Mode of action of the anilino‐pyrimidine fungicide pyrimethanil. 1. In‐vivo activity against Botrytis fabae on broad bean (Vicia faba) leaves

The effect of timed protectant applications of pyrimethanil on the natural infection process of isolates of Botrytis fabae Sardina on broad bean leaves was investigated using a range of light and electron microscopic techniques. At 400 mg litre −1 , there was some inhibition of spore germination, and a significant reduction in germ tube extension. These responses were variable between isolates, treatment times and dose rates. Appressorium formation and function (i.e. penetration of the host epidermis) were not affected. However, there was a significant reduction in the numbers of host cells killed at each penetration site, resulting in retarded, or, with some isolates, arrested lesion development. This response was similar for all isolates tested. The likely mode of action of pyrimethanil as a pathogenesis inhibitor is discussed.