Bacterial Spot

Xanthomonas campestris

Bacterial Spot is a significant disease of tomatoes and peppers, though it can also afflict squash and cucumber. Like most bacterial diseases it is more problematic in warm and wet or humid weather.

This disease first manifests itself as small, angular water-soaked spots on the undersides of leaves, often with a general yellowing. These spots slowly get larger and darker in color and may merge together to create larger necrotic areas. The infected leaves eventually die, exposing the fruit to sun scald. The green fruit develop small, raised, water-soaked spots up to a 1/4 inch in diameter. These eventually develop a rough, scabby surface.

The usual precautions apply for bacterial spot. Remove potential sources of infection such as crop debris. Rotate the crops annually, so it doesn’t get established in the soil (it can live in the soil for up to a year). Try and prevent any water movement between plants, as this can spread the bacteria (which enter plants through natural openings or wounds).

The most common source of infection is from seed. If you suspect seeds are infected, soak them in hot water (127 degrees F for 25 minutes) or in a solution of 1 part bleach to 10 parts water for 30 minutes.

A subspecies of this disease attacks beans and is known as Common Blight (Xanthomonas Campestris pv. Phaseoli). Symptoms are similar to those of Halo Blight.

Image: Howard F. Schwartz, Colorado State University, Bugwood.org