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Latency period (LP), infection frequency (IF) and urediosorus size (US) of leaf rust were determined on primary leaves and young flag leaves of 18 spring wheat cultivars . A large growth stage effect and a large cultivar effect on all three components were observed . Partial resistance as measured by the three components was generally better expressed in the adult plant stage than in the seedling stage . Associated variation of the components was observed : long LP, low IF and small US tended to go together . The association was not complete, cultivars with clear deviations of this association for one of the components were found suggesting the existence of at least partly different genetic factors controlling the respective components . LP measured on flag leaves gave the most reliable results and, therefore, could best be used as a selection criterion in breeding programs for partial resistance .

Eighteen spring wheat cultivars were tested in microfields and race nurseries for their partial resistance PR to wheat leaf rust under low and high disease pressure respectively Large differences existed between the 18 cultivars, Skalavatis 56 being the most susceptible and Ponta Grossa 1 being the most resistant cultivar Of the three epidemic parameters, disease severity (DS) at the time that the susceptible check was severely diseased and area under the transformed disease severity curve (AUTC) and the logistic growth rate (r), AUTC and DS were highly correlated Both seemed to be reliable estimators of PR but DS should be preferred for economical reasons The logistic growth rate seemed to be unsuitable as an estimator of partial resistanceHigh and low disease pressure gave similar cultivar ranking PR can be screened and selected equally well in race nurseries with low space, low time and low cost input as in microfields with high space, time and cost input Cultivar differences in development rate had a large impact on the cultivar differences for amount of disease and can therefore greatly bias the estimation of cultivar resistance The resistance of early cultivars tended to be underestimated whereas the resistance of late cultivars tended to be overestimated The effect of differences in developmental rate was most pronounced in the flag leaf It is advisable to avoid the assessment of disease levels on the flag leaf only and to incorporate in the tests several susceptible and resistant checks that cover the range of development rates in the material to be selected, because otherwise selection for resistance will tend to select also for lateness Regression of the epidemiological parameters on three components of partial resistance revealed that latency period (LP) is an important factor in determining the resistance observed in the field explaining on average 67% of the observed variation Adding infection frequency (IF) and urediosorus size (US) to the linear model increased the proportion of the observed variation in the field explained by the components to 80% This result supports the idea that the components of PR inherit independently, at least, in part 248

Three partially resistant spring wheat cultivars, with a long latency period were crossed among each other and with the highly susceptible Little Club, with a very short latency period . Parents, F3 and F5 plants have been inoculated with the leaf rust race Flamingo in the young flag leaf stage to determine the latency period .From the crosses with Little Club, it was concluded that Westphal 12A carries three, Akabozu two and BH 1146 two or three genes for a longer latency period . BH 1146 appears to carry also one hypersensitive resistance gene .Transgressive segregation occurred in crosses between partially resistant cultivars . From crosses between the partially resistant cultivars, it was concluded that the genes in Akabozu and Westphal 12A are different, while those in Akabozu and BH 1146 are at least partly different . The possibilities of accumulation of LP-prolonging genes are discussed .