Stability ofMicrosphaera diffusaand the Effect of Powdery Mildew on Yield of Soybean

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Abstract
Reactions of soybean cultivars to powdery mildew caused by M. diffusa collected in 1973, 1979 and 1982 were compared. There was no indication of any change in the strain of M. diffusa found in Georgia [USA] during this 9-yr period despite extensive planting of resistant cultivars and regular occurrence of the sexual stage of this fungus. A close negative correlation between powdery mildew rating and yield during 1 season indicated that yields in untreated control plots were 35% less than in plots where powdery mildew was controlled with fungicides. There was no significant difference between the yield of treated and control plots in 3 other years. During a 4-yr period when powdery mildew ranged from moderate to very severe, the powdery mildew ratings of plots treated with benomyl, chlorothalonil or thiabendazole at registered rates and intervals of application were significantly reduced but yields of these plots were not increased. Comparisons of weather conditions with disease severity during a 5-yr period indicated that high humidity, moderate rainfall, and low temperature were most favorable for disease development. Blue lupine [Lupinus angustifolius] appears to be a more favorable host than soybean for development of cleistothecia of M. diffusa.

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