Impaginato 213 Adv. Hort. Sci., 2020 34(2): 213­221 DOI: 10.13128/ahsc­7770 Effect of spread and shallow irrigation wetted area and application of organic mulch on citrus decline amelioration M.S. Tadayon (*), S.M. Hosseini Soil and Water Research Department, Agricultural Research, Education and Extension Organization (AREEO), Shiraz, Iran. Key words: Citrus decline, fibrous root, irrigation, orange, root decay. Abstract: Citrus decline threatens the orchards in the southern part of Iran. Roots begin to die­out before citrus decline deterioration. In this study, the effect of expanding the irrigation wetted area and decreasing the irrigation depth, and application of compost as organic mulch on root development and amelioration of citrus decline in Valencia orange trees (Citrus sinensis L. Osbeck) was investigated. Experimental factors contained the different per­ centage of irrigation wetted area and decreasing irrigation effective root depth by drip irrigation system at three levels ­ W0 (control): 30­40% with 60 cm effective root depth; W1: 50­60% with 45 cm effective root depth; W2: 70­80% with 30 cm effective root depth under tree canopy area and the factor of annu­ al application of compost as organic mulch at two levels ­ M0 (control): means no application of compost and M1: application of 80 kg compost under tree canopy area with 10 cm thickness as ground cover, after rotating the top soil at 10 cm depth for all treatments. The results showed that, annual application of compost as organic mulch under tree canopy area and expanding the irrigation wetted area with decreasing irrigation effective root depth, significantly improved fibrous root length and density at a lower soil depth and decreased the indices of citrus decline such as root rot percentage, leaf and fruit drop and shoot die back. Also these treatments increased the water productivity and fruit quality of in declining Valencia orange trees. 1. Introduction Citrus decline, commonly known as ‘dieback’, ‘chlorosis’ or neglecto­ sis, is not a specific disease but a syndrome expressing many disorders in the plant. Such syndrome leads to decline in productivity, reduced pro­ ductive life and poor fruit quality. The symptoms of citrus decline contain root rot and blackening, shoot die back, growth stunt, fruit drop, reduc­ tion of canopy, smaller leaf number and size and leaf blotchy mottle (Meena et al., 2018). Stress­sensitive trees fail to maintain sufficient car­ bohydrate availability resulting in the dieback of the stressed tissues (Kreuzwieser and Rennenberg, 2014). Also, the percentage of total solu­ ble solids and fruit juices in healthy trees is more than declined trees (Mauk and Shea, 2002). Declining trees have more water stress and more (*) Corresponding author: m.tadayon@areeo.ac.ir Citation: TADAYON M.S., HOSSEINI S.M., 2020 ­ Effect of spread and shallow irrigation wetted area and application of organic mulch on citrus decline amelioration. ‐ Adv. Hort. Sci., 34(2): 213­221. Copyright: © 2020 Tadayon M.S., Hosseini S.M. This is an open access, peer reviewed article published by Firenze University Press (http://www.fupress.net/index.php/ahs/) and distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Competing Interests: The authors declare no competing interests. Received for publication 1 January 2020 Accepted for publication 5 May 2020 AHS Advances in Horticultural Science http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ Adv. Hort. Sci., 2020 34(2): 213­221 214 affected trees tend to have higher percentage leaf and fruit drop rates than healthy appearing trees. Soil condition and water status (water stress and water logging) significantly reduce the citrus fibrous root density and increase the severity of citrus decline (Kozlowski, 1997; Graham et al., 2013; Morgan, 2015; Graham, 2017). Citrus decline is an issue that threatens the econ­ omy of various regions of the world, including south­ ern Iran. There are several factors that affect the inci­ dence of this complication. The most important influ­ ential factors on citrus decline are environmental stresses such as soil physicochemical