Bacterial Leaf Spot
Xanthomonas campestris, Pseudomonas syringae
Overview
Bacterial leaf spot diseases are caused by several genera, including Xanthomonas and Pseudomonas, on vegetables, herbs, and ornamentals. Pathogens often enter through stomata and wounds during warm, wet weather. Splash dispersal from rain or overhead irrigation spreads bacteria within a canopy. Seedborne infection is important for crops such as tomato, pepper, and brassicas. In humid gardens, repeated wetting cycles can turn a few spots into widespread defoliation and fruit blemishes.
Symptoms
Small water-soaked spots appear on leaves and expand into angular brown or black lesions bounded by veins. Spots may be surrounded by yellow halos. On fruit and pods, raised or sunken lesions reduce quality. Severe infections cause leaf yellowing, defoliation, and reduced photosynthesis. Symptoms often start on lower leaves after splash from soil. Lesions can look similar to fungal spots; bacterial spots often feel greasy when young and lack fungal fruiting bodies.
Environmental Conditions
Humid
Prevention
Use disease-free seed and clean transplants. Mulch soil to reduce splash onto lower leaves. Space plants for airflow and avoid dense, shaded canopies. Sanitize stakes and cages reused from previous crops. Keep nightshade and brassica weeds out of beds. Start with resistant varieties in humid climates where leaf spot is a yearly problem, and plan irrigation methods that keep leaves dry.
Management
Remove badly spotted leaves and severely infected plants to lower inoculum. Switch to drip or soaker irrigation and water early in the day so foliage dries quickly. Copper and other labeled bactericides can slow spread on some crops but work best as protectants before heavy rain. Avoid working wet plants. After harvest, remove crop debris. Rotate away from related hosts for two or more seasons in problem beds. Choose resistant cultivars when available for tomato, pepper, and leafy greens.