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Wheat ears were inoculated with conidia of Fusarium spp. at different growth stages between ear emergence and harvest and moist conditions were maintained for up to 7 days subsequently by mist irrigation. Of the fungi tested (Fusarium culmorum, F. avenaceum, F. tricinctum, F. sporotrichioides and Microdochium nivale), only F. culmorum produced ear blight symptoms and grain samples were found subsequently to contain deoxynivalenol. Most ear infection and deoxynivalenol formation occurred following inoculation at about mid-anthesis. Small amounts of deoxynivalenol were formed and some F. culmorum was isolated even in the absence of ear blight symptoms. An overnight wet period was sufficient to initiate infection and deoxynivalenol formation but both were increased by extending the wet period up to at least 3 days. Recovery of Fusarium spp. from harvested grain was usually possible whether or not symptoms developed. F. culmorum usually persisted and often increased to moderately high levels after storage for 7 wk in a range of moisture conditions.

Several DNA-based techniques, developed for identifying and differentiating fungi in the Gaeumannomyces-Phialophora complex associated with cake-all diseases of cereals and grasses, were used to compare fungi from maize. Maize isolates obtained as G. graminis (Sacc.) Ars Pr II Olivier var. tritici Walker, from the UK, having been identified by ascospore morphology and in pathogenicity rests on wheat, Mere indistinguishable from isolates of the same variety obtained from wheat. Isolates of G. graminis (Sacc.) Arx & H Olivier var. maydis Yao et al., recently described as the maize take-all fungus from China, were identical in DNA tests to the anamorphic fungus Phialophora radicicola Cain and almost identical to Phialophora zeicola Deacon & Scott, whose description was originally based on isolates from South Africa and France. These species appear to represent the holomorph of the same fungus. The late wile pathogen of maize, from India and Egypt, commonly known as Cephalosporium maydis Samra et nl., but suggested as being the: Phialophora anamorph of a Gaeumannomyces species, was closely related to other Gaeumannomyces species included in the tests

Field isolates ( n = 144) of the wheat take-all fungus Gaeumannomyces graminis var. tritici ( Ggt ) were tested for sensitivity to silthiofam, a take-all-specific fungicide used as a seed treatment, and identified as A-or B-type by PCR-RFLP analysis of nuclear rDNA. A possible association was identified between polymorphisms in ITS2 of the nuclear rDNA and sensitivity to silthiofam. A Ggt -specific PCR assay was developed which simultaneously identified isolates of Ggt as A-or B-type, based on the polymorphisms in the nuclear rDNA. A highly significant correlation between Ggt type using the PCR assay and sensitivity to silthiofam was demonstrated in a collection of 358 isolates from three field experiments designed to test the effects of seed-treatment fungicides on take-all and Ggt populations in winter wheat. In one experiment the percentages of silthiofam-sensitive and B-type isolates were significantly less in populations from plots sown with silthiofam-treated seed in two consecutive years than in populations from plots sown with nontreated seed. However, silthiofam still provided a significant amount of control of take-all. The natural occurrence of fungicideinsensitive isolates, up to about 30% in soils in which the fungicide had never been used, is unusual. The new PCR assay provides a useful tool for studying the population structure of Ggt , and may provide a novel method for assessing the incidence of insensitivity to silthiofam (the target site for which has not yet been identified) in field populations of Ggt .