Maataloustieteellinen Aikakauskirja Vol. 62: 275—284, 1990 Occurrence and pathogenicity of Pythium spp. in seedling roots of winter rye MAURITZ VESTBERG Agricultural Research Centre of Finland, Central Finland Research Station, SF-41340 Laukaa, Finland Abstract. Seedlings of winter rye collected from yellowing patches during October to Novem- ber 1985—1987 showed oospores of Pythium species in apparently healthy as well as in dis- colored roots. Examination of 1550 root pieces of rye on CMA yielded fungi belonging to 35 genera. The most commonly isolated ones were Fusarium spp, Penicittium spp, Mucor spp, Mortierella spp. and Cladosporium spp. Pythium spp. were isolated from 35 root pieces on CMA. Identified species were P. splendens, P. irregulare, P. dissimile, a species resembling P. ahstosporum and a species resembling P. ultimum. In in vitro and in vivo, tests on the cereals winter rye, spring wheat, oats and barley the pathogenicity of some Pythium isolates varied from high (P. splendens, P. irregulare) to moderate (P. irregulare, P. dissimile) and low (a species resembling P. ultimum). Index words: Pythium, pathogenicity, in vitro, in vivo winter rye, spring wheat, oats, barley Introduction Pythium species are common in agricultural soils and they parasitize a wide range of hosts (Domsch et al. 1980). All spring and winter small grains and forage grasses are suscep- tible to root rot caused by one or several Pythium species acting singly or in combina- tion (Wiese 1977). Root rot on rye caused by species of Pythium has been reported only in a few cases. The species Pythium aphanidermatum (Ed- son) Fitz (Sechler & Luke 1967) and P. my- riotylum Drechs. (Littrell & McCarter 1970, Mitchell 1975) have caused seedling damping-off and root rot in the southern parts of the U.S.A. Several species of Pythium cause a brown- ing root rot in barley. The disease has been encountered in the U.S.A. (Bruehl 1955, Kilpatrick 1968, Mathre 1982, Bratolov- eanu and Wallace 1985), Canada (McKeen 1977), Argentina (Frezzi 1956), England (Waller 1979) and Australia (Dewan and Sivasithamparam 1988). P. arrhenomanes Drechs., P. aphanidermatum, P. graminicola 275 JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND https://www.c-info.fi/en/info/?token=5eXSUFZA9J9IzM6H.dNOj4eOdkEhPudUSopxAeA.BM8p1OfZigMsMjea8NrxWHExp85Y79bEefqxXO9cZX-tZaskTuLjcs_XrobDhq0kcIHuCoBJGvk17KoYVKLmIl_OJGe3CvsEvFw3G1P6IcNrAKhGjw7MC4yFawSlTm2fgbPkUMsoaaWSRIBxnlm9cRpdpvJeoL6ewVSfe42B Subr., P. irregulare Buisman, P. splendens Braun, P. tardicrescens Vanterpool and P. volutum Vanterpool & Truscott are the most important species reported to be pathogenic on barley. Browning root rot of wheat also has a world-wide distribution, and it has caused damage to wheat seedlings in the U.S.A. (Kilpatrick 1968, Chamswarng and Cook 1985), England (Waller 1979), Austria (Glaeser 1979) and Australia (Dewan and Sivasithamparam 1988). More than ten spe- cies of Pythium have been reported to cause root rot, seed rot and damping-off in wheat seedlings. The most extensively documented wheat pathogens are P. arrhenomanes, P. aphanidermatum, P. graminicola, P. myrio- tylum Drechs. and P. volutum (Wiese 1977). In oats, root rot has been caused by the spe- cies P. debaryanum Hesse (Welch 1945), P. aphanidermatum and P. splendens (Kil- patrick 1968). In autumn 1984, patches of yellowed and stunted rye plants were observed in rye fields in southern Finland (Bremer and Vestberg 1986). Electron microscopy revealed two types of virus particles in the