Fungal diseases in orchids range from merely cosmetic to potentially fatal. Fortunately, there are treatments available to control most fungal infections, and many treatments don't require the use of chemicals. However, if you do use a chemical treatment, remember to always follow the label instructions.
With almost all fungal problems with orchids, the best way to prevent disease is to practice good orchid culture and housekeeping of the plants.
Anthracnose and Phytophthora
Anthracnose and phytophthora (also called black rot or black spot) are common fungal diseases affecting orchids.Anthracnose usually exhibits a problem by leaves turning black from the tips toward the base. Phytophthora is often displayed as dark lesions on the leaves that grow larger and spread toward the roots.
You can treat both conditions with baking soda mixed at a rate of four teaspoons per gallon of water. Also add two teaspoons of refined horticultural oil or insecticidal soap to help the solution stick to the leaves. Apply the solution to the affected areas every two weeks. If that does not work, try a copper fungicide.
Botrytis and Leaf Algae
Botrytis appears as small dark spots on the flower. As the problem worsens, the spots grow larger and may exhibit pink edges, eventually forming into mycelium growths. Leaf algae is not a fungus but it can lead to fungal diseases and should be controlled.
Botrytis can be treated with a solution of baking soda and horticultural oil or insecticidal soap, as described above. Both botrytis and leaf algae can be controlled with dimethyl and ethyl benzyl ammonium. Many professionals use products with ethyl benzyl ammonium as preventive sprays. Apply the treatment every seven to 10 days, diluted to one-third or one-half of the suggested product strength, or as directed by the product instructions.
Root rot typically appears first in the roots and spreads to the rhizomes, eventually turning the leaves yellow and wrinkled. It can be controlled with the beneficial bacteria Streptomyces griseoviridis. This is sold commercially as Mycostop, BioYield, Companion, and other brands. Follow product labels for application rates.
Fungus gnats are commonly found among orchids in homes and commercial greenhouses. They do not spread or promote fungal growth, but they do feed on fungus; therefore, their presence on your orchids indicates fungal growth, usually in the soil or other growing media of the plants.
Fungus gnats have a four-stage life cycle: egg, larvae, pupa, adult. Adult insects lay their eggs in the plant media. These hatch as larvae, or maggots, which feed on the fungus. The larvae become pupa in about two weeks, then turn into adult fliers within a few days. The larvae do the majority of the feeding and feed primarily on fungus when it is available. But they also may eat seedlings, leaves that touch the soil/medium surface, or decayed roots.
Fungus gnats can be controlled with BT (Bacillus thruingiensis israelensis), found in products such as Gnatrol. BT is a natural bacteria that gnat larvae will eat during the feeding stage. Follow the product's directions carefully for the best results.
Petal blight typically is not fatal to plants, but it commonly ruins the look of affected flowers. Pink, tan, or gray spots on flowers are sure signs of petal blight. Because this fungus thrives in humidity and cool conditions, the best remedy is to keep the plant cleaned up and to increase airflow to help control humidity.
Orchid Disease. St. Augustine Orchard Society
Fungus Gnats. Oregon Orchid Society Website
Fungus Gnats. Missouri Botanical Garden Website