Thermal thresholds for the germination of conidia of Exserohilum turcicum

ABSTRACT Corn (Zea mays L.) is cultivated throughout Brazil and presents great economic importance. Among the diseases that cause reduction in its productivity is helmintosporiosis. The objective of this study was to evaluate the conidial germination of the pathogen that causes helmintosporiosis, Exserohilum turcicum, in different temperatures and under different light regimes, to determine lower and upper temperature thresholds and optimum temperature for germination. The temperatures evaluated were 0, 5, 10, 15, 20, 25, 30, 35, and 40 °C, with exposure times of 3, 6, 9, and 12 h. After each exposure time, the number of germinated conidia and germinative tube length were determined. The experimental design was completely randomized with four replications and the data were subjected to analysis of variance and regression. For conidia of E. turcicum subjected to continuous light, it was found that most conidia germination occurred at the temperature of 23.3 °C for 3-h exposure; at 22.3 °C for 6 h; at 21.8 °C for 9 h; and at 21.7 °C for 12 h. For continuous darkness, the higher germination was observed in the temperature 24.0 °C for an exposure time of 3 h; at 22.5 °C for 6 h, at 21.8 °C for 9 h; in 21 7 °C for 12 h. Exserohilum turcicum spores germinated in a temperature range of 0 to 40 °C, the optimum temperature being 23 °C. The dark regime favored the germination of E. turcicum spores.