Bacterial infections and what to do with them
The most common diseases of orchid plants are fungal. These might be foliar blights, leaf spots, fungal rots, and flower blights. However there is also a bacterial rot which can diminish orchid health. Determining which disease your plant has is critical to treating orchid diseases. Most common orchid diseases can be prevented or cured, especially if it’s caught early. Just as with pests, it is important to monitor plant health frequently and act immediately if any abnormal conditions occur.
Orchids come in many sizes, colours, and growth forms. The majority of these wonderful plants in cultivation come from rainforest areas where temperatures are temperate to tropical. There are also species which thrive in arid conditions, but these are not widely grown. Orchid plant diseases are most likely to occur when excess moisture stays on leaves and flowers, and when soil has poor drainage. Cultural changes and even a site transfer can minimize disease as can good sanitation procedures.
Orchids are exposed to excessively high temperatures or humidity levels that if are higher than optimal for any length of time, they are at risk of developing orchid bacterial or orchid fungal diseases. Bacteria and fungus thrive and reproduce rapidly in hot humid environments. If they attack your orchid and are not treated promptly, bacterial and fungal diseases can spread quickly, possibly causing permanent damage or even killing your orchid.
If you’ve noticed that your plant has spots on it and it doesn’t appear to be hosting mites or another bug, it could be a fungal or bacterial infection. In this case, you can use your nose to help you determine the issue.
- Bacterial: An orchid with a foul smell and discolored leaves is likely the victim of a bacterial infection.
- Fungal: If your orchid has a spotty appearance but smells fine, it’s likely a fungal infection.
We are going to discuss bacterial diseases further now.
If the leaves of your orchid are becoming discolored or your plant has a foul smell, a bacterial disease has most likely infected your plant. While bacterial diseases will not respond to fungicides, they should be treated with a fungicide in the same manner as orchids infected with fungal disease. Bacterial diseases weaken orchid plants, making them more susceptible to secondary fungal infection. Applying a fungicide prevents secondary infection and increases your orchid’s chance of recovery.
Bacterial orchid diseases are caused by high temperatures and high humidity and usually have an obvious visual presentation. If daily inspection of your Orchid reveals brown or soft spots; a bacterial disease is the likely culprit
The most common bacterial orchid diseases are:
Bacterial Brown Spot - Acidovorax avenae subsp. catteyae (formerly Pseudomonas cattleya) a bacterium begins as a soft, watery lesion, usually on the orchid’s leaf, that eventually turns brown. In advanced stages, the brown spot will begin to exude a foul-smelling dark liquid.
Seedlings are infected through stomata, older plants through wounds. Bacteria in exudate spread from orchid to orchid by splashing water and from place to place with infected plants. Contaminated propagation tools are another way to transmit the bacteria. Warm, moist conditions and high nitrogen fertility favor the disease.
Starting as a soft, water-soaked spot that later becomes brown or black cavity. These spots expand and can involve the entire leaf. On older plants, it starts anywhere on a leaf and finally reaches the growing point, where a mucilaginous exudate may be produced. Plants may be killed.
On Cattleya, the disease advances more slowly and is limited to older leaves, producing clearly delimited, sunken, black spots; it is not fatal to the plant.
Bacterial Soft & Brown Rot - Pectobacterium (syn. Erwinia) 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.
Paphiopedilum. Leaves develop small, round spots often near the middle of the leaf. The spots 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.
How to treat bacterial orchid diseases:
- First, move your orchid away from other plants.
- Then, remove infected foliage using a sterilized razor blade or pair of scissors. Ensure that you have clean clear margins
- Dress the the cut foliage with Flowers of Sulphur powder to minimize further infections from entering.
- Spray the plant with a fungicide. While fungicide won’t affect the bacteria, it should still be applied to a bacterial infection to prevent a fungal infection from forming.
- Relocate your orchid to expose it to better air circulation, lower humidity and temperatures between 18ºC and 26ºC
Prevention:
The disease is commonly 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).
Practice good hygiene. Many diseases (bacterial, fungal or viral) can also be spread mechanically ie. by hands or tools. Sterilize your hands, tools and workbench between plant. Use household bleach to spray your benches down, soak tools in bleach between use and wash your hands with antibacterial soap.
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. 
Acidovorax avenae is a water-borne pathogen that prefers warm, moist conditions. Reduce humidity and temperature (if possible), eliminate overhead watering and increase air circulation.
In as little as two days, the virus can rot the extremely sensitive Phalaenopsis. In Vanda, the spots become translucent while in Dendrobium, the patches become black and sunken. Copper fungicides may be used except on Dendrobium and during flowering or you can use hydrogen peroxide. Simply spray hydrogen peroxide on the plant and any neighboring plants, as the infection can spread quickly.