Factors Related to Partial Resistance of Barley to Leaf Rust

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Abstract
Several components of partial resistance were studied in three barley (Hordeum vulgare) cultivars known to be widely divergent in their reactions of partial resistance to Puccinia hordei. Uredinia were counted daily to determine infection density and latent period. Inoculated leaf samples taken daily were cleared and stained for fluorescent microscopy. The factors studied histologically included proportion of successful infection units, penetration, number of haustorial mother cells (HMC), early abortion, late abortion, colony size, and uredinium sporulating area. Latent period (LP50) was longer for the cultivars with partial resistance. Infection density and proportion of successful units was lower for cultivars with partial resistance. The average percentage of germinated urediniospores that had not prevented was higher for the cultivar with the highest level of partial resistance (17-5-16) than for the very susceptible cultivar (L94). The percentage of early-aborted colonies was much higher for cultivars with parital resistance than for the susceptible cultivar. The percentage of late-aborted colonies was significantly greater for cultivar 17-5-16 than for the other cultivars. The average area of P. hordei colonies and the number of HMC was greater for the susceptible cultivar than for those with partial resistance. The size of the sporulating uredinium was not significantly different for any of the cultivars. The most definitive histological difference between susceptible and partially resistant barley cultivars to P. hordei was the amount of early abortion in the cultivars with partial resistance. The most important factor of partial resistance was the slower development of fewer sporulating colonies.