Sensitivity ofMonilinia oxycoccito Fenbuconazole and Propiconazole in vitro and Control of Cranberry Cottonball in the Field

The efficacy of fungicides in controlling cottonball disease of cranberry was tested during 1996 to 1998 at three locations in Wisconsin. For some fungicides, the efficacy of four applications, two each during shoot elongation and bloom, was compared with two applications during bloom only. Spraying twice during bloom was as effective in controlling secondary infection as spraying twice during shoot elongation plus twice during bloom. Azoxystrobin, cyprodinil, and propiconazole were equally effective. None of the treatments affected yield, fruit retention, or berry weight compared with the controls. Sensitivity of M. oxycocci, the cottonball pathogen, to fenbuconazole and propiconazole was tested in vitro by comparing the distributions of ED 50 values of populations collected from three sites that differed in previous exposure to fungicides. Median ED 50 values for fenbuconazole were significantly greater at sites where sterol demethylation inhibitor fungicides had been used compared with a site where fungicides had never been used, but median ED 50 values for propiconazole did not differ among sites. There was no correlation between the sensitivities to fenbuconazole and propiconazole. The data will form the basis of recommendations aimed at delaying the onset of fungicide resistance and will provide a baseline for monitoring resistance to fenbuconazole and propiconazole in populations of M. oxycocci in the future.