Effects of Milsana and benzothiadiazole on the ultrastructure of powdery mildew haustoria on cucumber

Disease assessments and cytological investigations provided valuable inform ation on the modes of action and efficacies of two prophylactic compounds, Milsana and benzothiadiazole (BTH), against powdery mildew development on l ong English cucumber. Milsana application significantly reduced disease inc idence relative to inoculated controls through induction of localized resis tance. Microscopic observations showed most haustoria had collapsed in the localized Milsana treatment and were encapsulated by an amorphous material impregnated by electron-opaque substances. The rapidity of haustorial colla pse (within 4 days of treatment application) together with the encasement b y electron-dense substances stained blue by toluidine blue O suggest that p henolics are possibly involved in the Milsana defense response. Cytochemica l labeling of chitin with a wheat germ agglutinin/ovomucoid-gold complex sh owed that complete cellular disorganization of the fungus had occurred with out disturbance to chitin in the walls of mycelia and haustoria. This may i ndicate that chitinolytic activity is not important in the Milsana-activate d defense response. Application of high doses of BTH induced occasional cel l wall thickening and accumulation of a compound that stained purple by tol uidine blue O, but the defense response was weak, sporadic, and insufficien t to reduce powdery mildew infection on cucumber. Responses to BTH could no t be differentiated in terms of timing of the initial application or system icity.