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Orchid Pests and Diseases - Orchid Diseases
Information assembled by Sue Bottom, symptom descriptions and prevention tips extracted from Orchid Species Culture by Margaret L. and Charles O. Baker, peer review by Fred Clarke. Fungicides and Bactericides for Orchid Diseases (follow label instructions and wear protective equipment).
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Bacterial Soft and Brown Rot - Erwinia
Symptoms: Small water-soaked spots appear on the leaves and often are surrounded by yellow halos. If unchecked, the infection will rapidly rot the leaves and roots and spread more slowly into the rhizomes or pseudobulbs. This wet rot may have a foul odor and has a water soaked appearance.
- Phalaenopsis. Disease spreads so rapidly that plants may be completely rotted in 2 to 3 days. The bacteria are opportunistic organisms that can enter through wounds.
- Dendrobium. Leaves appear yellow and water-soaked and become black and sunken.
- Vanda. Leaves develop translucent patches which become black and sunken.
- Paphiopedilum. Leaves develop small, round spots that are initially yellow and water-soaked but eventually become reddish brown and sunken. The spot enlarges in all directions and may reach the growing crown before the leaf tip is affected. If untreated, the disease quickly spreads throughout the plant, leaving it a dark, shriveled mass.
- Grammatophyllum. Leaves have water-soaked, browning spots which become black and sunken.
Treatment: Immediately remove infected tissue using a sterile instrument, spray bactericides like Physan or copper compounds on infected and adjacent plants following label instructions (copper should not be used on dendrobiums or blooming plants), disinfect growing area with 10% bleach solution. Treat nearby plants as well as those that are diseased.
Prevention: The disease is spread by splashing water so avoid overhead watering if the disease is present. The pathogen favors hot and moist conditions, so if infection occurs, keep leaves dry, increase air circulation and reduce temperature and humidity (if possible). Periodic preventive sprays with copper compounds help to prevent infection, particularly during hot and humid weather (do not apply copper to dendrobiums). Always follow label instructions.
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Bacterial Brown Spot - Acidovorax (syn. Pseudomonas)
Symptoms: The symptoms may appear anywhere on the leaf as a small, soft, water soaked blister. Initially dirty green in color, the infected spot enlarges, coalesces and eventually becomes brown or black, dried up and sunken. It oozes bacteria-laden liquid, particularly when the disease reaches the tip of the leaf. It is most prevalent during the warmer weather.
- Phalaenopsis. The blister-like spots may be surrounded with a yellowish or pale green halo. Spots coalesce, and the infection spreads rapidly. If the diseased area invades the crown, the plant will die.
- Cattleya. The infection enters through wounds on older plants and usually affects only older leaves. It advances slowly and is rarely fatal.
Treatment: Immediately remove infected tissue using a sterile instrument, spray bactericides like Physan or copper compounds on infected and adjacent plants following label instructions (copper should not be used on dendrobiums or blooming plants), disinfect growing area with 10% bleach solution. Treat nearby plants as well as those that are diseased.
Prevention: Pseudomonas cattleya is a water-borne pathogen that prefers warm, moist conditions. Reduce humidity and temperature (if possible), eliminate overhead watering and increase air circulation. |
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Black Rot - Pythium and Phytophthora
Symptoms: The infections usually starts on the leaves, new leads or roots, though all plant parts are susceptible. The disease spreads rapidly and will kill the plant unless treated promptly.
Leaf symptoms first appear on the underside as small, irregular, watery, brown spots which rapidly become purplish brown or purplish black. The spots may have a yellowish advancing margin. The lesions enlarge with age and may ooze water if pressed. Old lesions sometimes become dry and black, often allowing other diseases to attack the plant. The disease may spread rapidly to the rhizome and roots, particularly when the temperature and humidity are high.
New leads show a purple or purple-brown area with a yellowish advancing margin and may be pulled off easily.
Pseudobulbs, roots or rhizomes show infections as purplish-black, often sharply delineated, discolored area in the center of the plant. The infection often starts in the roots and may spread upward to the base of the pseudobulb or leaf, causing the plant to wilt.
- Cattleya. May show a creamy yellow discoloration on one or both sides of the pseudobulbs. The discoloration eventually turns black or brown and softens, and the bulb rots.
