Soybean Yield Response to Benomyl Fungicide Application under Maximum Yield Conditions

In 1980, after three consecutive years with soybean [Glycine max. (L.) Merr.] yields in excess of 5000 kg ha⁻¹ in surface-irrigated, maximum yield, research plots, yields dropped to 3000 kg ha⁻¹ due to severe development of Septoria brown spot (Septoria glycines Hemmi), which caused premature senescence and defoliation of the leaves. The purpose of this research was to determine the effect of benomyl (methyl-l-(butylcarbamoyl)-2-benzimidazole carbamate] control of natural infection of foliar diseases, primarily Septoria brown spot, on the yield of irrigated, high-yield soybean. Benomyl was applied at a rate of 2.2 kg ha⁻¹ at 2-wk intervals beginning in mid July (late flowering stage, R2-R3) with the last application in early September (late pod fill stage, R6). Two determinate semidwarf cultivars, Hobbit and Sprite, and two indeterminate cultivars, Pella and Williams 82, were grown in two row spacings, (75 and 17 cm) and compared with and without benomyl treatment for 3 yr. Averaged across the 3 yr, the determinate cultivar, Hobbit, and the indeterminate cultivar, Pella, were the most yield responsive to benomyl treatment (15.4 and 7.7% in 75-cm rows and 13.8 and 15.5% in 17-cm rows, respectively). The determinate cultivar, Sprite, and the indeterminate cultivar, Williams 82, failed to show a significant yield increase from benomyl treatment. These results indicate that yield reductions due to Septoria brown spot vary by genotype, but are not associated specifically with plant type (determine vs. indeterminate). Yield increases from benomyl treatment tended to be greater in 17-cm row width than in 75-cm row width. This research indicates that control of foliar diseases, such as Septoria brown spot, can significantly increase the yield potential of high yielding soybean under conditions of frequent wetting of the foliage, either by frequent rainfall or irrigation, and should be an important component of a maximum-yield research or production system. A joint contribution from USDA-ARS and The Ohio State Univ., Ohio Agric. Res. & Develop. Ctr. Research support was provided by State and Federal Funds appropriated to the Ohio Agric. Res. and Develop. Ctr., The Ohio State Univ. Journal Article No. 241-88.