conditions (compaction, high pH, bicarbonate, salinity) and soil nutritional status (Srivastava and Singh, 2009), soil moisture content (water stress and water logging with deficit or over irrigation), and biological stresses including rootstock, nematodes, Greening disease (Huanglongbing), Tristesa virus disease, phytophtho­ ra fungal infection and root fusarium (Johnson and Graham, 2015; Graham, 2017; Meena et al., 2018; Dewdney et al., 2019). According to the USDA, since 2004­2005 total Florida citrus production declined by Greening disease from 169.1 to 94.2 million boxes in 2015­2016, down 44.4% (USDA, 2017). The ideal environment for citrus root develop­ ment is a porous, medium­textured, well­drained soil, where water is easily available but not in excess (Dewdney et al., 2019). Mulching with organic matter helps retain moisture in the top soil by reducing sur­ face evaporation, as well as moderating soil surface temperatures. It also enhances the decomposition process of soil organic matter and improve the soil aeration and availability of nutrients in the soil (Gong et al., 2006; FAO, 2011). Increasing root water uptake efficiency and life span is possible by irrigation man­ agement such as time and duration (decrease water stress and water logging), also improving the growth environment and drainage, soil compaction, soil ther­ mal stress, bicarbonate and osmosis stress (Huber a n d H a n e k l a u s , 2 0 0 7 ; D e w d n e y e t a l . , 2 0 1 9 ) . Increased root density also increases water uptake (Morgan et al., 2006). In arid and semi­arid regions, the application of drip irrigation system results in root accumulation under emitter (Fernandez et al., 1991; Tanasescu and Paltineanu, 2004; Ruiz­Sanchez et al., 2005), so the expansion of irrigation wetted area could enhance the fibrous root density under declining condition. Increasing irrigation frequency and decreasing irriga­ tion depth stimulate root length density and increase water uptake and under these conditions, irrigation at the field capacity increases root density by 50% (Kadyampakeni et al., 2014 a). In citrus decline new root growth did not stop, but the root survival time was reduced from 9 to 12 months to 4 months and the root decay and dieback were increased (Dewdney et al., 2019). Citrus decline cause water stress and under such condition root growth is preferable to shoot growth (Hsiao and Xu, 2000). Root development dependent on soil aeration and moisture as two fundamental parameters for root healthiness, growth and distribution. Root growth in sandy and loamy soils with higher organic matter content is stronger, and increment in soil clay content has a negative relationship with the density of citrus roots (Koudounas, 1994) and the slope of clay content in soil profile is directly related to the effect of citrus decline deterioration (Srivastava and Singh, 2009). In citrus, the highest fibrous length den­ sities were observed in the humid part of the soil at a depth of 0 to 15 cm with higher root activity until 2 m horizontal distance from the trunk (Alves et al., 2012). The rate of soil water depletion is directly related to the abundance of fibrous roots and in gen­ eral pattern of water uptake in citrus fruits indicates that water is drained by surface roots with higher amounts of available soil water (Noling, 2003). Lack of soil aeration and excessive soil moisture causes more severe damage than lack of moisture in declining trees, due to stop breathing and the break­ down of root cells. There is also the growth of anaero­ bic microenvironments and the production of sub­ stances such as nitrite that is toxic for the roots. Centralize wetted area and excessive irrigation results in water logging, oxygen deficiency and root fungal infestationin the root zone (Johnson and Graham, 2015). Deficit irrigation and soil water stress stimu­ lates longitudinal and singular growth without lateral roots and repeated moisture stress causes thicker fibrous roots in citrus that could decrease the water uptake efficiency by the citrus roots (Castle, 1978). Increment of soil aeration and soil moisture distribu­ tion and retention could provide the better condition for root development and amelioration of citrus decline. So in this experiment, the effects of spreading irrigation wetted area and decreasing irrigation depth, and annual application of compost as organic mulch on root development and diminishing of citrus decline indices in Valencia orange was investigated. Tadayon and Hosseini ‐ Citrus decline amelioration in Valencia orange trees 215 2. Materials and Methods The experiment was conducted in a commercial orchard (28°38’13.12” N, 54°40’29.69” E and altitude 1138 m) of Darab region,with very hot and dry cli­ mate (Table 1), located in south west of Fars province in Iran, over three consecutive years 2016 to 2018. Ninty­six uniform 12­year­old Valencia orange trees (Citrus sinensis L. Osbeck), on lime (Citrus aurantifo‐ lia) rootstock with citrus decline symptoms (sparse foliage, chlorotic leaves, twig drying, premature leaf fall, reduced productivity and fruit size and die­back canopy complication) were selected based on canopy diameter and labeled based on experimental plan. The distance between the trees was 4×5 m and irri­ gated by drip irrigation system with a loop contained 6 emitters (4 liters per hour). Before the application of experimental treat­ ments, a composite soil sample were taken from 0­30 and 30­60 cm depths in Oct. 2014 (Table 2). Soil sam­ ples characteristics were determined at analytical laboratory of Soil and Water Research Department, Fars Agricultural and Natural Resources Research and Education Center, Zarghan, Iran. The soil of the experimental field site was calcareous with high pH, high total neutralizing value, and low amount of organic carbon, available P and Mn (Table 2). The fer­ tilizer application was conducted based on soil test and contained fertigation of ammonium sulfate (21­ 0­0­24S, 450 g tree­1), potassium sulfate (0­0­53+17S, 150 g tree­1) and triple superphosphate (450 g tree­1), manganese sulfate (32Mn, 18S, 250 g tree­1) and iron chelate (200 g tree­1 sequestrene 138­Fe EDDHA 6%, 150 g tree­1). A Factorial (3×2) experiment was conducted in a randomized complete block design with four replica­ tions and each plot with four declining trees over three years. Experimental factors contained the dif­ ferent percentage of irrigation wetted area and decreasing irrigation effective root depth by drip irri­ gation system at three levels as follows: • W0 (control)= 30­40% with 60 cm effective root depth; • W1= 50­60% with 45 cm effective root depth; • W2= 70­80% with 30 cm effective root depth under tree canopy area. Also, the factor of annual application of compost as organic mulch at two levels: • M0 (control)= means no application of compost; • M1= application of 80 kg compost with 10 cm thickness as ground cover under tree canopy area. Table 1 ­ Long­term mean of climatic elements of the study area (Darab) synoptic meteorological station statistical period (1997­2018) Table 2 ­ Soil characteristics in the experimental orchard Soil depth Bulk density (g cm­³) Field capacity (%) Wilting point (%) EC (dS.m­1) pH TNV (%) OC (%) P (mg kg­1) K (mg kg­1) Zn (mg kg­1) Mn (mg kg­1) Cu (mg kg­¹) Soil texture 0­30 1.25 19.5 10.3 1.32 8.15 35.2 0.76 7.3 264.5 0.76 3.12 0.57 Sandy loam 30­60 1.28 19.3 10.4 1.36 8.16 37.7 0.62 7.2 252.4 0.62 3.19 0.56 Sandy loam Temperature (°C) Relative humidity (%) Precipitation (mm) Evapo. (mm) Sunny day No. day Max wind (m s­¹) Min Max Ave. Abs. Min Abs. Max Min Max Ave. Amount No. rainy Max daily (montly sum) (montly sum) frost Direc. degree Velo. m/s April 11.5 26.4 19.0 3.8 34.7 26 70 48 32.50 6 43.70 182.0 276.0 0 253 13 May 16.7 33.6 25.2 9.4 41.6 16 51 34 6.46 2 24.80 286.6 331.4 0 240 15 June 21.6 39.3 30.4 15.6 44.4 11 37 24 1.10 1 9.60 382.8 356.6 0 200 14 July 25.5 41.7 33.6 15.4 46.5 12 38 25 0.92 1 5.80 428.3 341.3 0 171 15 August 26.2 40.9 33.6 19.2 45.2 14 40 27 5.76 2 20.50 422.3 339.8 0 207 16 September 21.9 38.5 30.2 16.2 42.6 14 44 29 0.82 1 4.70 336.0 327.5 0 180 10 October 16.0 33.6 24.8 7.6 38.8 16 48 32 0.32 1 2.60 228.4 308.0 0 226 10 November 10.4 26.5 18.5 2.0 33.5 22 62 42 8.48 2 50.20 138.2 