Construction of a bio-test for infection with Sclerotinia trifoliorum in red clover breeding programs

Sclerotinia trifoliorum causes clover cancer (clover rot, Sclerotinia crown or stem rot) in red clover crops, a major disease in European red clover crops. Clover cancer is difficult to control and completely resistant varieties are not available. Red clover breeding programs are confronted with the lack of a bio-test for infection with S. trifoliorum. The objectives of this research were to develop a high-throughput bio-test useable in breeding programs and to apply it in our red clover breeding program. We constructed a bio-test that infects young plants through ascospores in moist conditions. After two weeks incubation, plants are scored from 1 (no damage) to 5 (killed plant) and the disease index is calculated. The effects of ascospore concentration, plant age and cold treatment on the bio-test were evaluated and the repeatability was proven. 6000 plants from a red clover population were successfully inoculated: the average disease index was 51.4%. The 30% most resistant plants were selected as the new parent population and planted in the field. Seeds were harvested in bulk in August. The next two generations will again be subjected to the bio-test and we will compare their resistance levels with the parent population to evaluate the effectiveness of the bio-test in the breeding program.