Effect of Brown Rust on Yield of Sugarcane in Louisiana.

Brown rust, caused by Puccinia melanocephala, can cause severe epidemics in susceptible sugarcane cultivars during spring and early summer in Louisiana. The effect of the disease on yield was evaluated in field experiments conducted during three growing seasons. A mixture of three fungicides-azoxystrobin, propiconazole, and tebuconazole-applied biweekly during the spring epidemic period kept brown rust severity low (<5%), and plants protected by fungicide applications throughout the epidemic provided an estimate of attainable yield for comparison with plants naturally infected with rust. A combined analysis over three seasons estimated brown rust caused reductions of 16 and 14% in cane tonnage and total amount of sucrose produced, respectively, in cv. LCP 85-384. The greatest reduction in total sucrose yield of 22% resulted from the epidemic of longest duration, and stalk weight was negatively correlated with rust severity. Comparisons of the yields obtained from plots in which brown rust was controlled early versus late in the epidemic suggested that the impact of the disease is greatest from the middle to late epidemic period when stem elongation has begun.