leaves and roots of the yellowed rye seedlings. No virus vector, e.g. Polymyxa graminis Ledingham, was ob- served, only indications of fairly abundant oc- currence of Pythium spp in roots of yellowed rye seedlings. The aim of this study was to isolate and to identify species of Pythium from roots of rye seedlings. Introductory pathogenicity tests were conducted with some isolates on rye, bar- ley, oats and winter wheat. Materials and methods Sampling and sample treatment Seedling samples were collected from yel- lowing patches of winter rye fields during Oc- tober to November 1985—87 and in May 1987 in southern and central Finland. The locali- ties numbered 12, 23, 9 and 19 in autumn 1985, autumn 1986, autumn 1987 and spring 1987, respectively. The roots were rinsed thoroughly in tap wa- ter. One half of the root sample was exam- ined with a compound microscope. From the other half of the sample, discoloured pieces (approx. 0.5 cm long) of fine roots, including healthy and discolored ones, were transferred to water agar (WA) withoutany surface sterili- zation or antibiotic treatment. The fungi were transferred from the WA to corn meal agar (CMA) for identification. Studies on Pythium strains Morphological characteristics of the strains of Pythium spp. were determined on 4-week cultures on CMA and on 9-day cultures in wa- ter. The water cultures were established by transferring a small piece of culture (1 cm in diameter) on CMA beneath an autoclaved piece of young rye leaf, cv ’Dan Kowskie Zlote’ into a Petri dish containing autoclaved water. The water consisting of “pond water” and destilled water (1:1) (Van der Plaats- niterink 1981) was changed twice daily. On these occasions sexual and asexual structures of Pythium isolates were distinguished on the piece of rye. Pathogenicity of Pythium strains in vitro The pathogenicity of four Pythium strains (PI, P2, P3, P9) isolated from winter rye was tested in vitro in four replicates on barley cv ’Kymppi’, oats cv ’Hankkijan Vouti’, spring wheat cv ’Tähti’ and winter rye cv ’Dan Kowskie Zlote’. A system of hanging file folders made of filter paper was arranged in a water bath with 5 cm of water at the bot- tom. Due to water suction the filter paper re- mained wet when the water level was main- tained at 5 cm. In each folder five surface sterilized seeds (1 min in 1 °Io NaOCl) were applied between the two layers, 2 cm from the top. Inocula of Pythium (1 cm in diameter), 1-month culture on CMA, were placed 2 cm 276 3 beneath the cereal seed. Paper clips were used to bring the two layers of the hanging folder close to each other. The whole experiment consisted of six water baths with 16 hanging file folders in each. After three days of incu- bation in darkness the baths were placed into a growth chamber with a 16-h daylength and a light intensity of approx 5000 lux. The shoot lengths were measured seven days after onset of the experiment. Final observations of root and shoot lengths, disease symptoms and in- fection of Pythium were made four days later. Disease symptoms in shoots and roots of seedlings were estimated visually on a scale from 0 to 2: O= Healthy 1 = Moderately discolored 2 = Highly discolored The disease index (DI) of a treatment was calculated as the mean of altogether 20 seed- lings belonging to four replicates. Pathogenicity of Pythium strains in vivo The pathogenicity of three Pythium isolates (P2, P9, PI7) was tested in vivo in six repli- cates on the same species and cultivars of cereals as in the in vitro