From ONvegetables in The Grower, February 2014
Identification: Currently, bacterial leaf spot (Xanthomonas cucurbitacae) is not commonly found in Ontario. However, it is known to cause serious pumpkin crop losses in the Mid-western United States. Leaf lesions are dark and very small with yellow margins (figure 1). As the disease progresses they coalesce to form large necrotic areas. They are easily confused with several other foliar diseases including angular leaf spot.
Fruit lesions are circular in shape, small and slightly sunken. They often have a dark brown border (figure 2). As the lesions enlarge in size (up to 12 mm in diameter) they crack and become scab-like. Infected fruit often develop soft, watery rots in the field or in storage and collapse quite quickly.
Biology: Bacterial spot survives in crop residue. It is also seed-borne. It can tolerate high mid-summer temperatures. It spreads quickly within the field.
Management Notes: Use disease-free, certified seed. Follow a 2-3 year rotation away from all cucurbit crops. The bacteria are easily spread by splashing rain or irrigation and by machinery. Avoid working in the field when the foliage is wet.