Bacterial Leaf Spot

Pseudomonas syringae

This is a common disease of peppers in the Eastern United States, though it also attacks tomato and nasturtium. It first appears as irregular water soaked spots that eventually turn brown. Severely affected leaves may eventually turn yellow and drop and the accumulated leaf loss can result in a reduced yield. This disease prefers cool temperatures (50 to 75˚ F).

This bacterial disease is frequently seed borne, so if it is a serious problem you should treat seed with hot water (122˚ F for 25 minutes) or bleach (be careful however or you can damage the seed). If you are buying transplants make sure they are disease free. Bacterial leaf spot can also be spread by water splashing from soil to plant, so use mulch and drip irrigation. You should also ensure that leaves don’t stay wet for long periods (water early enough in the day so plants can dry out quickly). It’s also good to rotate crops on a 3 year rotation. If you see infected plants remove them immediately to reduce the spread of infection.

An interesting aspect of Pseudomonas syringae is that it has ice nucleation properties and can cause water to freeze at higher temperatures than it otherwise would. Some strains have been used commercially in artificial snow making compounds.

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