Effect of Mid‐Season Foliar Fungicide and Foliar Insecticide Applied Alone and In‐Combination on Soybean Yield

Abstract

Core Ideas Soybeans treated with fungicide + insecticide tank‐mixed yielded the same as fungicide only in the absence of insects and defoliation at the economic threshold. Fungicide increased yield at 4 of 12 site‐years, where leaf area affected by diseases tended to be > 6.5%. There was no yield response with insecticide due to low defoliation at the time of application. Due to the relatively low cost of foliar insecticide, farmers frequently tank‐mix insecticide with fungicide. The objective of this study was to evaluate the effect of mid‐season foliar fungicide and insecticide treatments on soybean grain yield, applied alone or in combination. A field trial was conducted in Ohio from 2015 to 2016 at six sites each year (12 sites total). Foliar treatments applied at the R3 soybean growth stage (beginning pod) included: 1) non‐treated control, 2) fungicide, 3) insecticide, 4) fungicide + insecticide, 5) fungicide + insecticide (applied separately to examine for synergistic effect), 6) fungicide + crop oil, 7) insecticide + crop oil, and 8) fungicide + insecticide + crop oil. Data were collected for percent leaf area affected by diseases (frogeye leaf spot caused by Cercospora sojina and brown spot caused by Septoria glycines) and insect defoliation at the R6 soybean growth stage (full seed). Soybean treated with fungicide + insecticide tank‐mixed did not yield differently than soybean treated with fungicide and fungicide + crop oil. Fungicide treatments increased soybean yield at 4 of 12 site‐years compared to the non‐treated control, and at three of these locations, brown spot and frogeye leaf spot disease in the non‐treated control accounted for ≥ 6.5% leaf area affected. There was no yield response associated with insecticide treatment due to sub‐threshold insect defoliation levels at the time of insecticide treatment. Application of fungicide alone, insecticide alone, or in tank‐mix did not improve yields when insects or foliar diseases were not present at threshold levels.