The durability of partially resistant wheat cultivars to wheat leaf rust depends on the amount of genetic variation in parasitic fitness within populations of the pathogen Pucciniareconditaf. sp.tritici. To assess the durability ofpanialresistance, phenotypic variation inlatentperiod (a major component of parasitic fitness) was quantified and partitioned into genetic and nongenetic components for isolates of P.reconditaf. sp.triticion susceptible and partially resistant cultivars. Latent periods among isolates differed by 24 to 27% on individual partially resistant cultivars. In simulated epidemics, isolates with short latent periods caused 2 to 2.5 times more disease and overcame 13 to 35% of the resistance of four partially resistant cultivars. Heritability estimates for latent period of isolates of P. recondita f. sp. tritici on partially resistant cultivars ranged from 0.28 to 0.76. These results suggest that isolates with short latent periods should be favored by natural selection and could overcome a portion of the resistance of partially resistant wheat cultivars provided that short latent period is unlinked to other traits that reduce fitness. Despite a long latent period, wheat cultivar Cl 13227 was anticipated to have the least durable resistance because pathogen isolates on 'Cl 13227' were the most variable for latent period and because 'Cl 13227' appeared to interact with pathogen isolates with the greatest specificity.
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Jeffrey S. Lehman (1996) studied this question.