The biology of Sclerotinia trifoliorum Erikss. and other species of sclerotium ‐forming fungi

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Abstract
SUMMARY: The survival in soil of sclerotia of Sclerotinia trifoliorum, S. sclerotiorum, Sclerotium delphinii and S. rolfii was studied. Those of S. rolfsii disintegrated in less than 1 month, while 95% of those of S. delphinii and Sclerotinia sclerotiorum were recovered after 2 years.Sclerotia of S. trifoliorum placed in soil in March slowly diminished in numbers over the summer months, and in late autumn there was a rapid decrease in those near the surface, as a consequence of apothecial formation. This rapid fall did not occur when sclerotia were buried under clover. Over the winter, sclerotia 1 and 2 in. deep increased in numbers by the formation of secondary sclerotia. Secondary sclerotia also occurred in samples of Sclerotinia sclerotiorum.Soil moisture content affected sclerotial survival, an accelerating decline in numbers being noted up to a soil moisture‐holding capacity of 30%. Above this level, increasing numbers of secondary sclerotia were formed, so that the numbers of sclerotia recovered were sometimes greater than those originally buried.Sclerotia became more liable to decay after producing apothecia, but they did not invariably disappear, and some were capable of forming two or possibly three crops of apothecia in successive years.