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Seed-borne Fungi, Quality, and Yield of Soybeans Treated with Benomyl Fungicide by Various Application Methods¹

Soybeans [Glycine max (L.) Merr.] are often damaged by the seedborne fungi Phomopsis sp., Diaporthe phaseolorum (Cke. and Ell.) Sacc. var. sojae (Lehman) Wehm. and D. phaseolorum var. caulivora Athow and Caldwell which inhibit germination and sometimes cause moldy seed. These fungi can be controlled with benomyl (methyl 1-[(butylamino)carbonyl]-1H-benzimidazol-2-ylcarbamate) fungicide when applied at the R₄-R₅ stage in relatively large, illegal, quantities, >1.1 kg/ha (a.i.), and in large volumes of spray solution, >230 liters/ha. The purpose of this research was to determine if Phomopsis and Diaporthe seed contamination and seed rot can be effectively controlled with legal rates of benomyl applied by aircraft or ground equipment at reduced volumes.

Aircraft application of benomyl at 0.56 kg/ha resulted in no overall improvement in seed germination, and on only 4 of 14 farms were there significant reductions in Phomopsis and Diaporthe seed contamination following benomyl application, but these were not large enough to be economical.

Ground application was with a backpack mistblower, a tractor-mounted Myers Downdraft sprayer, a high-clearance powered unit using hydraulic broadcast application with flat-fan nozzles, and the high-clearance unit with a special boom using a directed hydraulic spray.

Phomopsis + Diaporthe seed contamination in the controls ranged from 34 to 94% and germination, 10 to 71%. Benomyl decreased Phomopsis + Diaporthe seed contamination 20 to 67% and increased germination 16 to 61%. Incidence of these fungi in seed decreased and germination percentage increased with increasing spray volume where hydraulic flat-fan nozzles were used, but not with the other methods. With equal volumes (230 liters/ha) the mistblower and downdraft sprayer were superior to the hydraulic flat-fan broadcast method, and equal to a directed hydraulic spray for controlling Phomopsis and Diaporthe. Improvement in germination was sufficient to warrant application of benomyl with ground equipment.

Key Words: Glycine max (L.) Merr. • Diaporthe phaseolorum var. sojae. D. phaseolorum var. caulivora • Germination • Phomopsis sp. • Alternaria sp.

² Associate professor of agronomy, professor of plant pathology, Ohio Agric. Res. and Dev. Ctr., and Agricultural Engineer, USDA-ARS, Wooster, OH 44691.

Received for publication April 11, 1980.