experiment. Surface sterilized seeds (1 min in 1 % NaOCl) were pregerminated for five days at room temper- ature (about 22°C) until the seedling was ap- prox. 4 cm long; equally long seedlings were thereafter planted into white plastic pots (3 dl, 5.5 cm deep) at a depth of 2 cm, five in each. The pots were filled with a sand-vermiculite- leca gravel mixture (1:1:1). The inoculum, a piece of Pythium culture on CMA (1 cm in diameter, 14-dayold culture) was placed 1 cm beneath the seed. The pots were fertilized with the slow-release fertilizer osmocote, 2 kg/m 3 , and were kept in a glass house at a tempera- ture of about 20 0 C throughout the experi- ment. Supplementary light was given to get a daylength of 16 h. The experiment started on the 19th of Oc- tober, 1987, and ended eight days later, when the disease symptoms in roots and on leaves, as well as seedling lengths and dry weights of above ground plant parts and roots were de- termined. The disease symptoms in roots were esti- mated visually in the same way as in the in vitro experiment. The DI of a treatment was calculated as the sum of disease symptoms in all seedlings in that treatment, altogether 30 seedlings. Statistical analysis Statistical analysis of the results of the pathogenicity experiments was done using the analysis of variance. Means were compared with Duncan’s multiple range test. Results Direct microscopy of seedling roots of rye Although all seedling samples of rye col- lected in autumn 1985—1987 were taken from yellow patches in the field, the majority (70.5 %) of samples contained apparently healthy roots (Table 1). Highly discolored roots were found in five samples out of 44 samples. Oospores of Pythium were detected in healthy as well as in moderately or highly discolored roots. However, severe infection of Pythium was found only in samples with high- ly discolored roots. On the other hand, no oospores of Pythium were detected in two samples with highly discolored roots (Table 1). Including the seedling samples collected in spring 1987, which makes a total of 63 sam- ples studied by direct microscopy, the samples containing Pythium spp. in roots numbered as follows: No infection of Pythium detected: 34 samples Mild » » » : 20 »» » : 20 » Moderate » » : 7 » Severe » » » : 2 » 277 Table 1. Occurrence of oospores of Pythium in roots of rye seedlings collected from 44 localities in October — November 1985 —87. Root symptoms Number Number of samples with Pythium oospores of Severity of infectionsamples 0 + +++ + + Healthy roots Moderately dis- colored roots Highly dis- 31 15 12 2 0 8 4 2 2 ci colored roots 2 0 1 25 44 21 14 5 2 O =No oospores detected + =A few » » + + =A moderate number of oospores detected + + + =Large numbers of oospores detected Fungi identified on CMA From seedling samples collected in the autumn of 1985 and 1986 and in spring 1987 altogether 1550 root pieces were examined on CMA for fungi. Fungi belonging to 35 genera were found (Table 2). Among 884 root pieces studied from the autumn samples of 1985 Fusarium spp (84 pieces), Penicillium spp. (65 pieces) and Mucor spp. (45 pieces) were the most common fungi. In 1986, 423 samples were studied and the most common fungi were the same as in 1985, i.e. Fusarium spp. (31 pieces), Penicil- lium spp. (25 pieces) and Mucor spp. (24 pieces). The result for 243 root pieces evalu- ated in spring 1987 was Mortierella spp. (79 pieces), Cladosporium (76 pieces) and Fu- sarium spp. (74 pieces). The number of fungi per root piece aver- aged 0.4 and 0.3 for