, fruit setting and fruit production. Due to high temperatures, early bloom in dates was also noticed (Darfaoui and Assiri, 2009). Dates require prolonged summer to mature fruit, however, rainfall and humidity at fruit maturity cause fruit cracking and attract fungal dis- eases (Burt, 2005). Date palm can tolerate temperature up to 50oC under irrigation and as low as 0oC, however, growth is restricted below 7oC and above 40oC and it also affects the quality of date production (Al-Yahyai, 2018). The date palm biodiversity is on a steady decline in Oman due to certain biotic and abiotic factors. However, realizing the vulnerability of in-situ gene banks of date palm, newer approaches for preserving local varieties are being investigated. Detailed studies on date palm in Oman are under way at the Date Palm Horticulture Research Laboratory, Jimmah (MAF, 2006). It has been 28 SQU Journal of Agricultural and Marine Sciences, 2022, Volume 27, Issue 1 Biotic and Abiotic Stresses of Major Fruit Crops in Oman: A Review argued that Oman is prone to climate change effects and farming and precious plant genetic resources are under threat due to ever-changing climate, population influx, urbanization, and other factors along with environment are playing substantial roles in agricultural activities and Plant Genetic Resources (Hameed et al., 2017). In mango, temperature determines vegetative and flowering flushes. High temperature induces vegetative flushes (Laxman and Bhatt, 2017), temperature below 16 °C induces floral induction (Schaffer et al., 1994). Pan- icles that emerged below low temperature have more male flowers (Singh et al., 1974). The sudden variability in climate change may influence the vegetative and repro- ductive growth of mango by affecting its production. Re- cently extreme weather scenarios such as Gonu in 2007 and Phet in 2010, and torrential rains-caused flooding have damaged the date plantation. Events related to cli- mate change may be the next challenge to date palm sus- tainable cultivation in Oman. The government helped in replacing date palms destroyed by climatic catastrophes like cyclones and storms (Al-Yahyai and Khan, 2015). Each grape cultivar performs well in a suitable range of temperatures and changes in temperature of climate its physical and biochemical parameters (Akram et al., 2020). In grapes, the adaptability of genotypes of a partic- ular region enables it to produce fruit efficiently (Akram et al., 2019). High-temperature effects sugars, acids and antioxidant compounds in grapes, besides it may affect the aroma and color of grape vines (De-Orduna, 2010). In grapes, high temperatures lead plants towards prema- turity due to which berry size does not develop properly. Due to climate change, frost and storms have changed the citrus phenology which has increased the risk factor (Fitchett et al., 2014). The global increase in temperature has increased the distribution and be- havior of citrus insects. For example, the incidence of pathogens Candidatus and Diaphorina citri is highly increased with an increase in temperature (López-Ar- royo et al., 2009). Likewise, rainfall distribution high- ly affected the natural distribution of citrus black spot disease (Martı´nez-Minaya et al., 2015). In coconut, the high temperature reduces the pollen production and promotes male flowers induction (Burke et al. 2004), besides high temperature and excessive rainfall cause flower and fruit abortions (Nainanayake et al. 2008). Likewise, in banana, growth is also affected by high temperature and it was observed that leaf emergence and fruit production was highly reduced beyond 33.5°C and 35°C, respectively (Thornton and Cramer, 2012). In pomegranate, the ideal temperature for fruit growth is from 17 and 26°C, the high temperature at the fruit maturity increases oxidative stress by increasing peroxi- dase activity (Fischer et al., 2013). High temperature also varies antioxidant, anthocyanin, and the chemical com- position of pomegranate fruits (Fernandes et al., 2015). Conclusion The potential for expanding fruit crop production in Oman is great, especially since the country has the re- quirements, including diverse climatic conditions, edu- cated labor force, market chains, industrial infrastruc- ture, and logistical needs. Knowledge and cumulative experience that is based on research is also growing and aims to address fruit crop cultivation challenges. This re- view paper has highlighted the latest research on major challenges to fruit production in Oman, which include abiotic and biotic stress factors. In recent years, Oman witnessed a drastic change in climatic patterns and se- vere weather attributed to climate change, and existing adverse conditions, such as heat, salinity, and drought. Fruit production constitutes a large percentage of the total agricultural cultivated area in Oman, and almost 90% consists of date palm, banana, lime, and mango. In addition to the abiotic stress, major fruit crops have declined in recent years due to various biotic stressors, primarily pests and diseases including Dubas bug and Red Palm Weevil. Furthermore, lime has been infected with Witch’s Broom Disease of Lime (WBDL) caused by ‘Ca. Phytoplasma aurantifolia’, and large areas of man- goes have been lost to Sudden Wilt. Several solutions have been proposed to address these challenges, how- ever, further research is still needed as the challenges will continue to grow. 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