Fungal diseases in orchids range from merely cosmetic to potentially fatal. Fortunately, there are treatments available to control most fungal infections.
It should be said, however, that the best offense against fungal diseases in orchids is a good defense. Practice good orchid culture and you can prevent the worst of the fungal disorders most of the time. Also, many minor fungal issues, such as minor black-spotting on leaves, can be tolerated without using chemicals. Finally, if you do use a chemical, remember to always follow the label instructions.
The following list includes typical fungal diseases and recommends organic and biological controls treatments:
- Anthracnose, Phytophthora, Botrytis, mildew, black spot, and leaf spot can all be treated with baking soda mixed at a rate of 4 tsp/gal. Add 2 tsp./gal of refined horticultural oil or insecticidal soap. Reapply every two weeks. If this doesn't work, try copper fungicide.
- Root rot and pseudobulb rot can be controlled with the beneficial bacteria Streptomyces griseoviridis. This is sold commercially as Mycostop, BioYield, Companion, and other brands.
- Leaf-spotting caused by Anthracnose and Cercospora can be controlled with BT (Bacillus thruingiensis israelensis), which kills fungus gnats. Look for Gnatrol.
- Botrytis, leaf-spotting, and leaf algae (which can lead to fungal diseases) can be controlled with dimethyl and ethyl benzyl ammonium. Look for Physan 20 or Greenshield. Many professionals use these products as preventive sprays. Apply every 7 to 10 days, diluted to one-third or half of suggested product strength.