Influence of Disease Severity and Environmental Conditions on Low Receptivity of Oats to Crown Rust

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Abstract
Low receptivity, a major component of slow rusting, is characterized by fewer pustules on slow rusting cultivars [''Red Rustproof-14'', ''CI 4876''] than on fast rusting types [''Fulghum'', ''CI 708'']. Slow rusting and fast rusting cultivars of oats were quantitatively inoculated with crown rust urediospores. In the growth chamber, low receptivity was expressed when about 4 pustules/s cm2 developed on the slow rusting cultivar but not at greater densities, i.e., 6-8 pustules/s cm2. Spore germination and appressoria formation were not correlated with low receptivity. Spores that settled on the leaves quickly (1 min) were not as infective as lighter spores that settled more slowly (2 min). When comparable quantities of spores (about 50/cm2) germinated on leaves, the slow rusting cultivar had 50- to 65-fold fewer pustules in the field than in the growth chamber. The fast rusting cultivar had about the same number of pustules per square centimeter in the field and growth chamber.

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