Fusarium ear rot, caused by Fusarium verticillioides, is one of the most common worldwide diseases of maize, causing yield and quality reductions as well as contamination of grain by fumonisins and other mycotoxins. Drought stress and various insects have been implicated as factors affecting disease severity. Three separate field studies were conducted during 2002 and 2003. The first field study evaluated the relative influence of (1) drought stress at different stages of crop development, and (2) ear infestation by two species of insects, upon severity of Fusarium ear rot disease and fumonisin B1 contamination of commercial maize hybrids. In each year, plots of three commercial hybrids varying in partial resistance to Fusarium ear rot were sown on three planting dates and subjected to four irrigation regimes that resulted in differing levels of drought stress in the plants. A foliar-spray insecticide treatment (sprayed vs. unsprayed) also was imposed. Populations of thrips (Frankliniella spp.) and damage by corn earworm (Helicoverpa zeae) in open-pollinated hybrid ears were assessed following pollination for all treatment combinations. The results of this study agree with past results implicating thrips as a primary factor of Fusarium ear rot severity in California, and associate thrips with potentially high levels of fumonisin B1 in grain. Thrips appear to be a stronger influence on disease severity and fumonisin B1 contamination than earworm in this environment, and the historically higher disease severity in late plantings may be explained, at least in part, by higher thrips populations. Drought stress treatments did not influence ear rot severity in the presence of large populations of thrips and corresponding high disease severity and mycotoxin levels.
In the second study, plots were sown with a maize hybrid known to be susceptible to Fusarium ear rot. Half of the plots in each planting were treated with insecticides three times following pollination, while the remaining plots were not treated. Populations of intra-ear immature and adult thrips were subsequently evaluated at six ear developmental stages for the number of immature and adult thrips within developing ears. The results suggest that thrips are not simply occasional feeders on developing maize ears, but can complete a substantial portion of their life cycle on maize ears. The results also indicate that thrips oviposition and/or feeding may be implicated as causal factors of the silk-cut symptom that is common in many maize-growing areas, and this may be the mechanism that connects thrips activity with Fusarium ear rot and fumonisin contamination of grain.
The third study evaluated the associations of maize production regions, planting date, hybrid varieties, and ear infesting insect pests with Fusarium kernel rot symptoms, and fumonisin B1 contamination. In each season, plots of susceptible and resistant commercial maize hybrids were sown on 3 planting dates. Insects within open-pollinated hybrid ears at and following pollination were quantified for all genotype x planting date combinations at each location during each growing season. Grain subsamples were inspected to determine percentage of kernels that were either visibly molded or showed the starburst symptom of Fusarium ear rot. Grain samples were also analyzed by ELISA to quantify fumonisin B1 levels. Across locations, hybrid and planting date were both frequently significant effects (p ≤ 0.05). Insect populations varied across locations, and observed kernel damage was most attributable feeding intra-ear thrips infestations. Maximum average intra-ear thrips populations were 161.8 (hybrid A, 2003), and 99.8 (hybrid B, 2002), in California and Hawaii. Across the pooled dataset, intra-ear thrips population was more strongly correlated (R = 0.78) than percentage lepidopteran kernel damage (R = 0.37), with percent visibly molded kernels. A similar trend was observed for the relationship between these two types of insects and the starburst symptom of Fusarium kernel rot. The percentage of visibly molded kernels was highly correlated (R = 0.89) with fumonisin B1 concentration (Table 4). Though significant, the correlation between starburst kernels and fumonisin B1 (R = 0.18) was substantially weaker. Intra-ear thrips populations were more strongly correlated with fumonisin B1 concentration (R = 0.89), compared to the correlation between percentage of lepidopteran kernel damage and fumonisin B1 (R = 0.34). A multiple linear regression model accounted for 84% (P < 0.001) of the variation in fumonisin B1. Given the strong association between thrips, Fusarium ear rot, and fumonisin B1, producers in the global F. occidentalis host range should consider thrips as a factor in Fusarium ear rot and fumonisin B1 management. (Abstract shortened by UMI.)