269.6 0 249 8 December 5.7 20.1 12.9 ­2.2 30.0 32 76 54 40.25 5 51.80 79.1 234.9 0 207 7 January 3.8 17.0 10.4 ­2.6 25.6 36 81 58 64.77 6 65.40 64.4 226.1 3 205 8 February 5.0 17.7 11.3 ­2.6 25.8 35 81 58 49.88 6 68.20 79.3 238.4 1 240 10 March 8.0 21.8 14.9 0.2 31.6 30 74 52 37.26 7 63.00 116.7 241.1 0 238 12 Adv. Hort. Sci., 2020 34(2): 213­221 216 Compost applied 50 cm away from the trunk of the tree and over the soil surface, after rotating the soil at 10 cm depth for all treatments in March of each year. The physico­chemical properties of the applied compost contained organic carbon (22%), total nitrogen content (1.5%), C/N ratio (15), P2O5 (0.8%), K2O (0.7%), electrical conductivity (6.4 dS. m­1), pH (7.6), moisture content (12%), density (380 kg.m­3), particle diameter (6 mm). The amount of irri­ gation wetted area was adjusted by increasing the number of emitters (6, 9 and 12 per loop with 4 liters per hour discharge) and the irrigation interval was determined by placing tensiometers (Irrometer Tensiometer Model SR Manual Gauge 12 in) deep down at three irrigations effective root depth (60, 45 and 30 cm, respectively) according to the irrigation experimental factor. Each year in February, the com­ posite soil sample were taken from 0­10 cm and the percentage of soil organic matter, porosity and bulk density (g cm­3) were determined. The amount of organic matter was measured by weight loss of oven­ dried (105°C) soil sample after loss on ignition at 400°C. To determine the porosity of the samples, divide the pore space volume by the total volume and multiply the result by 100. Soil bulk density was deter­ mined by the weight of dry soil divided by the total soil volume. The amount of consumed irrigation water was measured by contours in each treatment. Over the three seasons, the indices of citrus decline were determined on tagged main branches on each tree quadrates. At the late spring, new flush lengths and the percentage of shoot dieback were measured. Leaf and fruit drop percentage were determined by counting their number at two separate time (after June drops and pre­harvest time). Leaf samples (con­ tain 100 leaves) were collected during July­August from fully expanded new flush sub­terminal leaves from non­fruiting from tagged branches. Total chloro­ phyll content was determined by the method worked out by Lichtenthaler (1987). Leaf fresh weight (Wf) were recorded quickly using an analytical balance (Mettler Toledo AL104, Switzerland). Then, they were dried at 120°C in a circulation oven for 20 minutes, and the temperature dropped to 80°C until the con­ stant weight (dry weight, Wd) was reached. Then the leaf relative water content (RWC) was calculated as: Leaf relative water content (%) = [(Wf ­ Wd)/Wf] x 100 Individualleaf areas (LA) were determined by Li­ cor leaf area meter, and sample were dried and spe­ cific leaf weights, SLW (mg cm­2) were measured as: Specific leaf weight (mg cm­²) = Wd/LA Root sampling was carried out under emitters from depth of 30, 45 and 60 cm soil depth, based on the effective root depth considered in the irrigation experimental factor, by auger (diameter 9 cm and height 25 cm), in Early­August (Alves et al., 2012). After washing the root samples, length, weight and number of fibrous root with less than 0.2 cm diame­ ter was determined by digital ruler and weight scale, and length density (cm cm­3), density (mg cm­3) and the percentage of decayed fibrous root were deter­ mined in the soil sample volume harvested by auger (1589.6 cm3) (Alves et al., 2012). At the harvesting time total fruit yield per trees was determined by the scale. A random sample of 50 fruits per plot was provided to determine fruit diam­ eter (average of two perpendicular diameters), total soluble solids, pH and titratable acidity of the juice. Total soluble solids (Brix) were determined by using hand refractometer (WYT portable model) and juice total acid was measured by titration method with 0.1 N s o d i u m h y d r o x i d e u n ti l p H m e t e r r e a d s 8 . 