autumns 1985 and 1986 respectively, while in spring 1987 there were 2.1 fungi per root piece. Pythium spp. were identified in altogether 27 root pieces, i.e. in five pieces in autumn 1985, 13 pieces in autumn 1986and nine pieces in spring 1987. Studies on Pythium spp Twenty-seven fungal isolates on CMA were identified as Pythium spp. Eight of these representing several species of Pythium were obtained in pure culture (Table 3). Pathogenicity of Pythium strains in vitro The “hanging file folder” method was suc- cessful. Infection of Pythium was noticed in all cereals tested. Of the isolates, PI (P. splen- dens) caused very extensive infection in all cereals, 87.5 °7o of seedlings in this treatment being affected. On the other hand, P 3 (iso- late resembling P. ultimum) caused very low or no infection at all. The isolates P 2 (P. ir- regulare) and P 9 (P. dissimile) gave rise to in- fection in about 50 % of the seedlings (Table 4). Oats was the healthiest cereal, infection occurring in 37.5 °7o of seedlings, while spring wheat was the most extensively infected, (60.3 %). Host specificity was observed especially for isolates P 2 and P3. P 2 infected winter rye by 67 %, while it caused no infection in barley. P 3 caused no infection in oats but a slight or moderate infection in barley (Table 4). Out of four isolates tested, only PI caused a significant reduction in shoot and root length (Table 5). It also caused more leaf symptoms than the control treatment. Inverse- 278 279 Table 2. Fungi isolated from roots of rye collected in autumn 1985 and 1986 and in spring 1987. Fungus Number of isolations 1985 1986 1987 Total Absidia sp 3 2 5 Acremonium spp 22 36 58 Alternaria alternala (Ft.) Keissler 15 4 10 Aspergillus sp. 1 1 Aureobasidium pullulans (de Bary) Arnaud 1 1 Broomella acuta Shoem. & E. Mull. 3 3 Chaetomium globosum Kunze ex Steud. 2 2 Cladosporium spp. 11 9 76 96 Cochliobolus sativus (Ito & Kuribayashi) Drechsler ex Dastur 10 5 15 Coniolyrium sp. 4 4 Cylindrocarpon spp. 3 I 8 12 Dendryphion nanum (Nees ex Gray) 1 1 Doratomyces sp. 11 2 Epicoccum purpurascens Ehrenb. ex Schlecht, 6 2 8 16 Fusarium avenaceum (Fr.) Sacc. 34 9 43 F. culmorum (W.G. Sm.) Sacc. 21 6 27 F. graminearum Schwabe 2 2 F. oxysporum Schlecht. 3 1 4 F. sambucinum Fuck. 4 4 F. solani (Mart.) Sacc. 1 1 Fuasarium sp, 21 13 66 100 Geotrichum sp. 1 12 Gliocladium sp. 2 6 19 Humicola fuscoatra Traaen 5 5 Mortierella spp. 25 79 104 Mucor spp. 45 24 52 121 Papulaspora sp. 19 5 10 34 Penicillium spp. 65 25 41 131 Pestalolia sp. 1 1 Phoma spp. 20 9 29 Pythium spp. 5 13 9 27 Rhizoctonia solani Kiitin 10 10 Rhizopus nigricans Ehrenb. 2 4 6 Scopulariopsis brevicaulis (Sacc.) Bain 1 1 Sporolrix schenckii Hektoen & Perkins 12 12 Toruta herbarum Pers. ex Gray I 3 4 Trichocladium asperum Harz 6 6 Trichoderma viride Pers. ex Gray 26 8 61 95 Ulocladium consorliale (Thiim.) Simmons 2 6 8 Verticillium sp. 16 3 19 Zygorhynchus sp. I 9 10 Total number of root pieces examined 884 423 243 1550 ly, isolate P 9 gave rise to the highest root symptom index, causing brown roots especial- ly in barley. Both P 9 and PI increased the severity of root damage. Pathogenicity of Pythium strains in vivo Isolate P 9 significantly decreased shoot dry weight, while the isolates P 2 and Pl 7 (P. ir- regulare) caused an increase as compared to the control. In root dry weight there were no decreases due to Pythium isolates, but Pl 7 sig- nificantly increased root dry weight (Table 6). All isolates slightly decreased seedling length, isolates Pl 7 and P 