December 2015 Plant Disease Advisory
Bacterial leaf blight, caused by Pseudomonas cichorii, is a disease that affects a number of herbaceous plants, such as coreopsis (Fig. 1) and chrysanthemum (Figs. 2 and 3). Symptoms include leaf spots that typically coalesce and progress to severe leaf blight under favorable environmental conditions for the bacterium. The problem is most commonly diagnosed in greenhouse or nursery situations when foliage experiences prolonged leaf wetness.
Once symptoms develop, the bacterial disease is typically difficult to control. Cultural controls that minimize periods of leaf wetness should be practiced to avoid conditions favorable for development of bacterial leaf blight: increase plant spacing and avoid overhead irrigation or at least water early in the day so that foliage dries relatively quickly. It is also important to adequately ventilate and heat the greenhouse to promote foliar drying. The bacteria are spread by splashing water and driving rain, so minimize water splash when possible. Chemical control with appropriately labeled copper or other bactericidal products can be used to help manage the disease, but will not be effective on plants severely affected and/or under conditions very favorable for spread and infection by the bacteria.