Efficacy of fungicide treatments for control of common rust and northern leaf spot in hybrid corn seed production

Field experiments were conducted during the 1992 and 1993 growing seasons in 13 commercial fields in Iowa to assess the efficacy of foliar applications of fungicides to seed corn for control of common rust, Puccinia sorghi, and northern leaf spot, Bipolaris zeicola. Chlorothalonil, mancozeb, and/or propiconazole sprays were initiated at growth stage (GS) 2.5, GS 4.0, or approximately 2 weeks before detasseling, and applications were repeated up to five times at various growth stages. The majority of fungicide treatments resulted in an increase in salable seed units (1 unit = 80,000 seeds) compared to untreated control treatments. The greatest increase in salable seed occurred in the large and medium seed size units. Although disease severity on the final assessment date in both years and standardized area under the disease progress curve in 1993 were significantly higher (P ≤ 0.05) in untreated control plots than in fungicide-treated plots in all experiments, the best disease control was provided by 5 applications of chlorothalonil or 2 to 4 applications of chlorothalonil, mancozeb, or propiconazole initiated on the first spray date.