Fungal Diseases of Strawberry

Abstract

Botrytis gray mold rot, caused by Botrytis cinerea Pers. ex Fr., is an important fruit rot wherever strawberries are grown. The fungus usually produces a velvety gray growth on the surface of the fruit, but, in high humidity, the surface growth may be cottony and white, with little or no spore production. Mycelium of the fungus is present in senescent petals, stamens, and calyces of marketable fruit (Powelson, 1960). In Ontario, Canada, 90% to 99% of the inoculum (conidia) was produced from mycelium in the laminae and petioles of dead strawberry leaves (Braun and Sutton, 1987). Humidity is the most important factor regulating the occurence of gray mold. Frequent rains induce maximum disease incidence. The optimum field conditions for the development of the disease are an environment at 15 to 20C with >90% relative humidity (Devaux, 1978). Furthermore, it seems that these conditions must be maintained for more than 28 hr for an epidemic development of the disease. In Ohio, optimum temperature for flower infection was 20C, with 100% infection-caused by 24 hr of wetness (Bulger et al., 1987). For inoculation of flowers at 20C, the highest incidence of fruit infection (60%) was recorded after 32 hr of wetness. Flower and fruit infection was greatly reduced above 25C and below 15C for all wetness durations. Protective fungicidal sprays are helpful for the control of gray mold fruit rot during periods of moderate disease incidence. During periods of continuous rainfall, however, control of the disease is almost impossible. Captan or thiram was effective for control in trials in Louisiana (Horn, 1961). Benomyl or thiophanate-methyl fungicides were effective for control during the late 1960s and early 1970s (Paulus et al., 1969; Gourley, 1974). Excellent control in California was obtained with vinclozolin or captan + benomyl, but the vinclozolin treatment was most effective for the reduction of the fruit rot (Paulus et al., 1978). Captan or thiram provided intermediate control. Vinclozolin alone and in combination with other fungicides such as captan consistently provided excellent gray mold control in Ohio trials (Ellis et al., 1987). Excellent control has been obtained with chlorothalonil, captafol, and dichlofluanid outside the United States, but these fungicides have never beenregistered by the EPA for use in commercial strawberry production fields (Peterson, 1973; Aharoni and Barkai-Golan, 1987). Repeated use of the benzimidazole and dicarboximide fungicides caused the development of isolates of B. cinerea resistant to the fungicides. Isolates of B. cinerea from slow-developing lesions on strawberries from plants treated with iprodione or vinclozolin fungicides grew on potato dextrose agar containing 10,000 ppm iprodione or 1000 pm vinclozolin. When isolates sensitive to iprodione and vinclozolin were placed on PDA containing between 1 and 5 ppm of one of these fungicides, a few of the cultures showed mycelial growth after incubation for 7 days (Dennis and Davis, 1979). During-the 1978 California strawberry season, benomyl used alone was not effective for the control of B. cinerea. In tunnel experiments, the efficacy of dicarboximide sprays in controlling gray mold of strawberries was greatly decreased by the presence of dicarboximide-resistant forms of B. cinerea (Hunter et al., 1987). Alternative “partner” fungicides (thiram, chlorothalonil) delayed build-up of resistance to dicarboximides.