9 significantly so, as compared to the control. The leaves and seedling bases showed no symptoms. Table 3. Morphological characteristics of Pythium isolates from rye seedlings. Characteristics of isolate Isolate Tentative name Growth 13.4 mm/day on CMA. Oogonia 34.3 pm (26.2—43.6). Several PI P. splendens in clusters. Older oogonia and oospores somewhat pigmented. Hyphal swellings. Oospores 26.3 m (22.5—35.9). Antheridia broad, sac-like, gener- ally 2—5/oogonium.No growth on rye leaf in water. No asexual structures. Growth 26.8 mm/day on CMA. Oogonia 22.0 pm (19.4—25.2), terminal P2, PlO P. irregulare or intercalary, of varying appearance, with one or a few finger-like projec- Pl 7 dons. Oospore 19.2 pm (15.5 —21.3). Antheridia I—2/oogoniura, mostly arising from the same hypha as the oogonium. Scanty growth on leaf of rye in water. No asexual structures. Hyphal growth 13.5mm/day on CMA. Small oogonia with thin wall. No P 9 P. dissimile antheridia or rarely 1/oogonium, big. Oospores 13.7 pm (8.7 —18.4). Scanty hyphal growth on rye leaf in water. Sporangia irregular, subglobose forming complex structures. No zoospores observed. Hyphal growth 27.1 mm/day on CMA. Oogonia 25.0 pm (17.2 —29.4). Pl3, Pythium sp. Antheridia almost thread-like with long stalk, several/oogonium, diffi- Pl 5 Resembling P. aris- cult to observe. Oospores 22.6 pm (19.4 —26.7). Sickle-shaped appres- tosporum soria at bottom ofPetri dish, in clusters. Moderate growth on leaf of rye in water. No asexual structures. Growth 15.4—17.5 mm/day on CMA. Oospores 18.8 pm (15.8—20.7) P 3 Pythium sp. with a thick wall. Antheridia seldom visible, with a long bent and irregu- Oospore resembling lar stalk, 1/oogonium. Club-shapedappressoria at bottom of Petri dish. P. ultimum Good growth on rye leaf in water. No asexual structures. Table 4. Percentage of cereal seedling roots infected by Pythium in the in vitro experiment. Cereal Percentage of roots infected Pythium isolates PI P 2 P 3 P 9 Mean Barley 92 0 25 75 48.0 Oats 75 50 0 25 37.5 Spring wheat 83 75 8 75 60,3 Winter rye 100 67 17 33 54.3 Mean 87.5 48.0 12.5 52.0 Seedlings showed few shoot and root symp- toms in the in vivo experiment. The disease severity index of roots (maximum 60) was cal- culated as the sum of DI of 30 seedlings. Iso- late Pl 7 had a somewhat higher root DI than the control and the other Pythium isolates, 3.8 as compared to 2.6, 2.8 and 2.5 for the con- trol, P 2 and P9, respectively. Barley had on average the most diseased roots, while spring wheat had the healthiest ones (Table 6). Discussion The aim of this investigation was to study the occurrence and the importance of Pythium spp. in roots of rye seedlings. Be- cause Pythium spp. are often relatively infre- quently isolated from surface sterilized roots (Waller 1968), an isolation method using no chemical compounds or antibiotics was cho- sen. The method yielded 35 genera of soil borne fungi. Most of them were classified as common saprophytic soil fungi, while other genera such as Fusarium spp., which was found quite frequently, and Pythium spp. of- ten act as pathogens or minor pathogens in cereal roots (Salt 1979). With the exception of P. dissimile, all the Pythium species identified, i.e. P. splendens. P. irregulare, P. dissimile, a species resem- bling P. aristosporum and a species resem- bling P. ultimum occur in roots of cereals in various parts of the world (Chamswarng & Cook 1985 Dewan & Sivasithamparam 1988, Kilpatrick 1968 Singleton & Ziv 1981). 