2 (Graham, 2017). Fruit juice percentage was calculat­ ed by fruit juice weight divided by fruit total weight. Water productivity (kg m­3) was calculated based on the yield of trees to the amount of consumed irriga­ tion water. The data set was auto scaled before analysis. All the parameters for three years were sub­ jected to combine analysis of variance (ANOVA) by M S T A T ­ C s o ft w a r e . M e a n s w e r e c o m p a r e d b y Duncan’s multiple range test and Pearson correlation were determined by SPSS software. 3. Results The results of the combine analysis of variance (ANOVA) over three consecutive seasonal growths from 2016 to 2018, indicated that there was a signifi­ cant effect of experimental factors on improvement of citrus decline deterioration indices in Valencia orange (Tables 4­8). Annual application of compost as organic mulch and adding them to 10 cm top soil in the following years had significant effect on the increment of soil organic matter and soil porosity up to 170.45, 13.18% respectively, and decreasing soil bulk density by 11.98% at 10 cm soil depth in com­ parison with control (without application of compost as mulch) (Table 3). Annual application of compost as organic mulch significantly increased the fibrous roots length densi­ ty and fibrous roots density of Valencia orange trees by 34.63 and 38.27% respectively. Also the interac­ Tadayon and Hosseini ‐ Citrus decline amelioration in Valencia orange trees 217 tion between expansion the percentage of irrigation wetted area along with and decreasing irrigation effective root depth with annual application of com­ post as organic mulch (W1M1 and W2M1), signifi­ cantly increased the fibrous root density up to 87.09 and 112.9% and decreased the root decay by 43.23 and 46.48% at a lower soil depth respectively in com­ parison with control (W0M0) (Table 4). Mean separation (± SD) within columns followed by different let­ ters are significantly different at P≤0.05 using Duncan’s new mul­ tiple range tests. Table 3 ­ Effects of experimental treatments on 10 cm top soil organic matter, porosity and bulk density over three consecutive seasonal growths from 2016 to 2018 Treatments Soil organic matter (%) Soil porosity (%) Bulk density (g cm­³) W0M0 1.42 ± 0.12 b 42.05 ± 1.21 b 1.49 ± 0.04 a W1M0 1.24 ± 0.14 b 41.86 ± 1.32 b 1.43 ± 0.04 a W2M0 1.30 ± 0.10 b 42.42 ± 1.15 b 1.42 ± 0.05 a W0M1 3.38 ± 0.07 a 47.08 ± 1.01 a 1.31 ± 0.06 b W1M1 3.84 ± 0.08 a 48.51 ± 1.02 a 1.27 ± 0.07 b W2M1 3.50 ± 0.07 a 47.39 ± 0.93 a 1.24 ± 0.06 b Mean separation (± SD) within columns followed by different let­ ters are significantly different at P≤0.05 using Duncan’s new mul­ tiple range tests. Table 4 ­ Effects of experimental treatments on root characteri­ stics of declining Valencia orange over three consecuti­ ve seasonal growths from 2016 to 2018 Treatments Fibrous roots length density (cm cm ­³) Fibrous root density (mg cm­³) Root decay (%) W0M0 0.043 ± 0.003 d 0.31 ± 0.08 d 66.24 ± 6.21 a W1M0 0.054 ± 0.002 c 0.48 ± 0.06 c 53.72 ± 8.60 b W2M0 0.056 ± 0.002 c 0.49 ± 0.06 c 54.25 ± 7.32 b W0M1 0.066 ± 0.003 b 0.53 ± 0.04 bc 48.07 ± 7.56 b W1M1 0.065 ± 0.004 b 0.58 ± 0.04 b 37.60 ± 8.14 c W2M1 0.075 ± 0.003 a 0.66 ± 0.03 a 35.45 ± 8.81 c Mean separation (± SD) within columns followed by different letters are significantly different at P≤0.05 using Duncan’s new multiple range tests. Table 5 ­ Effects of experimental treatments on vegetative growth indices of declining Valencia orange over three consecutive seasonal growths from 2016 to 2018 Treatments Total leaf chlorophyll content (mg g­¹ FW) Flush length (cm) Leaf drop (%) Shoot dieback (%) W0M0 0.29 ± 0.03 d 24.82 ± 2.31 d 35.30 ± 1.12 a 35.70 ± 2.11 a W1M0 0.34 ± 0.02 c 35.53 ± 3.78 c 27.63 ± 1.53 b 29.62 ± 3.10 b W2M0 0.36 ± 0.01 c 38.35 ± 3.81 b 21.30 ± 1.71 c 17.60 ± 2.27 d W0M1 0.42 ± 0.03 b 39.47 ± 3.15 b 24.70 ± 1.92 bc 25.25 ± 2.15 c W1M1 0.46 ± 0.02 a 43.63 ± 2.80 a 15.45 ± 1.65 d 15.60 ± 2.84 d W2M1 0.47 ± 0.02 a 44.90 ± 2.66 a 10.20± 1.86 e 8.41 ± 1.90 e In comparison with control (without compost as mulch), application of compost as organic mulchsig­ nificantly increased the total leaf chlorophyll content and new flushes length up to 36.3 and 29.67% respectively