Effects of fungicides on eyespot in five successive crops of winter wheat in plots with different initial or fungicide-selected populations of Pseudocercosporella herpotrichoides

The effects on eyespot of carbendazim, prochloraz or a mixture of carbendazim and prochloraz, applied twice yearly and to the same plots in each year, were studied in five successive crops of winter wheat. Some plots were uninoculated (with a background population of the eyespot fungus, Pseudocercosporella herpotrichoides, which was initially mainly MBC-sensitive W-type), whereas others were inoculated artificially in the first autumn of the experiment (1984) with mainly MBC-sensitive W-type, MBC-resistant W-type, MBC-sensitive R-type or MBC-resistant R-type fungus. Samples were taken in April (1985–1987) and July (1985–1989). When all inoculation treatments are considered together, carbendazim was ineffective at all sampling times after April 1985 because of its rapid selection for MBC-resistance, and increased eyespot incidence and severity in some samples. Prochloraz applied alone decreased eyespot incidence and severity at all sample times except July 1988. Where applied with carbendazim, prochloraz always decreased eyespot incidence and severity, sometimes more so than when applied alone. The control of eyespot by the fungicides was influenced by the initial populations in the first two seasons only. Carbendazim decreased eyespot incidence in 1985 only where no artificial inoculum was added (both samples) and where mainly MBC-sensitive R-type inoculum was added (April 1985). Prochloraz applied alone was ineffective in July 1985 where mainly MBC-sensitive R-type fungus was added, and, applied with carbendazim, was ineffective in July 1986 where mainly MBC-resistant fungus was added. Subsequently, the populations were unrelated to the initial populations and were mainly a result of selection by fungicides where these were applied. Although selecting for the R-type fungus and, where no carbendazim was applied, for MBC-sensitivity, prochloraz remained generally effective after repeated applications over five seasons.