Field performance of subterranean clover germplasm in relation to severity of Cercospora disease

Ninety six genotypes, including 14 cultivars, of Trifolium subterraneum var. subterraneum and var. yanninicum were screened in the field for resistance to Cercospora ( Cercospora zebrina). Seven genotypes, 84S43-13, 84S43-15, EP132Sub-E, 84Y32-59, 83Y83-23, 84Y32-42 and 83Y79-20, were totally resistant to Cercospora disease. A further 26 genotypes, including the cvv. Meteora and Napier, had an incidence of Cercospora of <1 (0-10 scale) and no consequent leaf collapse from the disease ( score 0; 0-10 scale), while 15 genotypes had incidence scores between 1 and 2 without any leaf collapse evident. There was excellent overall correlation between Cercospora incidence and leaf collapse across the genotypes tested, with both strong quadratic (y = -0.17x(2) + 2.50x + 1.33; R-2 = 0.89) and linear ( y = 0.96x + 1.63; R-2 = 0.82) components to this relationship. There was circumstantial evidence of ecogeographical differences for Cercospora resistance among ecotypes collected from different regions. Of eight overseas introductions with Cercospora incidence scores of <2 and a Cercospora leaf collapse score of 0, six were from Sardinia and one each were from Portugal and Greece. In contrast, all seven Sicilian ecotypes had Cercospora incidence scores of 7.75 or greater. The high degree of resistance observed in many of the genotypes to Cercospora highlights the existence of many excellent sources of resistance that could be exploited in breeding and development programmes to minimise production losses in Australian subterranean clover pastures.