Influence of temperature, cultivar, and time on sporulation of Mycosphaerella fragariae on detached strawberry leaves

The influence of temperature (5-35 degrees C), cultivar (Chambly, Kent, Oka, Tribute) and temperature, and time (0-108 h) on sporulation of Mycosphaerella fragariae was studied using a detached leaf assay. Sporulation was estimated as the number of conidia produced per lesion and transformed to relative sporulation. For all cultivars, except Oka, for which temperature was not significant, the highest sporulation occurred at 20 degrees C and no sporulation was observed at 35 degrees C. Relative sporulation varied with cultivar, Tribute being the most susceptible, followed by Kent and by Chambly with only few conidia per lesion. Sporulation started at 5 degrees C, increased with increasing temperature up to 20 degrees C, and then decreased. Relative sporulation was modeled using a second-order polynomial of temperature, and the models explained 85.7, 76.4, and 82.2% of the variation for cvs. Chambly, Kent, and Tribute, respectively. On cv. Tribute, less than 5% sporulation was obtained when leaves were exposed to wetness for 24 h or less. At temperatures of 10-25 degrees C, sporulation increased over time from 24 to 96 h, then stabilized. The Richards' function was used to describe the effect of temperature and time on sporulation (R-2 = 0.90) and to calculate relative risk of sporulation.