On this page we will look at the bacterial and fungal diseases that can affect orchids.
1. BACTERIAL SOFT AND BROWN ROT
Pectobacterium (syn. Erwinia)
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Symptoms:
• Phalaenopsis: Small, water-soaked spots appear on the leaves, often surrounded by yellow rings. If left untreated, the infection quickly causes leaves and roots to rot. This wet rot can have an unpleasant odor and a damp appearance. The disease spreads so quickly that plants can completely rot in 2-3 days. The bacteria enter through wounds on the plant.
• Dendrobium: The leaves appear yellow and waterlogged, later turning black and sunken. The infection progresses more slowly in pseudobulbs.
• Paphiopedilum: Leaves develop small, round spots, often near the center. Initially yellow and waterlogged, they later turn reddish brown and sunken. The spot spreads in all directions and can reach the growth point before the leaf tip is affected. If left untreated, the disease spreads rapidly, reducing the plant to a dark, shriveled mass.
• Grammatophyllum: The leaves show water-soaked spots that turn brown, black and sunken.
Treatment: Immediately remove infected tissue with a sterile instrument. Spray infected and adjacent plants with bactericides containing quaternary ammonium compounds or copper compounds, following the label instructions (copper should not be used on dendrobiums or flowering plants).
Prevention: The disease spreads through splashing water, so avoid overhead watering if infection is present. The pathogen thrives in warm, humid conditions, so if an infection occurs, keep leaves dry, increase air circulation, and reduce temperature and humidity (if possible). Periodic preventative spraying with copper compounds helps prevent infection, especially during hot, humid weather (do not apply copper to Dendrobiums). Always follow label instructions.
2. BACTERIAL BROWN SPOTS
Acidovorax (syn. Pseudomonas)
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Symptoms:
Bacterial brown spot is the most common and serious disease of phalaenopsis. Symptoms can appear anywhere on the leaf as small, soft, water-filled blisters. Initially dirty green, the infected spot grows larger, joins with others, and eventually becomes brown or black, dry, and sunken. It releases a bacterial-laden fluid, especially when the infection reaches the leaf tip. It is more common during warmer weather.
• Phalaenopsis: The blister-like spots may be surrounded by a yellowish or pale green halo. The spots merge together, and the infection spreads rapidly. If the infected area reaches the growth plate, the plant will die.
• Cattleya: The infection enters through wounds in older plants, usually affecting mature leaves. It manifests as sunken, well-defined black spots. It advances slowly and is rarely fatal. In young plants with more tender leaves, the infection spreads more rapidly.
Treatment: Immediately remove infected tissue with a sterile instrument. Spray infected and adjacent plants with bactericides containing quaternary ammonium compounds or copper compounds, following the label instructions (copper should not be used on dendrobiums or flowering plants). Treat adjacent plants as well as infected ones.
Prevention : Pseudomonas is a waterborne pathogen that prefers warm, humid environments. Reduce humidity and temperature (if possible), avoid overhead watering, and increase air circulation.
3. BLACK ROT
Pythium and Phytophthora
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Symptoms:
Infection usually begins in the roots or the basal part of the pseudobulb, although any part of the plant can be affected. The disease spreads rapidly, especially in high temperatures and humidity.
• Pseudobulbs, roots or rhizomes They show purplish-black areas of infection, often well-defined, that appear discolored in the center of the plant. The infection often begins at the roots and can ascend to the base of the pseudobulb or leaf, causing the leaf to drop at the slightest bump.
• Cattleya It may present a yellow-cream discoloration on one or both sides of the pseudobulbs. This discoloration then turns black or brown, softening the tissue and leading to bulb decay.
Treatment: If the plant is not valuable, it is advisable to remove it, as the disease is highly contagious and spreads rapidly between plants through splashing water. If the plant is valuable, isolate it from other plants, remove the infected tissue with a sterile instrument, and treat with an appropriate fungicide, carefully following the label instructions.
Prevention: High humidity and the presence of water favor the spread of the disease. During particularly humid or rainy periods, it is advisable to apply preventative treatments with suitable fungicides, always following the instructions on the label.
4. FUSARIOSIS
Fusarium
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Symptoms:
Fusarium blocks the flow of moisture through the plant's vascular system.
