Tomato fruit in home gardens and commercial fields in Denver and the northern Front Range may sometimes develop a black spot disease unusual for our area. The disease symptoms are dark specks that become raised and scab-like as they enlarge. Sunken centers on older spots are common. Lesions are brown turning black and can appear blistered. The round spots can merge causing irregular-shaped patterns.
Bacterial spot is caused by Xanthomonas campestrispv. vesicatoria (X.c.v.) bacteria. It is something seldom seen in Colorado due to the dry climate suppressing most bacterial diseases. Some years, a wet early summer and higher humidity favor it. If people overhead water instead of ground irrigating they tend to help the disease. Hail and heavy wind-driven rains may inflict injuries on plants and help disease spread via wound entry.
Avoid overhead watering. Clean plant debris from gardens and fields this fall. Don’t save seed as it can survive that way. Use disease-free seed and transplants next year. Rotate tomatoes and peppers (another host) to soil growing non-tomato family plants (disease survives in soil for up to 1 year). Eliminate tomato/potato family weeds.
Other measures probably apply to humid climates as we may see little or none of this in drier years, and when overhead watering is avoided. Copper/mancozeb sprays are applied to just-transplanted commercial fields. These protective sprays are most needed during early flowering and fruit setting periods.
This outbreak is a good example of an environmental trigger setting off a disease uncommonly seen even though the host plant and likely the bacteria were present in past years. The fruit is edible although many people may prefer to remove the skin with the spot.