Tomato fruit in home gardens and commercial fields in Denver and the northern Front Range have developed a black spot disease unusual for our area. Dark specks from earlier in the season become raised and scab-like as they enlarge. Sunken centers on older spots are common (see photo left). Spots are brown turning black and can appear blistered. The round spots can merge causing irregular-shaped patterns (photo on yellow tomato below).
Bacterial spot is caused by a Xanthomonas campestris pv. vesicatoria (X.c.v.) bacteria. It is something we seldom see in our area due to our dry climate suppressing most bacterial diseases. This year, the wet early summer and higher humidity favored it. If people overhead instead of ground irrigated they tended to help the disease. The widespread hail and heavy, wind-driven rains that inflicted injuries on plants helped disease spread via wound entry.
At this point there is no way to make the existing spots go away. Early in the season, avoiding overhead watering would have helped. Clean plant debris from gardens and fields this fall. Don’t save seed as it can survive that way. Rotate tomatoes and peppers (another host) to soil growing non-tomato family plants (disease survives in soil for up to 1 year). Eliminate weeds in the tomato/potato family.
Other measures to spray transplants after setting out probably apply to humid climates as we may see little or none of this next year if it is drier and overhead watering is avoided.
This outbreak is a good example of an environmental trigger (rain and humidity) setting off a disease rarely seen even though the host plant and likely the bacteria were present in past years.
Note that the fruit is edible although many people may prefer to remove the skin with the surface spot.
Photo credits: Sunken spots red tomato – Carl Wilson,
Merged spots yellow tomato – Robert Cox