Effects of temperature and wetness duration on infection of peanut cultivars by Cercospora arachidicola

The effects of temperature and duration of wetness (relative humidity great er than or equal to 95%) on infection of three peanut cultivars by Cercospo ra arachidicola were determined under controlled conditions. Plants of the Spanish cv. Spanco and the runner cvs. Florunner and Okrun were exposed to constant temperatures of 18 to 30 degrees C during 12-h periods of wetness each day that totaled 12 to 84 h following inoculation of leaves with conid ia. Severity of disease, measured by either lesion density (number per leaf ) or lesion size (diameter), was greatest for 'Spanco', intermediate for 'F lo-runner', and lowest for 'Okrun' in each of two experiments. Lesion densi ty was evaluated further because it was an indicator of both the occurrence and degree of infection. Nonlinear regression analysis was employed to eva luate the combined effects of temperature (T) and wetness duration (W) on l esion density (Y). In the regression model, the Weibull function characteri zed the monotonic increase of Y with respect to W, while a hyperbolic funct ion characterized the unimodal response of Y with respect to T. Parameters for the intrinsic rate of change with respect to W (b), the intrinsic rate of change with respect to T (f), the optimal value of T (g), and the upper limit (e) when T is optimum (T = g) were estimated for each cultivar and ex periment. The effect of cultivar was characterized primarily by differences in the upper limit parameter e. In each experiment, e was greatest for 'Sp anco', intermediate for 'Florunner', and least for 'Okrun'. The effect of c ultivar on b followed a pattern similar to that for e in experiment 1, but not in experiment 2. Differences among cultivars for estimates off and g we re small and inconsistent. Estimates for g were precise for each cultivar a nd experiment and fell within the range of 22.3 to 23.2 degrees C. Cultivar responses to T and W were further evaluated using data pooled over the two experiments. Parameter e was estimated for each cultivar, but common value s of b, f, and g were estimated. At e = 22.8 degrees C, lesion density appr oached an upper limit of 96, 17, and 6 lesions per leaf for the cvs. Spanco , Florunner, and Okrun, respectively. These fitted values approximated the observed values of 86, 25, and 9 lesions per leaf for the respective cultiv ars. Cultivars varied in their response to W at a given T. At 22.8 degrees C, one lesion per leaf was expected following 26, 30, and 36 h of wetness f or 'Spanco', 'Florunner', and 'Okrun', respectively. If temperature was inc reased to 28 degrees C, one lesion per leaf was expected following 36, 44, and 54 h of wetness for the respective cultivars.