Morphology, benomyl resistance and pathogenicity to wheat and rye of isolates of Pseudocercosporella herpotrichoides

Abstract

Seventy‐one isolates of Pseudocercosporella herpotrichoides, a non‐random sample obtained from cereal crops in Britain between 1981 and 1983, were classified as either sensitive or resistant to 2 mg/l benomyl. In agar culture, isolates were of two distinct morphological types, described as ‘fast‐even’ and ‘slow‐feathery’. Pathogenicity to wheat and rye seedlings was determined for 21 fast‐even and 32 slow‐feathery isolates. Fast‐even isolates were much more pathogenic to wheat than to rye; nearly all slow‐feathery isolates were about equally pathogenic to wheat and rye. Thus, in morphology and pathogenicity, irrespective of sensitivity to benomyl, fast‐even isolates conformed to the published descriptions of W‐types, while slow‐feathery isolates conformed to the descriptions of R‐types.Sensitivity to 2 mg/l benomyl was determined for 97 isolates, including W‐types and R‐types, collected between 1956 and 1983, mainly from England but including some from nine other countries. Only 16 isolates were resistant; three were fast‐even and 13 were slow‐feathery. All were collected in England since 1981, suggesting that resistance has been rare until recently. Resistance was not a feature of R‐type isolates collected before 1981. The increase in the proportion of resistant isolates since 1981 appears to have coincided with an increase in the proportion of R‐type isolates.