Treatment: Unless the plant is valuable, the best approach is to discard it, as the disease is highly contagious and will spread from plant to plant from splashing water. If the plant is valuable, isolate it from your other plants, remove infected tissue with a sterile tool, and drench with a suitable fungicide like Subdue or Banrot following label instructions.
Prevention: High temperatures and humidity contribute to the spread of the disease. Consider using a preventative fungicide drench like Banrot or Subdue, particularly during hot humid periods, following label instructions.
Read More: Black Rot by Robert Cating of IFAS, University of Florida and Black Rot by Susan Jones, American Orchid Society.
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Fusarium Wilt
Symptoms: Fusarium blocks the flow of moisture through the plant’s vascular system plugging the phloem. Infected leaves are yellow, thin, shriveled, wrinkled or wilted and eventually die.
The diagnostic symptom in the plant is a circle or band of purple or pinkish-purple discoloration on the outer layers of the rhizome evident when the rhizome is cut. If the disease is extensive, the entire rhizome may turn purple, and the discoloration may extend to the pseudobulbs. The pathogen is spread through improper hygiene, generally as a result of using nonsterile cutting tools, which transfers the fungus from plant to plant. Severely infected plants may die in 3-9 weeks, while mildly infected plants gradually decline over a year or so.
Treatment: Discard infected part of rhizome and pseudobulb if the purple band is evident. Repot only the part of plant showing no purple discoloration. Drench sanitized plants in a thiophanate methyl (like Cleary's 3336 or Banrot) following label instructions. Be diligent in disinfecting growing area and cutting tools. Each time the cutting tool contacts infected tissue, it should be sterilized before making a second cut.
Prevention: Prevention is a simple matter of following proper hygiene. Sterilize cutting tools after each use, preferably through flame sterilization.
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Leaf Spots - Cercospora, Guignardia & Phyllosticta and Septoria
Cercospora Symptoms: Infection shows first as a yellow spot on the underside of the leaf. Soon after infection occurs, the yellow-green area may be noted on the top surface of the leaf. The spots continue to enlarge in a circular or irregular pattern and may eventually cover the entire leaf. With age, the spots become slightly sunken and necrotic and change to purple-brown or purple-black. The advancing margin remains yellow. Heavily infected leaves usually fall from the plant prematurely, especially if the infection started near the base of the leaf. As the spots enlarge in irregular patterns, they become sunken and turn purplish brown to purplish black. The top surface of the leaf first becomes chlorotic and finally necrotic.
Guignardia Symptoms: The first signs of Guignardia infection are tiny, dark purple, elongated lesions on either leaf surface. These lesions run parallel to the veins and elongate into purple streaks or diamond-shaped areas. Spots often merge to form large irregular lesions that may affect a large part of the leaf. With age, the center of the lesion turns tan. Raised, black sporing bodies develop in the affected area. Affects mostly Ascocentrum and Vandas and their hybrids and may indicate insufficient light. This blight is also known as Phyllosticta; the names apply to two different sexual stages of the same fungus.
Phyllosticta Symptoms: Spotting from Phyllosticta may start anywhere on the leaf or pseudobulb. The lesions are tiny, yellow and slightly sunken. As they enlarge, they become round to oval and more sunken, especially if the infection is on the leaves. With age, they turn tan to dark brown and develop a slightly raised, red to purple-black margin. Eventually, tiny black, raised spore structures develop in the center of the spots. Individual spots are about ¼ in across. Severely infected leaves may drop prematurely. Its presence may indicate insufficient light. This blight is also known as Guignardia; the names apply to two different sexual stages of the same fungus.
Septoria Symptoms: The tiny spots may start on either leaf surface as sunken, yellow lesions. They continue to enlarge, becoming dark brown to black, circular or irregular lesions. Spots may merge to form large, irregular patches on the leaf. Heavily infected leaves fall prematurely.
Treatment: Remove infected leaves with a sterile instrument and reduce leaf wetness. Spray with a systemic fungicide such as thiophanate methyl (Cleary's 3336 or Banrot) or a protectant fungicide like Mancozeb, following label instructions. Alternate systemic and protectant fungicides.
Prevention: Good sanitation with good air movement. Reduce leaf wetness, water on the leaves may lead to infection. If the fungus is a continuing problem, monthly fungicide sprays may offer effective prevention.
Read More: Dr. Martin Motes has an excellent article on treating the Dreaded Thai Disease (Guignardia or Phyllosticta) that afflicts vandas.
