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

Clover rot, an important disease in European red clover crops, is caused by Sclerotinia trifoliorum or Sclerotinia sclerotiorum. Until today, little is known about the variation in aggressiveness among Sclerotinia isolates from red clover. Red clover cultivars are known to differ in susceptibility to clover rot, but few previous studies have phenotyped diverse red clover collections. It is unclear whether cultivars are generally more susceptible than wild accessions or landraces. Moreover, the relations between susceptibility to clover rot and other diseases have never been studied before. In this research we compared the aggressiveness of 30 Sclerotinia isolates from 25 locations in Europe. Aggressiveness was assessed in vitro on detached leaves from five genotypes and in a greenhouse on young plants from five cultivars. In a second part, we evaluated a diverse collection of 122 red clover accessions on the field. Susceptibility to mildew (Erysiphe polygoni) and rust disease (Uromyces trifolii) were scored visually on the field, along with other important traits such as plant yield, crown branching and flowering date. A similar series of accessions were scored for clover rot susceptibility in the greenhouse by inoculating young plants with ascospores from five aggressive Sclerotinia isolates. The susceptibility of cultivars was compared with wild accessions and landraces, and diploid and tetraploid accessions were compared. Correlations were estimated between susceptibility to clover rot and the other scored traits. Our 30 isolates differed significantly in aggressiveness. The most aggressive isolates were selected to screen our collection of red clover germplasm. Accessions differed in susceptibility to clover rot but no accession proved completely resistant. Diploid accessions were on average more susceptible than tetraploid accessions and cultivars were on average more resistant than wild accessions or landraces. Resistance to clover rot was negatively correlated with plant yield and resistance to rust disease. More resistant accessions can be used as starting material for resistance breeding, yet plant yield and susceptibility to rust disease should be carefully monitored when breeding for clover rot resistance.