Pollination can protect maize ovaries from infection by Ustilago maydis, the corn smut fungus

Ustilago maydis DC (Corda), the maize smut fungus, causes disease on maize (Zea mays L.) and related species. To determine whether pollination of maize ears affects their susceptibility to U. maydis infection, ears were treated in one of four ways: pollination only, inoculation with compatible haploid U. maydis cells only, pollination followed by inoculation 4 days later, or inoculation followed by pollination 4 days later. Combining a standard method of pollination with the silk channel method of inoculation resulted in reproducible, high levels of pollination and infection in controls. Seventy-seven percent of the kernels on ears pollinated only were fertilized, and 75% of the kernels on ears inoculated only were smutted. Ears pollinated 4 days before inoculation developed only 20% smutted kernels on average, with nearly all tumors forming at the tip of the ear where pollination was probably ineffective. Ears that were inoculated 4 days before pollination were 73% smutted, with only 8% average successful fertilization. Microscopic examination of silks after pollination and inoculation treatments indicated that an abscission zone formed at the bases of pollinated silks and may have prevented fungal infection filaments from growing into the ovaries. These results indicated that pollination rendered ovaries more resistant to infection by U. maydis.