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Botrytis Petal Blight
Symptoms: Very small, black or light brown, spots on the flowers. The spots may enlarge and cover the entire flower. If conditions are moist, a gray fungal growth may appear on severely infected or decaying flowers.
Treatment: Remove infected flowers, then spray with a protectant fungicide like Daconil. In enclosed areas, the smoke bomb Exotherm Termil (Daconil) can be used though be careful because this can coat the inside of your heater and cause firing problems. Always follow label instructions. This fungus is common in the environment and cannot be eradicated.
Prevention: Remove infected flowers since these are reservoirs of infection. Infection may be reduced through careful sanitation, increased air circulation, reduced humidity and warmer night temperatures (>68F).
Read More: Botrytis by Susan Jones, American Orchid Society.
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Microfungus - Microfungus or Phalaenopsis Chlorotic Spot Virus?
Fungus or Virus? From Bob Gordon Culture of the Phalaenopsis Orchid: "We still don't have a handle on what is causing the disease yet or even what
it is, but efforts are underway at two state universities. It may be a
fungal disease and virus in combination, confusing the diagnosis, but there
is little question that the disease weakens the plant and leaves it
susceptible to more common ailments such as Pseudomonas cattleyae."
Symptoms: In the order of appearance, the leaves display yellow chlorotic spots, then more defined yellow spotting that can grow into elongated yellow streaking, then pitting, and finally large areas of grayish tissue collapse. It can be confused with mesophyllic cell collapse caused by watering with cold water, although this weathers to dark rather than light sunken spots in phals. Paph infections weather to darkish sunken spots.
From Bob Gordon Culture of the Phalaenopsis Orchid: "sometimes a condition prevails that is caused by a systemic infection of microfungi. As there are literally hundreds of these, the symptoms vary from plant to plant. Some of the more common are a spotty, ill-defined chlorosis; a streaky chlorosis beginning at the edge of the leaf where it looks as if the leaf edge had been burned with a match or candle; a red-brown coloration appearing at the apical third or half of the lower leaves followed by a dehydrated and senescent (old) appearance and also mesophyll tissue collapse where deep pitting becomes apparent on the surface of the leaves. This latter condition can also be caused by cold water and by virus infections. However, in the latter instance, the pitting is usually dark-brown to black in appearance rather than the white to light fawn caused by fungi."
Treatment: Tom Nasser of Carolina Orchids recommends that infected plants be destroyed and the remaining plants treated with the copper fungicide phyton 27, a chelated copper fungicide that permeates the leaf more effectively than the more available copper fungicide Kocide. Alan Koch of Gold Country Orchids says to eradicate microfungus spray first with Banrot, then Cleary's 3336 plus Subdue, then Banrot plus Subdue. These sprays should be 7 days apart in summer and 10 days apart in winter, all at label strength - it results in a synergistic effect. Follow all label instructions and wear protective equipment.
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Fungal Root Rot - Rhizoctonia
Symptoms: Root rot occurs when the medium breaks down, drainage is poor and/or plants are overwatered. Rot sets in quickly when roots are damaged by injury or salt buildup from hard water or over fertilizing. Rhizoctonia is very contagious and if the disease is not controlled immediately, infected plants develop brown root rot and die.
Rhizoctonia is primarily a root disease, but the symptoms can be noticed on aerial parts of the plant. Leaves and pseudobulbs become yellow, shriveled, thin and twisted and new growths become progressively smaller. The roots usually show a brown rot with white or brown fungal growth. In severe infections, the fungus girdles and kills the plant. The infection quickly invades the lower leaves and rhizomes of small seedlings.
Treatment: Remove infected part of roots and leaves using a sterile cutting tool, drench the remaining plant in a protectant fungicide like thiophanate methyl (such as Cleary's 3336) or systemic fungicide (such as Subdue) following label instructions. Disinfect growing area with 10% bleach solution.
Prevention: Make sure your potting media is fresh and your plants are not overwatered. When disease is suspected in other plants or when repotting is overdue, unpot the plants, check their roots and repot as necessary. In hard-water areas, pots should be flushed at least monthly to prevent root damage by watering heavily to solubilize the salts and then watering heavily an hour later to flush the salts from the pot.
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 "Black Rot (Pythium, Phytophthora) on Cattleya Orchid")
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