FUNGAL COMMUNITIES ON ROOTS AND SHOOT BASES OF WINTER BARLEY AND WINTER WHEAT DURING CROP DEVELOPMENT
WAJM DAWSON and GL BATEMAN
IACR-Rothamsted, Harpenden, Hertfordshire, AL5 2JQ, UK
Background and objectives
Nonpathogenic fungi that occur commonly in the rhizosphere of cereal plants can influence the severity of take-all disease. Trichoderma spp., for example, are sometimes involved in suppression of the take-all pathogen, Gaeumannomyces graminis var. tritici. However, there have been few studies of the composition of fungal communities on roots or stem bases  or of the relationships between this and the severity of root and stem-base diseases. This presentation compares such communities in winter and summer on wheat and barley to determine those fungi that are likely to be associated with disease promotion or suppression and that may be affected by fungicides targeted at the pathogenic species.
Materials and methods
A field experiment, consisting of 32 plots (10X3 ;m) of winter barley cv. Pipkin and 32 similar plots of winter wheat cv. Brigadier, was sown on 14 October 1996 in four randomized blocks of 16 plots. Ten plants were sampled from each of 12 randomly chosen barley and 12 randomly chosen wheat plots (three plots of each cereal per block) on 21 January and 11 June 1997. The plants were washed thoroughly in tap water and from each plant three roots were selected at random. Root pieces (10 ;mm) were cut from 5-8 mm below the stem base and then serially washed 20 times, for 3 ;min, in 10 ;ml of fresh, cold (5°C) sterile distilled water . The root pieces were placed on sterile filter paper to dry in a sterile air flow prior to being cut in half. Half of each root piece was placed on potato dextrose agar (PDA) and half on low nutrient agar (SNA). There were five replicate plates and six root pieces per plate. The agars contained penicillin (0.3 ;g/l), streptomycin sulphate (1.33 ;g/l) and chloramphenicol (0.05 ;g/l). Shoot (January) or stem base pieces (15 ;mm) were treated in the same way. The plates were incubated for 3 ;days at 20°C, examined with a light microscope and then exposed to near-UV light for 7-10 ;days at 15°C. The plates were then reexamined and any nonsporulating fungi were subcultured onto SNA. Ten 20-cm lengths of row were taken from each plot on 26 June 1997. The plants were washed and assessed for eyespot, fusarium foot rot and sharp eyespot on the stem bases and takeall on the roots. Data was analysed by factorial analysis of variance.
Results and conclusions
Fungi of 41 genera were identified, 27 in January and 36 in June. The most abundant genera or orders were Alternaria, Cladosporium, Cylindrocarpon, Fusarium, Idriella, Mucorales (Absidia, Mortierella, Mucor), Penicillium, Phoma, Trichoderma and Verticillium. In January, Idriella bolleyi was predominant on roots and shoot bases and was more abundant on barley roots than on wheat. On shoot bases, Fusarium spp. were more abundant on barley and Alternaria spp. were more abundant on wheat. In June, there was a more even distribution of genera on roots. Cladosporium spp., Fusarium spp., -Penicillium spp. and Trichoderma spp. had increased considerably, while Phoma spp. had declined. Mucorales were more frequent on wheat roots. On stem bases Alternaria spp., Cladosporium spp., Fusarium spp. and Verticillium spp. increased considerably, while Cylindrocarpon spp., Idriella bolleyi, Mucorales and Phoma spp. had declined. Alternaria spp. and Idriella bolleyi were more frequent on barley and Fusarium spp. and Verticillium spp. were more frequent on wheat stems. Alternaria spp., Cladosporium spp. and Verticillium spp. were more frequent and Cylindrocarpon spp., Penicillium spp. and Trichoderma spp. were less frequent than on the roots. Unusual fungi included Acremoniella atra, Coemansia aciculifera and Exserohilum novae-zelandiae. On 26 June, eyespot, sharp eyespot and take-all were more severe on wheat and fusarium foot rot was more severe on barley. Further evidence of associations between fungal communities, crop species, disease severity and the effects of seed treatment fungicides, will be obtained from third cereals in the same plots in 1997-98.
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