Cucurbit angular leaf spot is a bacterial disease that can be economically damaging to leaves, stems, and fruit of cucumbers, honeydew melon, and zucchini squash growers in Wisconsin. This disease looks somewhat like downy mildew, but is caused by a bacterial pathogen Pseudomonas syringae pv. lachrimans. Initial lesions appear as small, tan, water-soaked spots that are delineated by the veins of the leaf giving the ’angled’ or straight edge appearance of lesions on the leaves. As the lesions expand and develop, they typically turn white and papery in texture until the dead tissue falls out of the leaf, leaving a hole and often a tattered appearance to the leaf. Under humid conditions, a milky substance (bacterial ooze) may be notes on the infected foliage. This ooze dries to a white crust around the lesions. While fruit may appear healthy during leaf infection, the infection may continue to develop later in the season or post harvest as the lesions develop into soft areas of fruit under the cuticle or skin.
The pathogen may be seedborne or debris-borne (for up to 2 years). To manage this disease, reduce the likelihood of infection by rotating out of cucurbit crops for at least 2 years and use disease free seed. In greenhouse cucumbers, lowering the humidity to 80-90% will help reduce the spread of the disease. Hot water and chemical seed treatments are not effective for totally eliminating the pathogen on infested seed (pathogen is beneath the seed coat). Breeding efforts have resulted in a large selection of cucurbits with tolerance to angular leaf spot. In some cultivars, tolerance to the disease is most strongly expressed in the fruit while significant infection can still occur in the foliage during cool, wet weather.
Prolonged leaf wetness favors infection. The bacterium enters the leaf through respiration pores (stomates), water excreting pores (hydathodes), or wounds. Infections spread from leaf to leaf by splashing rain, human activity, and cucumber beetles.
Copper containing fungicides can limit plant to plant spread of this disease. Treatments must be made every 5-7 days when the pressure is high and fruit require further expansion and maturity.
Written by Walt Stevenson, Karen Delahaut, and Amanda Gevens. Last updated Sep 2025.
