Assessment of ear rot symptom development in maize hybrids inoculated with Fusarium graminearum

Ear rot symptom development was evaluated in six maize (Zea mays) hybrids at six developmental stages, after inoculation with Fusarium graminearum. For each ear, the area affected by symptoms (lesion area) was measured with an image analysis system. Lesion width and lesion length were measured manually. Disease severity (percentage of the ear affected by symptoms) was assessed visually and by image analysis. Symptoms were more severe in 1993 than in 1994, perhaps because of warmer temperatures and more frequent rainfall. At the milk stage (10 days after inoculation), all hybrids exhibited small lesions around the inoculation point. Thereafter, the lesions expanded outwards. The rate of this expansion differed among hybrids, and by the early dent stage (20 days after inoculation), there were significant (P < 0.05) differences in lesion area. Lesion area increased more slowly in Pride K127 than in the other hybrids, resulting in a smaller area under the disease progress curve. The differences among hybrids were mostly due to differences in the rate at which the symptoms progressed around the ear than to differences in the rate at which symptoms progressed along the length of the ear. Estimates of disease severity obtained by visual assessment were usually slightly lower than estimates obtained by image analysis. Of the disease assessment methods used hen, image analysis of lesion area was the most precise, but also the most time-consuming and expensive. Visual rating required much less time and no equipment and gave adequate estimates of disease severity.