280 Table 5. The effect of inoculation with strains of Pythium isolated from winter rye on shoot and root length, shoot and root symptoms in four cereal crops in vitro. Cereal Treatment Inoculation with Pythium Control PI P 2 P 3 P 9 Shoot Differencefrom control length Barley 20.1“ 1.4“ +0.2“ —0.1“ —0.5“ Oats 18.3“ —4.3 b —0.8“ —0.8“ —0.9“ Spring wheat 21.4" —4.6 b +0.6“ +0.6“ —0.3“ Winter rye 17.8“ —2.7“ +1.7“ +o.4* —1.3“ Mean 19.4“ —3.3 b +0.4“ +0.2“ —0.7“ Root Differencefrom control length Barley 14.7“ —2.2“ —0.7“ 0.0“ —0.9“ Oats 9.4“ b —l.ob —o.4“b +1.0“ —o.l“b Spring wheat 15.1“ —1.8“ 0.0“ 0.0“ —2.9“ Winter rye 10.8* —0.4“ —0.6“ —0.6“ —1.2“ Mean 12.5“ —l.Bb —0.4“ +0.1“ —l.l“b Shoot Difference from control Dl Barley 0.0 +O.l 0.0 0.0 0.0 Oats 0.0 +0.2 0.0 0.0 0.0 Spring wheat 0.0 +0.4 +O.l 0.0 +O.l Winter rye 0.3 +O.B —O.l +0.2 +0.3 Mean 0.06 +0.31 +0.02 +0.07 +O.lO Root Difference from control DI Barley 0.1 +O.B +0.2 +0.2 +1.4 Oats 0.2 +0.4 —0.2 +O.l —O.l Spring wheat 0.1 —O.l +O.l +O.l +0.5 Winter rye 0.2 +0.2 —O.l —O.l 0.0 Mean 0.15 +0.31 —0.04 +O.OB +0.44 • Values in rows marked with the same letter do not differ significantly at p=0.05. Direct microscopy revealed easily oospores of Pythium in fine roots of rye seedlings, but a lower percentage of Pythium species was ob- tained on CMA and only a few isolates could be studied in pure culture. This indicates that on CMA the growth of the Pythium was dis- turbed or even overgrown by the saprophytic fungi, and some kind of surface sterilization of root pieces could have increased the num- ber of isolates obtained in pure culture. Oospores of Pythium were detected in brown as well as in apparently healthy roots, a fact which supports the findings of Waller (unpublished, ref. Salt 1979). This also sug- gests that in the roots of rye seedlings Pythium may rather be a minor pathogen than a 281 Table 6. The effect of inoculation with three strains of Pythium isolated from winter rye on shoot and root dry weight, seedling length and root disease index (DI) in four cereal crops in vivo. Cereal crop Treatment Pythium strain Control P 2 P 9 PI7 Shoot Difference from control DM, g Barley 0.50b + 0.20“ —0.05 b + 0.11“ Oats 0.43“ + 0.06a 0.9 b +O.O8a Spring wheat 0,49 a 0.00a —o.ll b 0.00a Winter rye 0.45ab +0.07“ —0.06 b +O.O1 ab Mean 0.47b +O.O8a —O.OB- +O.O5 a Root Difference from control DM, g Barley 0.21“ +0.02“ —o.ol a + 0.05“ Oats 0.18“ —0.04 a —o.ol a +O.O2a Spring wheat 0.21 ab —0.06 b —0.04 b +O.O9a Winter rye 0.16ab +O.O2ab —0.02 b + 0.06a Mean 0,19b —0.02 b —0.02 b +O.O5a Seedling Difference from control length, cm Barley 37.6a —o.7a —o.sa —l.2a Oats 35.4a + 1.0“ —l.la —1.5“ Spring wheat 43.9a —2.7 b —2.4 b —2.5 b Winter rye 34.2ab +0.9“ +l.O“b —2.2 b Mean 37.8“ Root DI Barley 5 5 5 7 Oats 2 3 2 4 Spring wheat 2 10 3 Winter rye 2 2 3 2 Mean 2.6 2.8 2.5 3.8 * Values in rows marked with the same letter do not differ significantly at p =0.05. real damage causing pathogen. This role of Pythium was supported also by the findings in the in vivo and in vitro pathogenicity ex- periments, in which the pathogenicity of some Pythium isolates varied from pathogenic to beneficial. This kind of variation in pathoge- nicity is commonly documented (Dewan and Sivasithamparam 1988, Kilpatrick 1968, Sin- gleton and Ziv 1981). 282 References Bratoloveanu, J. & Wallace, H.R. 1985. The influence of Pythium on the growth of barley seedlings as af- fected by soil water and inoculum density. Plant and Soil 85: 305—311. Bremer, K. & Vestberg, M. 1986. Two soil-borne viruses and their possible fungal vectors in Secale cereale in Finland. Research note. Ann. Agr. Fenn. 25; 31 —35. Bruehl, G.W. 1955. Barley adaptation in relation to Pythium root rot. Phytopath. 