Maple Leaf Extracts to Control Angular Leaf Spot of Cucurbits Caused by Pseudomonas syringae

Angular leaf spot (ALS) caused by Pseudomonas syringae is an important seedborne bacterial disease in cucurbits. Currently, growers have very few alternatives to copper‐based pesticides for the control of the disease in both conventional and organic growing systems. Recent work performed by our group revealed that sugar maple and silver maple leaf extracts show potential as alternatives to the use of copper‐based pesticides for the control of plant‐pathogenic bacteria. This study investigated the antibacterial and prophylactic activity of ethanolic sugar maple/silver maple autumn‐shed leaf extracts against P. syringae . Silver maple and sugar maple leaf extracts showed antibacterial activity under in vitro conditions against P. syringae with minimum inhibitory concentration (MIC) values of 0.39 and 0.78 mg/mL, respectively. The extracts were also very effective as seed treatments at concentrations of 50 or 100 mg/mL. Foliar applications of sugar maple leaf extracts at concentrations of 14.1 mg/mL (corresponding to phytotoxic dose 5% [PD5] value) and 28.2 mg/mL (corresponding to 2 × PD5 value) were either as effective (PD5) or more effective (2 × PD5) than copper octanoate to control ALS on cucumber plants. Silver maple leaf extracts used at a concentration corresponding to PD5 value were as effective as copper octanoate to control ALS on cucumber. Foliar applications of the extracts also showed efficacy in reducing the severity of ALS on butternut squash plants. This study opens new avenues for the management of ALS in cucurbits using silver maple and sugar maple leaf extracts as alternatives to the use of copper‐based pesticides.