Department of Plant Pathology University of Helsinki
It is generally considered at the present time that Sclerotinia trifoliorum survives during the summertime mainly in the form of sclerotia. These sclerotia can give rise to either mycelia or apothecia. It has been established that both the mycelia (7, 17) and the ascospores (3, 7, 10, 11, 15, 19, 21, 23) can infect clover. The possibility has also been pointed out that infection can take place by means of mycelia or spores which were present in the seed lot (3, 15, 16). The sclerotia have a durable outer covering consisting of 3—4 layers of cells which protects them from unfavourable external influences (9). Thus sclerotia are able to live for many years, although in general they are often rapidly destroyed in the soil. ERIKSSON (5) assumed that sclerotia in the soil remain viable during only one summer. At times sclerotia are completely destroyed within about two months (24), while often a large share of them survive in the soil for many years (3, 9, 19) and have been known to live for over seven years (16). The present investigation deals with the survival of sclerotia of Sclerotinia trifoliorum on the soil, their ability to infect clover, and the occurrence of clover rot at the Viik Experimental Farm of the University of Helsinki during the years 1954—1961. In addition, the appearance of apothecia in various years has also been studied.
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