Abstract
The development of a clover scorch epidemic was studied at Denmark, W.A., by using 62 lines of Trifolium subterraneum including 29 subspecies yanninicum genotypes. Total disease development (TDD), the intrinsic rate of disease development (Rate) and time to 50% disease development (Delay) were calculated from fortnightly observations from August to December. Principal component analyses of the data lent support to the view that TDD is a good measure of disease expression. TDD ranged from 16 to 139 disease days (maximum possible 140). Nine yanninicum genotypes that had TDD values of less than 28 disease days are considered to have potential in breeding for resistance. Within these nine genotypes there was, however, considerable variation in Rate and Delay. A positive correlation between early growth and TDD, and a negative correlation between flowering date and TDD, are seen as possible problems in breeding resistant cultivars. Scoring plants once in the middle of the period of maximum disease development and also at the end of the season should provide a reasonably accurate approximation of TDD and delay.
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