45: 97—103. Chamswarng, C & Cook, R.J. 1985. Identification and comparative pathogenicity of Pythium species from wheat roots and wheat-field soils in the pacific North- west. Phytopath. 75: 821 —827. Dewan, M.M. & Sivasithamparam, K. 1988. Pythium spp in roots of wheat and rye-grass in western Aus- tralia and their effect on root rot caused by Gaeuman- nomycesgraminis var. tritici. Soil. Biol. Biochem. 20; 801—808. Domsch, K.H., Gams, W & Anderson, T.H. 1980. Com- pendium of soil fungi. Academic press, London. 859 pp. Frezzi, M.J. 1956. Especies de Pythium fitopatogenas identificadas en la Republica Argentina. (Phytopatho- genic species of Pythium identified in the Argentine Republic). Rev. Invest, agric. 10: 113—241. Glaeser, G. 1979. Bericht über das Auftreten wichtige Krankheiten und Schädlinge an Kulturpflanzen in Ös- terreich im Jahre 1977. Pflanzenschutzberichte 45: 153—164. Kilpatrick, R.A. 1968. Seedling raction of barley, oats, and wheat to Pythium species, PI. Dis. Rep. 52: 209—212. Littrell, R.H. & McCarter, S.M. 1970. Effect of soil temperature on virulence of Pythium aphaniderma- turn and Pylhium myriotylum to rye and tomato. Phytopath 60: 704—707. Mathre, D.E. 1982. 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Pythium root necrosis of Oats. lowa St. Coll. J. Sci. 29: 361—399. Wiese, M.V. 1977. Compendium of wheat diseases. St. Paul, Minnesota. 106 pp. Ms received April 6, 1990 SELOSTUS Rukiin oraiden juurissa esiintyvät Pythium sienet ja niiden patogeenisuus viljalajeille Mauritz Vestberg Maatalouden tutkimuskeskus, Keski-Suomen tutkimusasema. 41340 Laukaa Etelä-Suomen ruispelloilla oli syksyllä 1984 monin pai- koin kellastuneiden oraiden muodostamia laikkuja. Kel- lastuneiden oraiden juurissa havaittiin kahdenlaisia virus- hiukkasia. Samanaikaisesti etsittiin rukiin juurista virus- vektoreiksi sopivia sieniä. Niitä ei kuitenkaan löytynyt. Sen sijaan juurissa oli usein havaittavissa Pythium-sienen rakenteita. Rukiin oraita kerättiin kellastuneista peltokohdista vuo- sina 1985—1987. Keruupaikkoja oli syksyllä 1985 12, syk- syllä 1986 23, syksyllä 1987 9 ja keväällä 1987 19. Osa 283 näytteiden juurista tutkittiin suoraan stereomikroskoo- pilla ja osa juurista laitettiin maissiagarille. Pythium- sienten rakenteita esiintyi sekä terveen- että sairaannäköi- sissä juurissa. Kuitenkin hyvin runsaita Pythium esiinty- miä löytyi ainoastaan voimakkaasti tummuneista juuris- ta. Maissiagarilla esitetyt sienet kuuluvat 35 sienisukuun. Näistä suurin osa lienee saprofyyttisiä maasieniä. Pythium-sientä eristettiin 27;stä juurenpalasta.Lajit oli- vat P. splendens, P. irregulare, P. dissimile, P. aristos- porum 'ia muistuttava laji sekä P. ultimum ’ia muistutta- va laji. Neljän viljalajin (ruis, kevätvehnä, ohra, kaura) pato- geenisuustesteissä tulokset olivat vaihtelevia. In vitro tes- tissä P. splendens oli hyvin patogeeninen kaikille vilja- lajeille, kun taas P. ultimum’ia muistuttava laji ei juuri infektoinut. In vivo testissä P. irregulare hieman lisäsi juurten tautisuusindeksiä. Tutkimus osoitti, että Pylhium-sieniä esiintyy melko yleisesti rukiin juurissa. Sienten todellista merkitystä on kuitenkin vaikea päätellä. Todennäköisesti ne kuuluvat siihen suureen maasienten ryhmään, jotka ovat taudin- aiheuttajia vain tietyissä sienille edullisissa olosuhteissa, ovat n.s. ’’minor patogeenejä”. 284