Partial resistance (PR) in barley to leaf rust is assumedly a case of durable resistance. PR is characterized by a reduced rate of epidemic development in spite of a susceptible infection type. One of the components of PR, low infectibility, was studied histologically by means of fluorescence microscopy. Quantitative analyses of the phases of the infection process beyond appressorium formation showed that the reduced infectibility of partially resistant barley seedlings rests on a significant "early abortion" of colonies. This type of abortion occurs at about the moment of the formation of the first haustoria, when the young colonies have formed up to five or six haustorial mother cells. Early abortion is only incidentally associated with the collapse of host cells. Not only the variation in infectibility among barley genotypes but also the variation in infectivity among leaf rust isolates is based mainly on differences in the degree of early abortion. The occurrence of a high degree of early abortion in several unrelated barley genotypes indicates that the PR genes are part of a generally occurring system.
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