Most orchid diseases are caused by either fungi or bacteria. Identifying which disease is affecting your orchid plants is critical if you are wanting to give the plant the correct treatment. Before we discuss the fungal and bacteria diseases, we ought to first take a look at other, non-disease, factors that maybe affecting the plant. If the leaves are streaked either brown or black, or become mottled, then this could be a sign of a virus infection. There is no cure for orchid viruses, so they best thing you can do is remove the infected plant away from other healthy plants and see if it recovers in isolation. Probably the most common sign of an unhealthy plant is yellow leaves. More than likely these are just old leaves dying back, or the plant is being fed too little nitrogen. Other causes of leaf yellowing are too much light, loss of roots or the temperature of the room is too low. If the pseudobulbs are starting to shrivel, this could be sign that the plant is not getting enough water.
Fungal infections usually manifest themselves as small brown spots that eventually turn black. Yellow spots, with a black edge, are another sign of a fungal infection. The main causes of fungal disease in orchids are poor air circulation and a too high humidity. Fungi infections in orchids can be treated with any plant fungicide sold at your local garden store. Another well-used treatment, also available at garden stores, is Bordeaux mixture – a combination of water, hydrated lime and copper sulfate.
If your plant has brown/black smudges on the leaf edges, this is a sure sign of bacterial blight. Providing the infection is not too advanced, treat the plant with Physan. If the orchid’s leaves are turning black and wilting, then the best you can do is immediately remove it from other plants and remove all infected areas. Repot the remaining healthy plant into a new pot with new potting mix.
Unhealthy plants are more susceptible to parasites and disease. Low humidity and poor air circulation weaken plants. Be sure to address any of these underlying problems while treating specific issues:
Scale -- These small, immobile brown lumps on the stems, leaves, or flowers can become a persistent problem.
Aphids - Soft-bodied, small insects can be green or black. They usually attack new leaves or flowers.
Mealybugs -- These white parasites look like little pieces of cotton.
Spider mites -- While these insects are too small to see, they can still cause a lot of damage. Fine webs on the undersides of leaves and a bronze or yellow color to the leaves can indicate their presence. They like dry air, so keeping the humidity high will discourage them.
Snails & slugs -- These can eat leaves, flowers, or roots.
Viruses -- Unfortunately viruses in orchids are difficult to identify and are usually incurable. Symptoms to watch for include deformed flowers, streaking patterns on leaves, and deformed growth. Discard infected plants.
Pets -- Some animals do not bother plants, but others may chew them. Keep plants out of reach. Protect new flower spikes.
Remedies Always try to limit use of artificial chemicals, which can be dangerous for you and the environment. Remember that insecticides often kill beneficial insects as well as harmful ones. Using artificial chemicals inhibits Mother Nature's ability to take care of herself.
Beneficial insects are one of the best solutions. Many garden stores sell ladybugs and praying mantises to release in your garden. Spiders eat many plant pests.
Soapy water sprayed on the tops and bottoms of leaves can safely and effectively kill pests. You can buy insecticidal soaps at a gardening center, or make your own using a little mild, unscented soap mixed with water. Insecticidal soaps only kill insects they have direct contact with.
Rubbing alcohol is effective for controlling Scale and Mealybugs. Use a cotton swab dipped in rubbing alcohol to wipe the bugs off. Rinse the alcohol off the plant with clean water afterwards.
Insecticides with natural ingredients that decompose quickly, such as pyrethrins, should be used only when absolutely necessary. Always follow the instructions.
Horticultural oils are available at garden stores and kill many pests. Always follow the instructions.
Copper strips are effective barriers for slugs and snails, which can be very destructive for outdoor orchids.