Evaluation of a diverse red clover collection for clover rot resistance (Sclerotinia trifoliorum)

Sclerotinia trifoliorum Erikks. causes clover rot (clover cancer, Sclerotinia crown and root rot), an important disease in European red clover crops (Trifolium prat-ense L.). The fungus infects plants in autumn through ascospores and entire fields can be destroyed by early spring. Although previous studies have evaluated various red clover populations for clover rot resistance, screening was often performed with one local isolate on just a few local varieties, often cultivars. Until today, no large collections of diverse red clover accessions have been screened. In this study, we studied the variation in clover rot susceptibility among 122 red clover accessions, including 85 accessions from the NPGS-USDA core collection. Cultivars (both diploid and tetraploid), landraces and wild accessions were included and different S. trifoliorum isolates were used.
In a first experiment, susceptibility to clover rot of the 122 accessions was deter-mined with a bio-test on detached leaves from plants in the field. Leaves were inoculated with a mixture of four S. trifoliorum isolates. Plant yield, branching and susceptibility to mildew, rust and virus disease were scored in the field. In a second experiment, a similar collection was screened in the greenhouse with a bio-test on young plants using a mixture of five aggressive S. trifoliorum isolates. The effects of the variety type, ploidy level, growth habit, resistance to other diseases and levels of isoflavones (described for the NPGS-USDA collection) on clover rot susceptibility were determined. Possible sources of resistance were identified.
Our red clover accessions differed significantly in susceptibility. No accession was completely resistant, but there was a large variation in resistance among genotypes within accessions. Three tetraploid accessions were significantly less susceptible than the other accessions. Intensive branching or a creeping growth habit did not render plants more resistant. Also accessions resistant to mildew or viruses were not more resistant to clover rot and accessions with high levels of isoflavones were not better protected against clover rot. On the other hand, tetraploid cultivars were on average 10% less susceptible than diploid cultivars. Cultivars were generally less susceptible than landraces and wild accessions.
Allocating sources of resistance for breeding purposes is difficult. The best way to improve clover rot resistance may be to select and intercross resistant plants from cultivars with low susceptibility.