Using controlled enviroment studies the influence of temperature (15 °C - 32.5 °C) and the duration of leaf wetness (2 h - 24 h) were quantified for the development of Alternaria leaf spot, caused by Alternaria helianthi in sunflower (Helianthus annuus). The effect of temperature on the mycelial growth rate and conidia germination was also quantified by in vitro trials. Relative lesion density and severity were influenced by temperature and leaf wetness duration. The disease was more severe at a temperature of 25 °C. The minimum temperature for disease development, estimated by generalized beta function, was 13.0 °C, and the maximum was 35.8 °C. Relative lesion density increased with increasing periods of leaf wetness, as described by a logistic model. The response surface obtained by the combination of generalized beta and logistic functions provided a close fit to observed data (R²=0.87 for relative lesion density and 0.92 for severity). The generalized beta function also described fungus growth and conidium germination as a function of temperature. The estimated minimum temperatures for mycelial growth rate and for conidium germination were 5.5 °C and 7.9 °C, respectively, while the maximum temperatures were 32.9 °C and 40.0 °C, respectively. Ten sunflower hybrids artificially inoculated, under controlled conditions, showed differing degrees of resistance to infection and colonization by A. helianthi.
