Control of cucumber powdery mildew caused by Sphaerotheca fuliginea by potassium phosphite

Powdery mildew is a major damaging disease of cucurbits worldwide. Phosphite derived fungicides are environmentally safe chemicals possessing direct fungicidal effect and indirect activation of plant defense responses. The preventative and curative effects of potassium phosphite (KPhi) on cucumber plants infected by Sphaerotheca fuliginea, was investigated. Cotyledons and true leaves of greenhouse grown cucumber plants were inoculated with S. fuliginea in separate experiments and treated with KPhi solution at different times before and during pathogen inoculation. The results showed that KPhi foliar spray significantly prevented the incidence of the disease symptoms (P ≤ 0.01). Foliar 20mM KPhi applications 2 days before as well as 2 and 4 days post-inoculation significantly prevented the incidence of the disease compared to the control. Particularly, foliar application of KPhi after S. fuliginea inoculation dramatically decreased the mean number of colonies per either cotyledon or true leaf. Also, KPhi application after pathogen symptom appearance considerably prevented the further development of the pathogen colonies. The findings of the present investigation would be implemented in integrated disease control programs in cucurbits.