Cause Acidovorax avenae subsp. cattleyae (formerly Pseudomonas cattleya), a bacterium. Seedlings are infected through stomata, older plants through wounds. Bacteria in exudate spread from orchid to orchid by splashing water and from place to place with infected plants. Contaminated propagation tools are another way to transmit the bacteria. Warm, moist conditions and high nitrogen fertility favor the disease. Phalaenopsis is especially susceptible.
Symptoms Especially severe in California on Phalaenopsis seedlings, starting as a soft, water-soaked spot that later becomes brown or black cavity. These spots expand and can involve the entire leaf. On older plants, it starts anywhere on a leaf and finally reaches the growing point, where a mucilaginous exudate may be produced. Plants may be killed. On Cattleya, the disease advances more slowly and is limited to older leaves, producing clearly delimited, sunken, black spots; it is not fatal to the plant.
Cultural control Early detection of disease outbreaks through regular scouting is essential to minimize losses.
- Avoid overhead irrigation that wets leaves or any practice that keeps plants wet for extended periods.
- Provide good aeration to promote drying.
- Keep benches clean and reasonably dry.
- Reduce nitrogen and increase potassium fertility.
- Separate diseased and healthy plants. Remove and destroy severely infected plants.
- Disinfect cutting tools after cutting flowers or making divisions. Disinfect tools after each plant. Soak in trisodium phosphate or 10% bleach or Physan 20 helps. Use two knives so a worker can cut with one while the other soaks. When done with a plant switch one knife for the other and allow the used one to soak.
Chemical control Focus on cultural controls first as reliance on chemical control alone is not effective.
- KleenGrow at 6 to 38 fl oz/100 gal water. 48-hr reentry.
- Phyton 27 at 1.5 to 2 oz/10 gal water. Group M1 fungicide. 48-hr reentry.
- ZeroTol 2.0 at 0.5 to 1.25 fl oz/gal water. 1-hr reentry.
Reference Smitamana, P., and McGovern, R.J. 2018. Diseases of Orchid. In McGovern, R.J. and Elmer, W. H. (eds.) Handbook of Florists' Crops Diseases. Springer Int.