EAR ROT DEVELOPMENT AND MYCOTOXIN PRODUCTION IN CORN IN RELATION TO INOCULATION METHOD, CORN HYBRID, AND SPECIES OF FUSARIUM

We determined the effects of inoculation method, plant growth stage at inoculation, and species of Fusarium on severity of com ear rot, colonization of kernels by Fusarium, and contamination of kernels by deoxynivalenol (DON), zearalenone, and fumonisin. Conidia of Fusarium graminearum, F. moniliforme, or F. subglutinans were placed in the silk channel or on the wounded surface of cobs of two hybrids of corn, one resistant (P3790) and one susceptible (P3737) to F. graminearum. The plants were inoculated 1, 3, or 6 weeks after silking, irrigated by overhead misting, and assayed at harvest for disease, infection, and toxin concentrations. Ear rot tended to be most severe in ears inoculated with F. graminearum and least severe in ears inoculated with F. moniliforme. Hybrid P3790 generally developed less ear rot than P3737 following inoculation with each of the species tested. Among 12 comparisons obtained over two years of testing and three times of inoculation each year, significant differences between the hybrids in ear rot severity were detected seven times for F. graminearum and five times for F. moniliforme and F. subglutinans. Disease severity was significantly increased by wounding in both hybrids. In 18 comparisons, wounding revealed the difference between hybrids 10 times and inoculation through the silk channel showed hybrid differences seven times. Ear rot severity following wounding was little affected by time of inoculation but, after inoculation through the silk channel, tended to decline as the time of inoculation after silking was delayed. Silk channel and wound inoculations 1 wk, and 1 to 3 wk, after silking, respectively, are both recommended for evaluating resistance of com to ear rot. DON was found in all samples in both years. Zearalenone was detected in most ears in 1990 but only in ears inoculated with F. graminearum or F. moniliforme one week after silking in 1989. Generally, inoculations near silking with F. graminearum that resulted in greater ear rot severity also led to higher levels of DON. Severity of ear rot may be a useful indicator of contamination of ears by mycotoxins. Fumonisin, a mycotoxin produced by F. moniliforme, is reported from inoculated corn in Ontario for the first time. Fumonisin and F. moniliforme occurred in all samples tested, regardless of the species of Fusarium used as inoculum.