Angular Leaf Spot on Cucurbits

Kate Everts, Vegetable Pathologist, University of Maryland;

A common bacterial disease of cucurbits is angular leaf spot, caused by Pseudomonas syringae pv. lachrymans. Angular leaf spot can be seed borne and its hosts include cucumber, zucchini, yellow squash, pumpkin and winter squash. Foliar lesions are initially small, water soaked, and green, but become tan with dark brown margins. Holes or tears often form in the tan lesions, and yellow haloes occur around the lesion. Fruit also develop lesions, which are small and have tan centers. Because the disease is seed borne the symptoms occur on young leaves and the disease is spread by splashing rain, workers hands and other contact when the leaves are wet. Within a field, the first flush of leaves may be heavily infested, but if conditions don’t favor spread and the plant has indeterminate vines, the plant may “outgrow” the symptoms and produce normal fruit. However, fruit can also develop lesions that compromise their marketability. Fruit lesions are small (up to 1/8 inch diameter) tan lesions, which can penetrate the fruit and may predispose fruit to secondary rot.

Winter squash leaves infected with angular leaf spot. Lesions have tan centers, dark edges, ragged appearance and yellow halos.

Control measures begin with purchasing disease free seed and planting into land that has not had a cucurbit crop in two or more years. Seed treatments are available that can reduce the likelihood of bringing in the angular leaf spot on seed. Avoid overhead irrigation to reduce disease spread. In season, apply fixed copper on a 7 to 10-day schedule to reduce disease spread and protect fruit from lesion development. Some copper formulations are available for organic producers. Conventional growers should apply fixed copper plus mancozeb.