Effect of environment on efficacy of seed-treatment fungicides for control of common bunt in spring and winter wheat

Abstract

The effectiveness of protectant and systemic seed-treatment fungicides for control of common bunt caused by Tilletia caries (DC.) Tul. and T. foetida (Wallr.) Liro. in spring and winter wheats was evaluated at different locations from 1981 to 1987. The fungicides, tested at two rates (I and II), were of three types: (1) fungicides which were effective in all years and locations at levels approaching 100%; (2) fungicides which provided uniform bunt control in all years and locations at levels less than 100%, and (3) fungicides which were sensitive to environmental conditions within years and locations. Fungicides which were effective in all years and locations at levels approaching 100% were triadimenol II, myclobutanil I and II, diniconazole I and II, and flutriafol II, on spring wheat, and etaconazole and furmecyclox in the seed-(SD), and seed- and soil- (SS) inoculation treatments, and triadimenol in the SD-inoculation treatment on winter wheat. Fungicides which provided uniform bunt control in all years and locations at levels less than 100% included flutriafol I and triadimenol I in spring wheat, applied at rates below optimal, and carboxin dust and nuarimol on winter wheat, in SS- and SD-inoculation treatments. Fungicides which were sensitive to environmental conditions within years and location included maneb, carboxin I and II on spring wheat, and carboxin solution and dust in the SD- and SS-inoculation treatments on winter wheat. Comparisons of bunt levels in fungicide treatments with those in the infested controls revealed that some fungicides did not perform consistently across years and locations. Variation in efficacy of fungicides was classified into three types: (1) random environmental variation which occurred with all fungicide treatments tested; (2) non-random environmental variation which occurred with maneb and carboxin fungicides, and (3) variation attributed to increasing bunt severity which occurred with the SS and SD carboxin fungicides used to control bunt in winter wheat. The implications of inadequate protection from currently registered seed-treatment fungicides on wheat varieties that are highly susceptible to bunt are discussed.