Wet, humid, overcast conditions outside favors plant disease development inside greenhouses because plant foliage and soil remain wet for extended periods of time. Plant wetness and high humidity within warm greenhouses coupled with young, succulent seedlings and cuttings in production and poor sanitation issues and the introduction of propagation material fulfills all components of the Disease Triangle (host, pathogen, and environment interacting all at the same time).
Bacterial leaf spot diseases on ornamental plants are usually caused by Xanthomonas, Pseudomonas, or Acidovorax species. Identification of which bacterium is causing the problem usually requires culturing on agar medium. However, there are certain symptoms that suggest bacterial infection such as brown to black, round to angular (when vein-delimited), greasy or oily-looking (water-soaked) leaf spots that may or may not have chlorotic halos.
Bacterial pathogens are spread within greenhouses mostly by splashing water and water activity of handling wet plants. Overhead irrigation can easily spread the bacterium to adjacent plants on the same bench. Bacteria can also be spread on contaminated hands and tools. Bacterial leaf spot diseases are very difficult to control. Infected plants should be discarded. It is almost impossible to cure a plant of a bacterial disease. Copper containing fungicide/bactericides can help protect plants from infection, but they will not stop the spread on already infected plants.
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