Changes in populations of Pseudocercosporella herpotrichoides in successive crops of winter wheat in relation to initial populations and fungicide treatments

Abstract

The effects of two applications annually of carbendazim, prochloraz or a mixture of carbendazim and prochloraz on five populations of the eyespot fungus, Pseudocercosporella herpotrichoides, were studied in successive crops of winter wheat. Initially, populations were established by inoculation of plots with mixtures of isolate types which were mainly MBC-sensitive W-type, MBC-resistant W-type, MBC-sensitive R-type or MBC-resistant R-type. Further plots which were not inoculated had, initially, a mainly MBC-sensitive W-type population. Plants were sampled each April and July for 3 years. Carbendazim treatments resulted in an almost entirely MBC-resistant population by July of the first year, regardless of the proportion of MBC-resistant isolates in the initial inoculum. Although treatment with prochloraz greatly decreased populations of the eyespot fungus, its effect on the number of MBC-sensitive isolates was usually not significant. Prochloraz effectively controlled the MBC-resistant fungus and was usually more effective against the W-type than the R-type of the fungus. Treatment with the mixture of carbendazim and prochloraz also greatly decreased populations, but the selective effect of each component fungicide was less marked. Regardless of fungicide treatment, a smaller proportion of R-type isolates was recovered from severe than from slight stem lesions in July, indicating that the times of lesion establishment or rates of development of the R-type and W-type were not the same. Migration of fungal types between plots appeared to have little influence on the outcome of selection.