Bacterial leaf spot diseases are prevalent in wet greenhouses

Wet, humid and 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). In several greenhouses I have visited, bacterial leaf spots and blighting were seen on Calibrachoa (Fig. 3), Dieffenbachia (Fig. 2) and geranium (Fig. 5).

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 (Figs. 1, 2, 3 and 5).

The best approach to dealing with bacterial plant diseases is to get a sample to a diagnostic lab for confirmation quickly. One simple test to determine if bacteria may be causing the leaf spot is to slice through a leaf spot in a droplet of water on a glass microscope slide and viewing it for signs of bacterial streaming, which results from pooling of bacterial cells from the cut edges (Fig. 4). Any time water sits on a bacterial spot, bacterial cells may ooze out of the tissue into the water droplet and be easily water-splashed to adjacent plants.

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 help protect plants from infection, but they will not stop the spread on already infected plants. Bacterial pathogens are often introduced with contaminated seed or seedlings; therefore, inspect all incoming plants for symptoms of infection. It is often a good idea to segregate incoming plants from different suppliers and monitor them for symptoms prior to transplanting. Handling of infected plants can contaminate worker’s hands and tools, which can spread the bacterium as well.

Always handle healthy plants first, and then suspect or infected plants to reduce spreading diseases through worker activity. Disinfesting hands and tools with 70% ethanol or isopropyl alcohol can reduce disease spread. Ready-to-use (RTU) Lysol or Clorox disinfectant aerosol spray contains mostly ethyl alcohol and can be used to disinfect tools and gloves. It is also a good idea to provide alcohol gel dispensers (Fig. 6) throughout the greenhouse, especially when growing crops that are very susceptible to bacterial diseases.