EVALUATION OF A DIVERSE RED CLOVER COLLECTION FOR CLOVER ROT RESISTANCE(SCLEROTINIA TRIFOLIORUM)

摘要

Sclerotica trifoliorum Erikks. causes clover rot (clover cancer, Sclerotinia crown and root rot), an important disease in European red clover crops [Trifolium pratense L). The fungus infects plants in autumn through ascospores and entire fields can bedestroyed 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 5. trifoliorum isolates were used. In a field experiment, plant yield, branching and susceptibility to mildew, rust and virus disease were scored for 122 red clover accessions. A similarcollection of germplasm was screened for clover rot resistance by 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 levelsof isoflavones (available 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 but no accession was completely resistant. Three accessions (Maro, Tedi and No. 292) were significantly less susceptible than the other accessions. Intensive branching or a prostrate growth habit did not render plants more resistant. Accessions resistant to mildew or viruses were not moreresistant 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.