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Managing Cranberry Cottonball Caused by Monilinia oxycocci with Fungicides. S. N. Jeffers, Department of Plant Pathology, University of Wisconsin, Madison 53706. Plant Dis. 75:502-506. Accepted for publication 29 October 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0502.

Over a 3-yr period (1987–1989), 11 fungicides were evaluated for efficacy in managing cottonball in a commercial cranberry bed heavily infested with Monilinia oxycocci. Three applications, beginning at budbreak, were made to control primary infection of shoots by ascospores, and two applications were made during bloom to control fruit rot caused by secondary infection of flowers by conidia. Three demethylation-inhibiting fungicides—triforine, tebuconazole, and RH-7592—were most effective at controlling both primary and secondary infections. Two dicarboximide fungicides—vinclozolin and RH-3486—also controlled both disease stages but less effectively. Benomyl and chlorothalonil were very effective against secondary infection only. Yield was significantly affected by treatments in 1987 and 1988 but not in 1989, when yields overall were reduced markedly. Plots treated with benomyl had the highest yields, whereas those treated with copper hydroxide or not treated had significantly lower yields because of fruit rot. In 1988, treatment with chlorothalonil resulted in only 2.4% fruit rot but significantly reduced yield (49% compared with the benomyl treatment), which was attributed to reduced fruit retention and reduced berry weight. Tebuconazole, triforine, vinclozolin, and the control treatments also reduced berry weight in one of the years. Copper hydroxide was the only fungicide other than chlorothalonil to significantly reduce fruit retention. Fungicides had no effect on the number of flowers produced per flowering shoot. Triforine is the only fungicide currently registered for cottonball management in Wisconsin and has been used exclusively since 1982. Registration of additional fungicides is needed to ensure effective management of cottonball in the future.

Keyword(s): fungicide resistance, iprodione, myclobutanil, thiophanate-methyl, Vaccinium macrocarpon.