and decreased leaf and fruit drop (Table 5, 6) and shoot dieback by 40.21, 38.18 and 40.59%, respectively (Table 5), in declining Valencia orange trees, respectively. Increment the irrigation wetted area percentageand decreasing the effective root depth for irrigation, significantly increased leaf chlorophyll contentand flush lengthand reduced leaf and fruit drop and shoot dieback. The interaction between the expansionof irrigation wetted area with annual application of compost as organic mulch (W1M1 and W2M1) had the highest impact on the increment of total leaf chlorophyll contentup to 58.62 and 62% and flush length by 75.78 and 80.9% and decreasing leaf drop 56.17 and 71.1%, fruit drop by 44.62 and 63.3% and shoot dieback by 56.3, 76.44%, respectively, in comparison with control (W0M0). Leaf relative water content, specific leaf weight, fruit diameter and tree yield increased with annual application of compost as organic mulchanddevelop­ ing the irrigation wetted area (Table 6). The interac­ tion between irrigation wettedarea and annual appli­ cation of compost as organic mulch (W1M1 and W2M1) had the highest impact on the increment of leaf relative water content up to 11.91 and 14.25% and specific leaf weight up to 18.75 and 22.66%, respectively, in comparison with control (W0M0) (Table 6). Also Fruit diameter significantly increased by increment of wetted area and application of mulch by 15.9 and 14.11%, respectively (Table 6). The highest yield of Valencia orange trees was belonged to the interaction between irrigation wet­ ted area of 70­80% with 30 cm effective root depth for irrigation and application of compost mulch 218 Adv. Hort. Sci., 2020 34(2): 213­221 (Table 7). Annual application of compost as organic mulch had significant effect on the reduction of con­ sumed irrigation water by 17.39% in comparison with control (without mulch). The interaction between irrigation wetted area and annual application of com­ post as organic mulch (W1M1 and W2M1) had the highest impact on the increment of water productivi­ ty up to 53.06 and 49.79% respectively, in compari­ son with control (W0M0), and there was no signifi­ cant difference between them (Table 7). Expansion of irrigation wetted area and decrease effective root depth for irrigation under the condition of compost mulch applicationhad the greatest effect on water productivity in Valencia orange. Annual application of compost as organic mulch sig­ nificantlyincreased the fruit juice content by 11.98%, total soluble solids (Brix) by 10.96%, Brix/Titratable acidity ratio by 24.52% in declining Valencia orange trees. Fruit acidity was significantly reduced by annual application of mulch. Increment of irrigation wetted area percentage significantly, increased fruit juice and brix/titratable acidity ratio. The interaction between increased irrigation wetted area with mulch application (W1M1 and W2M1), resulted in the highest increase in fruit juice percentage by 35.43 and 34%, total soluble solidsby 23 and 26.36% and brix/titratable acidity ratio by 49.6 and 57.6%, respectively in comparison with control (W0M0) (Table 8). Mean separation (± SD) within columns followed by different letters are significantly different at P≤0.05 using Duncan’s new multiple range tests. Table 6 ­ Effects of experimental treatments on the leaf relative water content, specific leaf weight, fruit drop and fruit diameter of declining Valencia orange over three consecutive seasonal growths from 2016 to 2018 Treatments Leaf relative water content (%) specific leaf weight (mg cm­²) Fruit drop (%) Fruit diameter cm) W0M0 83.50 ± 5.30 d 3.84 ± 0.12 d 31.15 ± 1.10 a 7.23 ± 0.10d W1M0 89.64 ± 5.24 c 4.12 ± 0.08 c 25.12 ± 1.32 b 7.58 ± 0.08 c W2M0 91.25 ± 4.22 bc 4.36 ± 0.07 bc 21.84 ± 1.25 bc 8.20 ± 0.07 b W0M1 88.36 ± 5.75 c 4.04 ± 0.09 cd 19.61 ± 1.74 c 7.71 ± 0.08 c W1M1 93.45 ± 4.12 ab 4.56 ± 0.06 ab 17.25 ± 2.11 c 8.38 ± 0.06 a W2M1 95.48 ± 4.53 a 4.71 ± 0.06 a 11.43 ± 2.73 d 8.25 ± 0.07 ab Mean separation (± SD) within columns followed by different letters are significantly different at P≤0.05 using Duncan’s new multiple range tests. Table 7 ­ Effects