Aggressiveness study on Sclerotinia isolates from red clover crops

Sclerotinia trifoliorum Erikks. causes clover rot (clover cancer, Sclerotinia crown and root rot) in red clover crops (Trifolium pratense L.), an important disease in Europe. Little is known about the aggressiveness of Scierotinia isolates and aggressiveness studies were never conducted on a European scale. In this study we compared the aggressiveness of 30 Sclerotinia isolates isolated from red clover crops in 25 locations in 12 European countries using a plant-based bio-test. Plants from 6 red clover cultivars with different resistance levels were spray inoculated at the age of 12 weeks with 1 to 1.5 ml mycelium fragment suspension per plant. After 10 days incubation, plants were scored on a scale from 1 (healthy plant) to 5 (dead plant) and the disease index was calculated. The experiment was repeated 3 times and all repetitions were highly correlated. Average disease indices ranged from 52.6% to 82.7%. Significant differences were detected between isolates and between cultivars, but there was no isolate--cultivar interaction. Based on these results, the most aggressive isolates can be selected for resistance breeding. Future work should investigate whether the differences in aggressiveness are due to a higher growth speed or due to a higher secretion of cell-wall degrading components and pathogenicity factors.