AbstractFour grain sorghum (Sorghum bicolor (L.) Moench.) lines showing distinct reactions to sorghum rust (Puccina purpurea Cooke) were planted under subhumid tropical oceanic conditions in northwestern Puerto Rico in an oxisol of pH 5.5 in May 1985. Disease development was periodically monitored until harvest. Rust levels were established using either no application or two applications each of either oxycarboxin, triadimefon or mancozeb fungicide. Noticeable rust was found at boot stage and increased with maturity. Rust differences were most apparent at physiological maturity. At that stage foliar rust coverage was estimated at 20, 16, 1 and 0% for SC 212, SC 307, TAM 428, and SC 120, respectively. Greatest rust control was found for oxycarboxin, triadimefon, and mancozeb in that order of efficacy. Oxycarboxin treatment resulted in over 80 and 60% less rust; 25 and 40% more grain yield; 0 and 20% greater seed density; and 80 and 71% more foliar sorghum anthracnose in SC 307 and SC 212, respectively, in comparison with those of nontreated plots. Fungicide treatments did not increase yields of either TAM 428 or SC 120, sorghum cultivare showing moderately resistant and very resistant rust reactions, respectively.
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