• Cattleya: Fusarium can infect a plant through the roots or by invading the rhizome of recently divided plants. The pathogen is often spread by poor hygiene, usually due to the use of unsterile cutting tools, which transfer the fungus from one plant to another. Infected leaves become yellow, thin, shriveled, withered, and eventually die. Severely infected plants may die within 3 to 9 weeks, while those with milder infections gradually decline over about a year. The diagnostic symptom in cattleya is a ring or band of purple or pinkish-purple discoloration on the outer layers of the rhizome, visible when cut. If the disease is advanced, the entire rhizome may turn purple, and the discoloration may extend to the pseudobulbs.
• Phalaenopsis and Paphiopedilum: Infestation can be caused by excessively high salt concentrations, low substrate temperatures, and excessively moist or peat-rich substrates. Roots begin to rot, and the base of the plant develops a dry, black rot. Sunken spots on the leaves enlarge, forming yellow streaks, while older leaves become leathery, and young ones may take on a reddish hue. The flower stem can also develop sunken rot spots, resulting in premature bud drop.
Treatments:
• Cattleya: Remove infected parts of the rhizome and pseudobulbs if the purple band is visible. Repot only the part of the plant that is not discolored. Dip the sanitized plants in an appropriate fungicide, following the label instructions. Thoroughly disinfect the growing area and cutting tools, sterilizing them after each contact with infected tissue.
• Phalaenopsis and Paphiopedilum: Remove severely infected plants, remove damaged tissue and all infected roots, and treat with an appropriate fungicide, carefully following label instructions.
Prevention:
• Cattleya: Prevention depends primarily on proper hygiene. Sterilize cutting tools after each use, preferably by flame.
• Phalaenopsis: Maintaining an adequate balance of temperature, substrate humidity, and fertilization helps prevent infection.
5. COLLAR ROT
Sclerotium
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Symptoms:
The main symptom is the rapid collapse and decay of roots, pseudobulbs, and the lower parts of leaves. Roots, pseudobulbs, leaf bases, and the lower parts of stems turn a creamy yellow. Infected tissue turns brown due to the invasion of secondary pathogens, collapsing and rapidly rotting. The disease eventually envelops and destroys the entire basal portion of the plant. Leaves turn yellow, wilt, and die. Small yellow or light brown sclerotia, resembling mustard seeds, form on the infected tissue: this is the dormant form of the fungus.
Treatment:
Remove infected root and leaf parts using a sterile cutting tool. Dip the remaining plant in an appropriate fungicide, following the label instructions.
Prevention:
Proper hygiene, good air circulation, and lower temperatures (if possible) can help reduce the spread of the disease. The pathogen is most active in warm seasons and high humidity.
6. ROOT ROT
Rhizoctonia
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Symptoms:
Root rot occurs when the substrate degrades, drainage is poor, and/or plants are overwatered.
The infection develops when the roots suffer damage due to wounds, salt accumulation from hard water or excessive fertilization, allowing the fungus Rhizoctonia solani to reach dangerous levels.
• Cattleya: Rhizoctonia spreads slowly from older pseudobulbs to younger ones, moving up the stem. Infected pseudobulbs become brown, hard, and eventually fibrous. The roots of older, infected parts of the plant die. Although Rhizoctonia is primarily a root disease, symptoms can also appear on the above-ground parts of the plant.
resembling Fusarium wilt. Leaves turn yellow, shrivel, and eventually fall off, while older pseudobulbs yellow and brown from the base upward.
Treatment: Remove the infected parts of the plant with a sterile cutting tool. Dip the remaining plant in an appropriate fungicide, carefully following the label instructions.
Prevention: Make sure the substrate is fresh and the plants are not overwatered. If you suspect the disease in other plants or if repotting is late, remove the plant from the pot, check the condition of the roots, and repot if necessary. In hard water areas, rinse the pots at least once a month: water thoroughly to dissolve accumulated salts, then repeat the watering process an hour later to remove the salts from the substrate.
