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.). This disease is important in Europe. Ascospores infect clover plants in autumn and if winter weather condi-tions are favourable, entire fields can be destroyed by early spring. Little is known about the aggressiveness of Sclerotinia isolates and aggressiveness studies have never been 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.
Mycelium was grown for 5 days in PDB medium. In preparation for infection, the mycelium was rinsed, homogenised and diluted to 5000 fragments/ml in infection solution. 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 suspension per plant. After 10 days of incubation at 15-25 °C and 100% relative humidity, 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% and significant differences were detected among the 30 isolates.
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.