Bacterial Spot symptoms on a tomato leaf. Photo: Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org
Updated: February 20, 2023
Two types of bacterial diseases occasionally damage the foliage and/or fruits of tomato plants: Bacterial Spot and Bacterial Speck.
Symptoms: small, dark spots (⅛ inch or less), sometimes water-soaked or wet-looking spots; leaf yellowing and leaf loss; fruits develop small, dark raised blisters that can enlarge and become scab-like.
Cause: Four types of Xanthomonas sp. bacteria. These pathogens can be transmitted by seeds and transplants.
Symptoms: small black spots (⅛ inch or less), more prominent on leaf undersides, older spots may develop yellow rings; tiny spots or specks on the surface of tomato fruit, may be raised or sunken.
Cause: Bacterium, Pseudomonas syringae pv. Tomato
For both diseases
Spots coalesce during warm, wet, and humid weather and turn brown.
Leaves will drop with severe infections, resulting in sunscald.
The bacteria overwinter on crop residue and are seed-borne.
The disease is spread by splashing rain and working with wet, infected plants. Hot, dry weather will slow the spread of this disease.
Select disease-resistant tomato cultivars. Buy clean, disease-free seeds from a reputable supplier. If the seeds have not been chemically or heat-treated, soak the seeds for five minutes in one pint of water and add two ounces of bleach. Rinse seeds thoroughly with clean, cool water and dry thoroughly.
Cover the soil around plants with mulch and do not crowd plants together. Avoid overhead watering and working with plants when the foliage is wet. Fertilize plants to support new growth. Spray with a labeled copper fungicide when symptoms are first observed. Remove infected plants and dispose of all infected crop residue.