7. ANTHRACNOSE
Colletotrichum and Glomerella
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Symptoms:
This fungal disease affects the above-ground portion of the plant, primarily the leaves. Dark brown or light gray spots develop, sometimes arranged in concentric rings or dark bands along the leaf. The infected area is usually well-defined and slightly sunken, while the rest of the leaf appears normal. Fruiting bodies of the fungus form in the affected areas. Flowers develop watery, black or brown pustules, usually raised, which appear on the underside of the older sepals and petals. The spots can expand and cover the entire flower. The pathogen most commonly associated with anthracnose is Colletotrichum gloeosporioides (asexual stage, while the sexual stage is known as Glomerella cingulata). Another pathogen is Colletotrichum theobromicola, previously unreported as an orchid pathogen. This fungus causes a chlorotic mottling on the leaves.
upper surface of the leaf, with corresponding finely dotted spots on the underside. Necrotic or sunken spots may also occasionally appear.
Treatment:
Spray an appropriate copper-containing fungicide, following label instructions carefully.
Prevention:
Proper hygiene, good air circulation, lower temperatures (if possible), and increased light can help reduce the spread of the disease. The pathogen is most active in warm weather, low light, and high humidity.
8. LEAF SPOTS
Symptoms of Cercospora:
The infection begins with a yellow spot on the underside of the leaf. Shortly thereafter, it becomes visible on the upper surface as well. As the spots enlarge into irregular shapes, they become slightly sunken and necrotic.
taking on a brown-purple color.
The spots continue to expand in a circular or irregular pattern and may cover the entire leaf, with a yellow margin advancing. Severely infected leaves tend to drop prematurely, especially if the infection began near the base of the leaf.
Symptoms of Pseudocercospora:
Cercosporoid fungi primarily cause leaf spots and irregular blemishes. Depending on the seudocercospora species and the type of orchid affected, the spots may be circular or nearly so, reflecting the growth pattern of the fungal colony. Sometimes, the spots are purple or black with
a greater number of brown-black spots as they expand. Other species cause smaller, irregular spots, usually in large numbers. On the upper surface of the leaf, the infection may appear as a mosaic pattern, while on the underside, dots form.
corresponding to the spore structures of the fungus.
Symptoms of Guignardia:
The first signs of Guignardia infection are tiny, elongated, dark purple lesions on both sides of the leaf. These lesions develop parallel to the veins and elongate into purple streaks or diamond-shaped areas. The spots may merge to form large, irregular lesions that affect most of the leaf. Over time, the center of the lesion turns leathery. Raised, black fruiting bodies develop in the infected area, with a sandpaper-like texture. This infection is also known as Phyllosticta, as the two names refer to different sexual stages of the same fungus.
Symptoms of Phyllosticta:
Phyllosticta spots can appear on any part of the leaf or pseudobulb. Initial lesions are small, yellow, and slightly sunken. Over time, they become round or oval and deeper, especially if the infection is severe.
It affects the leaves. As they age, they take on a color ranging from light brown to dark brown and develop a margin
slightly raised, red or purple-black in colour.
Eventually, small, black, raised spore-like structures appear in the center of the spots. Individual spots are about 6 mm in diameter. Severely infected leaves may drop prematurely. This infection is also known as Guignardia, as the two names refer to different sexual stages of the same fungus.
Symptoms of Septoria:
The initial small spots appear on both leaf surfaces as sunken, yellow lesions. As the infection progresses, the spots enlarge, turning dark brown or black and taking on circular or irregular shapes. They may merge and form large patches on the leaf. Severely infected leaves drop prematurely.
Treatment:
Spray a suitable fungicide containing copper or quaternary ammonium compounds, carefully following the label instructions.
Prevention:
Maintain good hygiene and ensure proper air circulation. Reduce leaf wetting, as water on the leaves promotes infection. If the fungus is a recurring problem, apply preventative fungicides monthly for effective control.
9. BOTYRITIS
Botrytis
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Symptoms:
Small black or light brown spots appear on flower petals. The spots may enlarge to cover the entire flower. In high humidity, gray mold may appear on the surface of severely infected or decaying flowers.
Treatment:
Remove infected flowers, then spray with an appropriate fungicide. Always follow the label instructions. This fungus is widespread in the environment and cannot be completely eliminated.
Prevention:
Remove infected flowers, as they act as a breeding ground for infection. To reduce the risk of infection, maintain proper hygiene, increase air circulation, reduce humidity, and maintain warmer nighttime temperatures (>20°C).
Collaboration between Criplant and Giulio Celandroni Orchids
Info: St. Augustine Orchid Society
