| Summary: | Natural epidemics of brown rust, caused by Puccinia melanocephala, affecting sugarcane were studied to determine the crop and/or environmental factors that affect epidemic onset, severity and eventual decline. Environmental and crop growth variables were monitored along with disease severity in two susceptible cultivars, LCP 85-384 and Ho 95-988, each grown at a different location in Louisiana. During two seasons, correlation and multiple regression analyses identified leaf wetness and temperature as important determinants of disease severity for both cultivars. The results suggested that crop growth variables were not determinants for epidemics. Controlled conditions experiments assessing the interaction of leaf wetness and temperature demonstrated that changes in one variable will influence the effect of the other and identified minimum and maximum values required for infection. Increasing leaf wetness duration from 7 to 10 or 13 hours resulted in greater infection at an optimal temperature range of 17 to 27 C. Minimum requirements for leaf wetness and temperature were 7 hours and 17 C. Minimal infection occurred at 29 and 31 C. Severe epidemics in both cultivars began to decline once maximum ambient daily temperature was 32 C or higher. Lower disease severity during the 2010 epidemic in Ho 95-988 allowed an analysis of the effects of conducive and limiting conditions on brown rust severity. Lower severity resulted from a combination of unfavorable temperature and leaf wetness conditions that delayed onset then reduced the rate of disease increase. An accumulation of 23-25 leaf wetness conducive days after the daily minimum temperature exceeded 17 C preceded the onset of disease on young leaves in all three epidemics suggesting cumulative leaf wetness days might provide an epidemic predictor. The study results suggest that the occurrence of limiting temperatures determines the initiation and decline of a brown rust epidemic under Louisiana climatic conditions. The availability of leaf wetness is then the most important determinant of disease severity during the epidemic period. The study results suggest that temperature and leaf wetness can provide the basis for a disease advisory or forecasting system that predicts the threat of a severe epidemic and improves recommendations for fungicide use.
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