Bacterial pathogen: Acidovorax on petunias
Water early in the day to reduce leaf wetness and destroy plants infected with bacterial pathogens. Copper-based products can reduce spread to non-infected plants.
Bacterial leaf spots often occur when a bacterial pathogen is present where disease pressure is high. For example, growing environments with high humidity where plants stay overly wet for extended periods of time are ideal for development of bacterial leaf spots. Bacterial leaf spots on floriculture crops may be caused by several species of Pseudomonas and Xanthomonas. While not extremely common, Acidovorax spp. was recently detected on petunias, where it was causing leaf spot symptoms.
Affected plants were under areas where there was moisture persistently dripping from above. The symptoms of the bacterial leaf spot were necrotic spotting, plant stunting and plant dieback. Disease symptoms typically progress as bacterial populations increase in or on the host. As many of the bacterial leaf spot diseases produce similar symptoms, consider sending a sample to a diagnostic lab such as Michigan State University Diagnostic Services to get a final diagnosis.
Bacterial pathogens are spread easily through handling and via splashing water when irrigating crops. Michigan State University Extension reminds growers that asymptomatic plants can be infected and can be a source of inoculum. Plants with bacterial leaf spot symptoms should be removed and discarded. To prevent further spread of bacterial pathogens, do not handle infected plants until the foliage is dry. Upon removing the infected material, employees should wash their hands. Neighboring plants to those infected should be quarantined.
Growers should water plants early in the morning and try to minimize durations of leaf wetness, which promote both bacterial and fungal pathogens. In addition, consider reducing the humidity in the greenhouse and increasing air flow so that your growing environment is not as optimal for pathogens. If it is possible, try to move plants from under areas where there is constant dripping (i.e., leaky plumbing, baskets hung above).