Differential responses of red clover cultivars to Sclerotinia trifoliorum under diverse natural climatic conditions

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.