Comparative Grain Yields of Corn Hybrids in Normal and in Texas Male‐Sterile Cytoplasm (cms‐T) Infected with Helminthosporium maydis Race T and Disease Components of cms‐T Corn Hybrids1

Abstract

The amount of Helminthosporium maydis (Nisikado and Miyake) race T infection on corn (Zea mays L.) hybrids with Texas male‐sterile cytoplasm (cms‐T) (resulting from artificial inoculation or from natural infection) was significantly greater than on their normalcytoplasm counterparts. Early infection of cms‐T corn hybrids after artificial inoculation was significantly more severe than infection which took place naturally later in the season. Both 1,000‐kernel weight and grain yield of artificially inoculated cms‐T corn hybrids were reduced significantly more than those from the same cms‐T hybrids exposed only to natural infection. Grain yields and 1,000‐kernel weights of most cms‐T corn hybrids, exposed only to natural infection, were significantly reduced in relation to their normal‐cytoplasm counterparts even though the epiphytotic was delayed until late during physiological maturation of the plants. In the same field, however, there was no significant difference in grain yield or in 1,000‐kernel weight between cms‐T hybrids and their normal cytoplasm counterparts of highly resistant corn hybrids. Early infection, resulting from artificial inoculation, had no appreciable influence on amount of disease infection or on 1,000‐kernel weights of most normal‐cytoplasm hybrids. The amount of disease infection, grain yield, and 1,000‐kernel weight of cms‐T hybrids were significantly correlated with those of their normal‐cytoplasm counterparts. Cms‐T hybrids which had a high disease tolerance produced about twice as much grain as those of low tolerance in the presence of disease.All disease components (leaf, sheath, and husk infection, and lesion sizes) were highly intercorrelated at each disease rating date: before silking, midsilk, and physiological maturity. The contribution of the lesion size component to the standardized regression weight () and incremental proportions of variance (ΔR2) values for reductions both in 1,000‐kernel weights and in grain yields was always higher than that of other disease components at each disease rating date. Lesion sizes measured at midsilk had higher and ΔR2 values in grain yield reduction than measurements before silking or at physiological maturity. The r values between lesion sizes and the entire plant infection were highest at each disease rating date.