RELATIONSHIPS BETWEEN TEMPERATURE AND LATENT PERIODS OF RUST AND LEAF-SPOT DISEASES OF GROUNDNUT

The effect of temperature on the latent periods of rust, late leaf spo t and early leaf spot diseases of groundnut caused by Puccinia arachid is, Phaeoisariopsis personata and Cercospora arachidicola, respectivel y, was studied. The latent periods (LP) of rust, late leaf spot and ea rly leaf spot ranged from 12-49 days, 13-38 days and 13-39 days, respe ctively, between 12 degrees C and 33 degrees C. An equation relating t he rate of pathogen development (1/LP) to temperature was fitted using daily mean temperatures to provide three cardinal temperatures: the m inimum (T-min), optimum (T-opt), and maximum(T-max). T-min was about 1 2 degrees C for rust and about 10 degrees C for the two leaf-spot dise ases. T-opt for all three diseases was close to 25 degrees C. T-max wa s 31 degrees C for early leaf spot, and extrapolated values for late l eaf spot and rust were about 35 and 40 degrees C, respectively. For P. personata, a temperature response curve was fitted using data only fr om controlled environment experiments. This curve was used to simulate latent periods from both mean daily and mean hourly temperatures in t he field. There was substantially better agreement between observed an d simulated latent period with hourly temperatures, provided the devel opmental rate of the pathogen was determined at a constant temperature .