How to Fix It
What it looks like:
Often confused with cloudy spot disease, bacterial cankers start as yellow dots on ripening red tomatoes. If you look carefully at the spots, using a magnifying glass if you have one, you’ll see a dark, birds-eye-type rim around each of the yellowed spots. This is what distinguishes bacterial canker from cloudy spot disease.
What causes it:
A bacteria called Clavibacter michiganensis. The bacteria occurs naturally but can be brought into the garden on infected plants or tools. Once it gets into the soil, rainwater splashes it up onto the plants. If there’s an open sore, such as insect damage or a leaf missing from pruning, it can enter the plant and infest it.
What to do about it:
Remove the infected plants immediately and do not plant tomatoes again in that soil for at least three years. Rotate your crops regularly to prevent these and other diseases from taking hold in the soil. Don’t compost the dead plants — instead, put them in the trash to avoid spreading the bacteria.