EFFECT OF SILK AGE ON RESISTANCE OF MAIZE TO FUSARIUM-GRAMINEARUM

The effect of silk age on the resistance of maize to ear rot caused by Fusarium graminearum was investigated. Resistant and susceptible maize hybrids (Pride K127 and Pioneer 3953) and inbreds (CO272 and CO266) were inoculated at intervals from 0 to 18 days after silk emergence by injecting a macroconidial suspension into the silk channel. Five inbreds (CO272, CO266, F7, F2, and A641) were inoculated at three stages of silk development using two techniques: injection into the silk channel and spraying onto exposed silks. At harvest, the severity of ear rot symptoms was assessed on a seven-class rating scale. In an additional experiment, silk tissue sampled from within the silk channel and from the exposed silk of the inbreds at three stages of silk development was examined in vitro for resistance to degradation by F. graminearum. Susceptibility increased immediately after silk emergence, then declined. The best differentiation among genotypes was obtained when inoculation involved injection into the silk channel at an intermediate stage of silk development. When maize ears were inoculated later than 7 days after silk emergence, it was not possible to detect differential resistance of genotypes. Fungal degradation of detached silk tissue also decreased as silk aged. Decreases in silk channel length and in water content of exposed silk as silk ages may partially account for changes in resistance observed over time within individual inbreds. The results of these experiments indicate that when silk inoculation is used to screen for resistance to F. graminearum, all plants should be inoculated at the same stage of development, preferably at the stage when the silk has elongated, has been pollinated, and may have some brown tips.