Bacterial leaf spot represents a common challenge for tomato growers, impacting plant health and reducing fruit yield. Caused by specific bacteria, this disease can lead to significant damage if not addressed. Understanding its characteristics and how it affects tomato plants helps in effective management.
Identifying Bacterial Leaf Spot
Bacterial leaf spot has distinct visual cues on tomato plants. Initial symptoms often appear as small, dark, water-soaked spots, less than 1/8 inch (3 mm) in diameter, on older leaves. These spots gradually enlarge and become dark brown to black, often developing a greasy or shiny appearance on the underside of the leaves. A yellow halo may surround these spots, though it can sometimes be absent.
As the disease progresses, multiple spots merge, leading to larger, irregular blighted areas on the leaves. Severely infected leaves turn yellow and drop prematurely, leading to significant defoliation. Similar dark, elongated lesions also appear on stems and petioles. On the fruit, symptoms begin as small, raised, dark spots that may develop into a scabby texture, sometimes surrounded by a green halo on unripe fruit.
Understanding the Cause and Spread
Bacterial leaf spot on tomatoes is caused by several species of bacteria, notably Xanthomonas euvesicatoria, Xanthomonas gardneri, Xanthomonas perforans, and Pseudomonas syringae pv. tomato. These pathogens survive from one growing season to the next in infected plant debris left in the soil, on volunteer tomato plants, or within contaminated seeds. They also persist on certain weed hosts as reservoirs for the bacteria.
The bacteria spread through splashing water from rain or overhead irrigation, carrying bacteria from infected plants to healthy ones. Wind also plays a role in disseminating airborne droplets containing the bacteria. Contaminated tools and human handling of wet plants facilitate the rapid spread of the disease within a garden. Warm temperatures, ranging from 75 to 86°F (24 to 30°C), combined with periods of high humidity and prolonged leaf wetness, create an ideal environment for bacterial multiplication and infection.
Preventing Bacterial Leaf Spot
Preventative measures are the most effective strategy against bacterial leaf spot. Begin by sourcing certified disease-free seeds or healthy transplants from reputable suppliers. When planting, ensure adequate spacing between tomato plants to promote good air circulation, to help foliage dry quickly. Avoiding overhead irrigation in favor of drip irrigation or watering at the base of plants reduces leaf wetness and the spread of bacteria.
Crop rotation is another preventative step; avoid planting tomatoes or other solanaceous crops in the same spot for at least two to three years. Promptly remove and dispose of any plant debris at the end of the growing season, as bacteria overwinter in infected material. Regularly sanitize gardening tools using a disinfectant between uses and between plants. Some tomato varieties offer varying levels of resistance to bacterial leaf spot, and selecting these enhances preventative efforts.
Managing an Existing Outbreak
When bacterial leaf spot is already present, immediate action helps limit its spread and impact. Remove and dispose of any visibly infected leaves, stems, or fruit as soon as symptoms are identified. Avoid composting diseased plant material, as the bacteria can survive and re-infect future crops; instead, bag and discard it away from the garden. Minimize contact with plants when their foliage is wet to prevent inadvertently spreading bacteria from infected to healthy tissues.
Applying copper-based bactericides offer a suppressive effect on the disease, especially when used preventatively or at the first sign of symptoms. These products work by creating a protective barrier on the plant surface, hindering bacterial entry, but they do not cure existing infections. Consistent sanitation of gardening tools and hands is important throughout the growing season to avoid further spread of the pathogen.
References
Bacterial Spot of Tomato. The Ohio State University. https://ohioline.osu.edu/factsheet/plpath-veg-23. Accessed July 26, 2025.
Bacterial Spot of Tomato and Pepper. University of Florida. https://edis.ifas.ufl.edu/publication/PP127. Accessed July 26, 2025.