Evaluation of a diverse collection of red clover germplasm for susceptibility to clover rot (Sclerotinia trifoliorum) and other important traits

Clover rot, caused by Sclerotinia trifoliorum or S. sclerotiorum, is an important disease in European red clover crops. Little is known about the differences in aggressiveness among Sclerotinia isolates from red clover. No completely resistant cultivars are available and few previous studies have screened large red clover collections for resistance. Moreover, the relations between susceptibility to clover rot and architectural and biotic factors were never studied before. In this study we compared the aggressiveness of 30 Sclerotinia isolates from 25 locations in Europe. Aggressiveness was assessed on young plants from five cultivars with different resistance. Additionally, we evaluated a diverse collection of 121 red clover accessions. Yield, branching, flowering date, susceptibility to mildew (Erysiphe polygoni), rust (Uromyces trifolii) and viral disease were scored on the field. Clover rot susceptibility was determined by inoculating young plants with five aggressive Sclerotinia isolates. Our 30 isolates differed in aggressiveness. Red clover accessions differed in susceptibility to clover rot, but no accession proved completely resistant. Cultivars were more resistant than landraces and wild accessions. Tetraploid cultivars were more resistant than diploid cultivars. Clover rot susceptibility was not correlated with the degree of branching or plant yield. On the other hand, late flowering and erectly growing accessions were less susceptible to clover rot. Clover rot susceptibility was not correlated with susceptibility to rust or viral diseases, but negatively with mildew susceptibility. The best way to improve resistance may be to recurrently select for resistant plants among diverse cultivars and landraces with lower susceptibility. Tetraploidisation of diploid populations with a higher resistance level can provide an additional level of protection.