Effects of silk maturity and pollination on infection of maize ears by Ustilago maydis

Host resistance is the most efficient method of controlling common smut of maize (Zea mays), caused by Ustilago maydis. Precise timing of ear inoculations with U. maydis relative to silk maturity and pollination may improve the ability to screen maize germ plasm for resistance. The objectives of this study were to determine the length of time maize kernels can be infected by U. maydis through silks, and to examine the effects of pollination on infection through silks. Two field studies were done in 1995, 1996, and 1997 at the University of Illinois South Farms. In the date-of-inoculation study, ears were inoculated at 2- to 3-day intervals from early silk emergence until 16 days after silk emergence. In the date-of-planting study, hybrids were planted on four dates and ears were inoculated on the same day for all planting dates. In each study, ear shoots were covered with shoot bags prior to silk emergence to prevent pollination, or ear shoots were left uncovered to allow silks to be pollinated normally. Maize ears were susceptible to infection by U. maydis from silk emergence until 8 to 14 days after silk emergence. During this period of susceptibility, incidence of ears with galls decreased as silks aged. Incidence of ears with galls on plants inoculated 7 days apart differed by as much as 70%. The period that maize ears were susceptible to infection by U. maydis was shorter and incidence of ears with galls decreased more rapidly when silks were exposed to pollen than when silks were not exposed. The silk channel method of inoculating for common smut does not appear to be practical for large-scale evaluations of numerous lines. The method is practical for evaluating a limited number of lines or for inducing ear galls for commercial production of huitlacoche (smut galls eaten at an immature stage).