Ohio ranks 9th among leading U.S. corn-producing states in total grain corn acreage. The corn crop adds an estimated $3 billion to Ohio’s economy each year. Since the 1980s and 1990s when gray leaf spot, caused by Cercospora zeae-maydis, provoked widespread yield and economic losses in Ohio, foliar diseases have only been a concern in localized areas of the state. In most years, producers effectively manage foliar diseases by practicing some form of crop residue management, rotating corn with soybean, and planting resistant hybrids. However, without a major corn disease epidemic in more than a decade, the focus has shifted away from production practices geared to minimized disease-related losses. Since 2006, growers across the Corn Belt have been investing heavily in fungicide application. This is due in part to recent corn prices and the potential for greater disease development in a continuous-corn conservation-tillage cropping system, but most importantly, to industry claims of substantial yield increases due to the use of foliar fungicide, particularly the Quinone Outside Inhibitors (QoI). In Ohio, more than 650,000 hectares acres of corn were sprayed with fungicide during 2007. At an estimated application cost of $57 per hectare, this represented a total expenditure on fungicide of $37 M. Based on grain prices in that year, it would have required an estimated 375-501 kg/ha increase in yield to offset the cost of fungicide application. Results from university-based replicated trials suggest that the effect of fungicide application on yield is less clear-cut than industry data seem to suggest. In some university trials, positive yield responses were associated with disease control, in others there were no yield benefits even when diseases were controlled, yet in other trials positive yield responses were observed in the absence of high disease pressure. Since production practices have changed substantially since the available disease management recommendations were developed, there is a need to revisit these recommendations and develop sustainable management guidelines suitable for current production systems. Hence the objectives of the research presented in this thesis were to: investigate the effects of hybrid gray leaf spot resistance and yield potential, and fungicide application (product and timing) on the development of gray leaf spot and grain yield and yield components in a minimum-till/continuous-corn and a corn-soybean rotation cropping systems.
A total of five field trials were conducted in Ohio in 2009 and 2010. In two of those trials, the effects of hybrid characteristics and application timing of a QoI and a DMI (Demethylation Inhibitor) fungicide on GLS severity, grain yield and yield components were evaluated. Four hybrids with different yield potential, maturity, and GLS resistance profiles were used. Plots were treated with either Domark (20.5% tetraconazole; a DMI), Headline (23.6% pyraclostrobin, a QoI), or left untreated. Applications were either made at the silking (R1), blister (R2) or milk (R3) growth stage, with each product being applied at label-recommended rate, along with a nonionic surfactant. In the remaining three trials, the effects of cropping sequence (crop rotation) and fungicide application timing were evaluated. Plots were established in sections of fields previously planted with corn or soybean, and a QoI/DMI combination fungicide, Stratego (10.8% prothioconazole + 32.3% trifloxystrobin) or Stratego YLD (11.4% propiconazole + 11.4% trifloxystrobin), was applied at label-recommended rates at growth stage R1, R2, or R3, along with a nonionic surfactant. Untreated checks were used as references. In all five trials, a randomized complete block design was used, with a split-plot arrangement of hybrid or previous crop (whole plot) and fungicide application timing (sub-plot). Plots in the first two trials were inoculated with C. zeae-maydis, while those in the last three were not. GLS severity was assessed between silking and maturity in all plots. Grain yield was estimated and yield components determined by weighing five ears and then counting the number of kernel rows and the number of kernels in two arbitrarily selected rows on each of the five ears. Linear mixed models were used to analyze the data from all trials. GLS severity varied among trials, with only nominal levels of disease developing in one of the uninoculated trials. In general, fungicide treatments had a significant effect on GLS severity, with applications made a R1 resulting in the best control of GLS relative to the check. The R3 applications were not significantly different from the check. Fungicide effects on GLS, however, did not translate into an effect on grain yield, since the effects of fungicide treatment on yield was not statistically significant. However, hybrid, previous crop, and the interaction between previous crop and fungicide treatment had significant effects on yield in at least one trial. One of the late-maturing hybrids with high yield potential out-yielded all other hybrids in one of the trials. Plots planted following soybean yielding significantly more than plots planted following corn in one of the cropping sequence x fungicide treatment trials, whereas in another, the response depended on the fungicide treatment. For plots planted into corn residue, only the R2 treatment resulted in significantly higher yield than the check, whereas for plots planted after soybean, the R1 and R3 treatments yielded significantly more than the check. Yield components varied substantially among trials, hybrids, and fungicide treatments, with the hybrid x fungicide interaction being statistically significant for all yield components in at least one of the two trials in which these factors were treated. Cropping sequence did not affect yield components. In general, hybrid maturity and yield potential had a greater influence on yield components than fungicide application (product or timing). However, neither the main effects of hybrid nor its interaction with fungicide timing were consistent between trials or among yield components. For a given yield component, the most effective treatment or treatment combination in one trial was not necessarily the most effective in another trial. Moreover, the most effective treatment or combination for one yield components within or between trials was not necessarily as effective for another yield component. The results from this investigation clearly emphasize the complexity of yield response to fungicides and serve to reiterate the fact that such a response is highly variable, unreliable, and as such, may not always be profitable.