of experimental treatments on the leaf characteristics, fruit quantity and water productivity of declining Valencia oran­ ge over three consecutive seasonal growths from 2016 to 2018 Mean separation (± SD) within columns followed by different letters are significantly different at P≤0.05 using Duncan’s new multiple range tests. Table 8 ­ Effects of experimental treatments on the fruit quality indices of declining Valencia orange over three consecutive seasonal growths from 2016 to 2018 Treatments Total tree yield (kg) Consumed irrigation water (m³ ha­¹ year­¹) Water productivity (kg m­³) W0M0 62.33 ± 2.14 e 11625.28 ± 136.58 c 2.45 ± 0.11 c W1M0 71.05 ± 2.71 d 13857.14 ± 122.61 b 2.39 ± 0.16 c W2M0 83.10± 1.50 c 14631.67 ± 121.25 a 2.66 ± 0.14 bc W0M1 68.47 ± 2.43 d 10363.64 ±136.84 d 3.01 ± 0.12 b W1M1 89.40± 1.37 b 10904.76 ± 132.70 d 3.75 ± 0.11 a W2M1 94.40± 1.15 a 11869.57 ± 126.39 c 3.67 ± 0.12 a Treatments Fruit juice (%) Titratable acidity (g 100­¹ ml) Total soluble solids (Brix) Brix/Titratable acidity ratio W0M0 42.64 ± 2.02 d 1.16 ± 0.01 a 9.56 ± 0.61 d 8.94 ± 0.22 f W1M0 48.80 ± 1.81 c 0.94 ± 0.03 b 11.13 ± 0.35 c 10.26 ± 0.19 e W2M0 53.71 ± 1.60 b 0.95 ± 0.02 b 11.87 ± 0.24 b 11.87 ± 0.17 c W0M1 47.65 ± 2.17 c 0.82 ± 0.02 c 12.29 ± 0.73 a 11.23 ± 0.18 d W1M1 57.75 ± 1.61 a 0.74 ± 0.03 d 11.76 ± 0.40 b 13.37 ± 0.15 b W2M1 57.14 ± 1.55 a 0.84 ± 0.03 c 12.08 ± 0.37 b 14.09 ± 0.14 a Tadayon and Hosseini ‐ Citrus decline amelioration in Valencia orange trees 219 Results showed that fibrous root density had a significant negative correlation with leaf drops (r= ­ 0.791) and shoot dieback (r= ­0.612) (Table 9). Also fibrous root density had positive and significant cor­ relation with flush length (r= 0.624), leaf water con­ tent (r= 0.732) and specific leaf weight (r= 0.631). The root decay percentage had significant negative corre­ lation with leaf chlorophyll content (r= ­0.643), flush length (r= ­0.632), leaf water content (r= ­0.603), leaf specific weight (r= ­0.638), yield (r= ­0.691), water productivity (r= ­0.602), and Brix/TA (r= ­0.684). Also there was a positive and significant correlation between the percentage of root decay with leaf drops (r= 0.784), shoot dieback (r= 0.692), and fruit drops (r= 0.704) (Table 9). Specific leaf weight had positive and significant correlation with leaf chloro­ phyll content (r= 0.482) (Table 9). There was a nega­ tive correlation between fruit drops and relative water content of leaves (r= ­0.774). Tree water pro­ ductivity had a positive and significant correlation with the leaf chlorophyll content (r= 0.612), leaf rela­ tive water content (r= 0.766), and specific leaf weight (r= 0.677) (Table 9). 4. Discussion and Conclusions Citrus decline was directly related to root decay and decreasing of fibrous root density and root expansion in Valencia orange trees. Tree perfor­ mance is a function of how the root system is distrib­ uted over a large volume of soil to absorb water and nutrients (Lehmann, 2003). In arid and semi­arid regions, the highest density of fibrous roots in the drip irrigation system is located under the emitters and soil wetted area (Ciancio and Mukerji, 2008; Alves et al., 2012). The rate of water adsorption by the tree is reduced when the soil oxygen level is low (Levy, 1998; Boman et al., 1999) and the most influ­ ential soil condition on citrus decline are soil mois­ ture condition (Meena et al., 2018), and soil com­ paction (Srivastava and Singh, 2009). Root develop­ ment is related to the hydrophilicity of roots, soil aer­ ationand distribution of soil moisture (Ruiz­Sanchez et al., 2005). Our results showed that Application of compost as organic mulch and rotating them with 10 cm top­ soil improved soil physical properties such as soil organic matter, porosity and bulk density which could provide the best conditions for root development. This treatment could increase the soil aeration and soil moisture retention at topsoil, as two key factors for root health and growth (Boman and Parsons, 2002; Nelson et al., 2008; Johnson and Graham, 2015). Meanwhile, it could reduce surface evapora­ tion, moderate soil