Two complementary experiments were conducted in a controlled environment to elucidate the interactions between the fungus Sclerotinia trifoliorum Erikss. and red clover (Trifolium pratense L.). In one of these studies, two hardened diploid red clover cultivars (cvs) were inoculated with 20 isolates of S. trifoliorum of various geographic origins. In the other study, 20 red clover cvs, diploid or tetraploid, including late and medium-late flowering types, were inoculated with two isolates of the fungus. Prior to inoculation, some plants were hardened by subjecting them to a low temperature and light treatment mimicking autumn conditions. Late flowering cvs were found more resistant than medium-late ones. Isolates collected in the northern region, where late cvs are grown, were significantly more aggressive than isolates from southern locations, where medium-late cvs are more prevalent. Such an adaptation has not previously been reported for this fungus. This is the first report concerning flowering type and resistance in red clover. Tetraploids were generally not more resistant than diploids. A hardening procedure for red clover plants was found to be a prerequisite for detecting the differences in disease resistance.www.blackwell-synergy.com J. Phytopathology 153, 505-511 (2005) Ó 2005 Blackwell Verlag, Berlin

The susceptibility of 15 red clover ( Trifolium pratense ) cultivars to Sclerotinia trifoliorum under natural conditions in field experiments was investigated at sites in Sweden located at latitudes ranging from 60 to 64 ° N, with corresponding ranges of climatic regimes and lengths of vegetative growth and winter periods. At each site, 10 red clover cultivars were included: five were common, and five were selected according to the regional climate. Clover rot caused by S. trifoliorum from natural sources occurred in all geographical areas studied, and all cultivars included were affected. The numbers of plants of each cultivar killed by the pathogen was estimated at five sites over 2 years. Severe effects were observed in the first winter at these sites, but while incidence of clover rot increased and diseased areas expanded substantially in the second winter in northern Sweden, this pattern was not observed in southern regions. Tetraploidy, late flowering type and previous selection for resistance among cultivars positively influenced survival rates in the field. Estimates of susceptibility of cultivars obtained under field conditions after 2 years showed strong correlation when compared with rankings of susceptibility caused by an aggressive isolate of S. trifoliorum in previously conducted controlled environment laboratory tests. Yields were correlated to the proportion of gaps caused by the pathogen, indicating its economic importance.

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