Bacterial Spot

Small black spots on tomatoes and tomato leaves often indicate bacterial spot.

Bacterial leaf spot (Netty4299) Public Domain

Those black spots might not look like anything important, but this bacterial disease can also affect peppers, eggplant, groundcherries, and tomatillos, along with your tomatoes. Close cousin to the bacterial spot of almonds and practically impossible to differentiate from bacterial speck without a microscope, bacterial spot (Xanthomonas campestris pv. vesicatoria) is perfectly capable of killing your tomato plants.


​Bacterial spot symptoms

Symptoms of bacterial spot can appear on all life stages, from seedlings to mature plants, and on all aboveground plant parts. Bacteria enter through wounds and stoma. Infected younger plants can be completed defoliated by bacterial spot. Older plants exhibit insignificant looking water-soaked areas on mature leaves, usually near the bottom of the plant. making it easier to dismiss this disease. This would be a mistake.


Closer inspection of these lesions shows that they start out yellow or light green, turning dark brown or black. Older spots may be raised areas that average 1/3” across. Larger damaged areas may be seen at the margins (leaf edges). ​

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Immature fruit can also be affected by bacterial spot. Bacteria enter through tiny hairs, called trichomes. Infected areas start out looking like tiny raised black dots which then become sunken or dimpled, and surrounded by a white halo, similar to bacterial canker. The halos eventually disappear as the spots get larger and become scabby. These fruits, if they are able to mature at all without rotting on the vine, are still edible. Simply cut out the diseased areas. Just be sure to dispose of the infected parts in the trash. Do not add them to your compost pile.


Controlling bacterial spot

Because these bacteria overwinter in infected plant debris, you can protect next year’s crops by clearing infected plant tissue out of your garden completely each fall. The disease can also appear on volunteer tomato plants, so watch rogue tomato plants closely for signs of infection. Splashing rain, irrigation water, and contaminated tools can also spread the disease, so avoid overhead watering and be sure to sanitize your garden tools regularly.


Since these bacteria need humidity and water droplets to survive, pruning for good air flow can go a long way toward preventing this disease.


If you are like me and save seeds from each year’s crops, be sure you don’t use seeds from an infected plant, as you will be perpetuating the disease. As always, only buy certified disease-free plants and seeds and always quarantine new plants. Fixed copper sprays can be used in areas where bacterial spot has been a significant problem, although there have been some cases of copper-resistant bacteria. Crop rotation can also be used to break this disease triangle.


Protect next year’s crops by tossing plants infected with bacterial spot into the trash and providing good air flow around future plants.