by Lia Andrews
Expert grower and Southwest Florida Orchid Society president Barbara Murza explains identification, prevention, and treatment of the mortal enemy of our orchids: fungus. This is perhaps the most important topic we have covered this year, because a fungal infection can take out one of your beloved orchids in a matter of days. Barb focused on the two primary diseases: water molds (the cause of black rot and root rot) and fusarium wilt (causative fungi in vanda root rot).
3 Main Categories of Orchid Diseases:
- leaf spotting (fungal infection)
- water molds
Prevention is better than treatment. Healthy orchids and good cultural practices minimize opportunities for fungi to sneak in.
Orchids evolved the ability to withstand drying out to keep themselves disease-free. Fungi, which require consistent moisture, are naturally kept in check. The primary cause of fungal infections is overwatering, either due to our South Florida monsoon season or being excessive in our watering practices. Water orchids thoroughly in the morning allowing them to dry out thoroughly by nightfall. Having good air movement and good drainage (mounting orchids, using clay pots, and using potting media that does not readily decompose) are ways to ensure that your orchids are drying out between waterings. Bright light (appropriate to the species), proper plant spacing, and elevation above ground are also key. These factors become problematic as your collection increases and you run out of space. Practice excellent hygiene: keep your plant area clear of debris and disinfect your tools. Barb uses a torch she purchased at Home Depot to sterilize cutters while John Hampton uses disposable razors.
You will need to find your own balance. You need to now your orchids, your growing environment, and your habits. For example, some growers who water very sparingly and keep their orchids in very high light get away with using sphagnum moss. I love to water my plants and I quickly learned that I cannot use sphagnum except when I use it to help establish an orchid on a mount.
At certain times of year many orchid growers use a chemical or organic prophylaxis. Barb recommends a fungicide application every 3 months or so beginning in January to prepare for the rainy season.
Barb says it takes time and experience to identify disease and catch it quickly before it starts spreading through your collection. As a general rule with fungus you want to cut off the diseased parts if you can and allow the orchid to dry out.
BLACK ROT (see more pictures)
These are water molds caused by Pythium and Phytophthora spp. This group of fungi is a scourge on many of our beloved plants and caused the 19th century potato famine.
Symptoms: begins as brown spots that turn black. All parts of the orchid are susceptible but it usually begins at the base of the pseudobulb or the roots. The organisms require water to thrive and spread so black rot tends to occur during the hot humid summer monsoons. The infection can spread throughout the plant quickly causing death. It is very contagious. Quarantine the plant and wash your hands before toughing other plants.
Treatment: immediately cut off the diseased portion of the plant if possible. Use sterilized cutters and be sure to carefully sterilize afterwards. There are 2 systemic fungicides which are effective. The first is Aliette WDG. The second is thiophanate methyl sold under the names Cleary’s 3336 WP, Banrot, and Thiomyl. Mix one of the listed fungicides at a ratio of 1 tsp/gallon of water. Spray the entire plant (except the flowers) saturating thoroughly. Repeat in 2 weeks. Follow up every 4-6 weeks to prevent recurrence.
Root rot is caused by the Rhizoctonia spp. This is a common issue in humid south Florida where lack of water is seldom an issue. I learned very quickly to remove every orchid I purchase from moss and plastic pots immediately, even if they are flowering. Sphagnum moss can be reduced to mush in a year in our climate and plastic does not allow the roots to breathe. I have had to recover too many orchids. Though all survived, it set them back tremendously.
Symptoms: roots become yellowed and eventually black. Eventually the leaves begin to wilt. The pathogenic brown mycelium infects the roots and progresses through the rhizome and lower parts of the pseudobulb.
Treatment: Remove the orchid from the media. Cut away rotted roots. Saturate the roots with thiophanate methyl systemic fungicides (Thiomyl or Banrot) mixed at a ratio of 1 tsp/gallon of water. (I have used Garden Safe Brand Fungicide 3-in-1 concentrate successfully. I soak the orchid after cutting off the diseased roots. After I repot, I soak the pot and media.) Repot the orchid in media that provides excellent drainage such as these from Broward Orchid Supply or mount the orchid. Keep the orchid on the dry side until it recovers. Remember, air flow, oxygen, and dryness inhibit fungi.
FUSARIUM WILT (see more pictures here)
Fusarium wilt is also known as vanda root rot. Caused by Fusarium spp. which blocks the flow of moisture up through the roots to the leaves. Excessive fertilizing tends to reduce pH levels (acidity) leaving orchids more vulnerable to fusarium.
Symptoms: infected leaves are yellow, thin, shriveled/wrinkled, an die off. The roots appear as desiccated and woody. This dry rot will advance upward quickly and eventually kill the plant when all the roots have been desiccated. A tell tale symptom is a band of purple or pink discoloration on the outer layers of the rhizome when the rhizome is cut.
Treatment: there is no available chemical fungicide that can satisfactorily cure a plant of fusarium. Thiomyl or Cleary’s 3336 WP will work to some extent, but the the fusarium will make a comeback in a plant that has been infected before. The best treatment is to catch the infection early. Cut the bad roots and/or stem, sterilizing between each cut. Treat the remaining plant with Thiomyl.
NOTE: chemical fungicides are nasty stuff. Only use in well-ventilated areas. Wear gloves, long sleeves, and a mask and wash your clothes right after. Also be cautious mixing chemicals. It is best to use them separately unless you are given special instructions from a knowledgeable source.
The best bang for your buck is Banrot. This contains Truban (etridiazole) plus thiophanate methyl (Cleary’s 3336 WP or Thiomyl). Naples vanda grower and hybridizer David Genovese uses a mix of Thiomyl and Dithane.
The following chart by Sue Bottom breaks down fungicides and bactericides and the diseases they treat: Orchid Fungicides and Bactericides
ORGANIC FUNGAL CONTROL
The increasing interest in organic gardening as well as the health hazards associated with the harsh fungicides many growers use has led to increasingly experimentation in natural fungicides. Cinnamon powder can be sprinkled on orchid roots before repotting to prevent disease. I use the neem oil-based Garden Safe Fungicide 3-in-1 as a preventive soak every time I repot or bring in a new orchid. It is also highly effective on aphids and thrips. In some cases fungicides are harmful to the orchids. For example, Gary Murza attributes his success with phragmapediums with his use of mycorrhiza which would be destroyed by chemical fungicides.
Trichoderma and Biofungicides
Some growers have started using a trichoderma-based protectant every 2 months or so. There has been a great deal of research on the use of beneficial fungus in agriculture, and especially in hydroponics. It has been used with neem oil to prevent/treat ganoderma infection in coconut palms, for example. Cultivating an environment with naturally occurring beneficial bacteria and fungi creates competition that edges out the bad ones. This reduces the dependence on chemical fungicides. You can purchase a trichoderma-based biofungicide from many sources such as Subculture B and Root Shield. Trichoderma grows readily on cornmeal and powdered beans. An excellent description of how to use cornmeal to innoculate your orchids can be found by on the Orchid Board here. As an aside, cornmeal can be used as a soak to eradicate foot fungus and a recent study suggests internally cornmeal and beans can help re-establish beneficial gut bacteria.
LINKS FOR FURTHER READING
The St. Augustine Orchid Society has excellent information and pictures of orchid diseases. Barb recommends reading through.
A PDF on orchid pests from the Missouri Botanical Garden
Here is detailed instruction of saving an orchid from fusarium rot.
A popular book on beneficial microorganisms and gardening – Teaming with Microbes