Department of Plant Pathology, University of Helsinki
At the experimental station of the University of Helsinki located at Muddusniemi, Inari (69°5'N), it was observed in the spring 1953 that the roots of dead red clover plants contained small, darkcolored formations resembling sclerotia (5). The same fungus occurred even more abundantly in the spring 1961. During both winters, 1952—53 and 1960—61, there were especially large amounts of low temperature parasitic fungi. Thus, in the spring 1961 Sclerotinia borealis Bubak & Vleugel caused almost the complete destruction of first-year timothy leys and had badly damaged older leys. In addition, stands of red clover, which generally overwinter well at Muddusniemi (5, 6, 7), were found to be severely injured. For example, in the red clover sowing time trial (sowing dates 15.6.1960 and 15.7.1960), all the clover varieties, including the Finnish variety Tammisto, were completely destroyed in the course of the winter, In investigating the cause of the clover destruction, it was found that the fungal growths observed on the roots of dead clover plants were black, hard, slightly flattened spherical formations, 5—11 mm in diameter. They occurred in all parts of the surface of dark-colored, dead roots, but they were most numerous in the upper parts of the roots, where they appeared as dense clusters (Fig. 1). The fungus was isolated on Henneberg agar (2) in Petri dishes and was cultured at temperatures of +2°C, +7°C and +22°C. On the basis of the culture tests as well as the symptoms observed in the roots of infected clover plants, it was possible to determine this fungus as being Plenodomus meliloti Dearness & Sanford (1, 4). This fungus belongs to the Sphaeropsidales order of the Fungi Imperfecti.
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