surface temperatures and provide the best condition for root development for declining Valencia orange trees. Application of organic mulches on Eureka lemon (Citrus limon Burm) significantly increased the soil moisture status in various soil Table 9 ­ Pearson correlation coefficients between the means of citrus decline indices of Valencia orange over three consecutive seaso­ nal growths from 2016 to 2018 Chlorophyll content Flush length Leaf drop Shoot dieback LRWC 1 SLW 2 Fruit drop Fruit diameter Yield Water productivity Brix/TA. Fibrous roots density Fibrous roots length density Root decay Chlorophyll content 1.00 Flush length 0.64 ** 1.00 Leaf drop ­0.51 * ­0.64 ** 1.00 Shoot dieback ­0.55 * ­0.74 ** 0.78 ** 1.00 LRWC 0.54 * 0.64 ** ­0.77 ** ­0.75 ** 1.00 Fruit drop ­0.66 ** ­0.25 NS 0.67 ** 0.64 ** ­0.71 ** ­0.69 ** 1.00 Fruit diameter 0.43 * 0.49 * ­0.52 * ­0.51 * 0.52 * 0.53 * ­0.53 * 1.00 Yield 0.59 ** 0.65 ** ­0.76 ** ­0.67 ** 0.66 ** 0.60 ** 0.26 NS 0.75 ** 1.00 Water 0.61 ** 0.50 * ­0.75 ** ­0.65 ** 0.77 ** 0.68 ** ­0.75 ** 0.78 ** 0.78 ** 1.00 productivity Brix/T.A. 0.63 ** 0.52 * ­0.70 ** ­0.68 ** 0.15 NS 0.57 ** ­0.66 ** 0.13 NS 0.57 ** 0.46 * 1.00 Fibrous roots density 0.71 ** 0.62 ** ­0.79 ** ­0.61 ** 0.73 ** 0.63 ** ­0.71 ** 0.64 ** 0.70 ** 0.71 ** 0.62 ** 1.00 Fibrous roots length density 0.58 * 0.48 * ­0.59 ** ­0.54 * 0.68 ** 0.59 ** ­0.54 * 0.61 ** 0.62 ** 0.59 ** 0.52 * 0.78 ** 1.00 Root decay ­0.64 ** 0.63 ** 0.78 ** 0.69 ** ­0.60 ** ­0.64 ** 0.70 ** ­0.66 ** ­0.69 ** ­0.60 ** ­0.68 ** ­0.64 ** ­0.71 ** 1.00 1 Leaf relative water content. 2 Specific leaf weight. *, ** significant at 5 and 1% statistical levels respectively; NS = not significant. Adv. Hort. Sci., 2020 34(2): 213­221 220 depths and farmyard manure were found to be more effective in producing maximum growth extension (Kumar et al., 2015). The irrigation wetted area, also affect citrus root system development and distribu­ tion (Alves et al., 2012). In our experiment, the expansion of irrigation wetted area and reduction the effective root depth for irrigation with annual appli­ cation of compost as organic mulchand its rotation in the 10 cm soil depth, increased the fibrous root length and root densityat lower soil depths and decreased root decay in Valencia orange trees. Also, leaf relative water content, specific leaf weight and f r u i t d i a m e t e r i n V a l e n c i a o r a n g e s i g n i fi c a n t l y increased with developing the irrigation wetted area. Increment of fibrous root length and root density at lower soil depths with decreasing the irrigation depth and consequent improvement of tree water status were in agreement with the results that showed more root distribution in the drip irrigation method was at the depth of 15 cm, with high water uptake efficiency (Kadyampakeni et al., 2014 b). Annual application of compost as organic mulch significantly decreased the consumed irrigation water in Valencia orange trees. Increasing soil organic carbon improve the healthy root system of citrus (Sharma et al., 1986). There was a positive and significant correla­ tion between leaf relative water content and leaf chlorophyll content (r= 0.540), and flush lengths (r= 0.643) and a significant negative correlation between leaf water content and leaf drops (r= ­0.771), and shoot dieback (r= ­0.748) (Table 8). It has been shown that clementine ‘Nules’ vegetative growth and fruit size was higher with increasing numbers of emit­ ters on the double drip­lines treatments (Abouatallah et al., 2012). We conclude that annual application of compost as organic mulch under tree canopy area and its rota­ tion at 10 cm soil depth at following years and expanding the irrigation wetted area and decreasing the irrigation effective root depth, significantly improved the indices of citrus decline and increased the water productivity and fruit quality in Valencia orange trees. Acknowledgements The authors greatly appreciate Mr. Mojtaba Dadgar who provided the commercial orchard for this experiment and Fars Agricultural Research, Education and Extension center. 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