SEASONAL-CHANGES OF SENSITIVITIES TO STEROL DEMETHYLATION INHIBITORS IN VENTURIA-INAEQUALIS POPULATIONS

During three seasons and at three different orchard sites, the sensiti vities of V. inaequalis populations to sterol demethylation inhibitors were reduced over the course of the summer and in the absence of expo sure to these fungicides. A seasonal fluctuation was not observed for dodine with a mode of action unrelated to inhibition of sterol demethy lation. Seasonal fluctuations of population sensitivities to sterol de methylation inhibitors are of theoretical and practical importance. Th ey not only reflect a rarely reported population response to changing environmental conditions over a growing season, but also have implicat ions regarding the development of anti-resistance strategies for the c ontrol of apple scab with sterol demethylation inhibitors. As the freq uency of the least sensitive population segment increases in late summ er, the performance of these fungicides is most likely decreased. The resulting effect on disease incidence and selection of a resistant sub population can be expected to accelerate the speed of resistance devel opment.