Expression of Partial Resistance to Common Rust in Sweet Corn Hybrids at Various Host Growth Stages

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Abstract
Six sweet corn [Zea mays] hybrids varying in levels of partial resistance to Puccinia sorghi, causal agent of common maize rust, were inoculated simultaneously in the field at eight stages of growth. Both partial resistance and an adult plant resistant reaction were observed. Differences in partial resistance were measurable at all growth stages. Miracle and Sugar Loaf displayed high levels of partial resistance, whereas Stylepak and Florida Staysweet were highly susceptible. All hybrids were most resistant to rust when inoculated at the late silk stage and most susceptible when inoculated a the five- to six-leaf stage. As plant age increased, rust severity decreased. This adult plant resistant reaction was most apparent after the onset of reproductive growth of the host. Regressions of rust severity on plant age were fit best by a quadratic model for five of the six hybrids. In general, the rate at which hybrids with high levels of partial resistance displayed the adult plant resistant reaction was slower than that for hybrids with low levels of partial resistance. The adult plant resistant reaction is apparently a universal property of sweet corn and is a function of plant age, whereas partial resistance is a genotype-specific trait and functions at all growth stages. Partial resistance and the adult plant resistant reaction were similar in that both reduced the number of uredinia per leaf area. Both should be more durable than race-specific resistance.