I don't mean to be bringing you more doom and gloom. Orchids are actually very tough plants, and if you grow them in the correct cultural conditions and take the preventative measures mentioned, they'll rarely suffer from fatal diseases. Still, being aware of what can happen when things go wrong is a good idea.
Diseases are somewhat trickier than bugs to deal with because you really can't see them. You just see the damage they leave behind — such as rotten plant centers or spots on the leaves.
Look for the Symptoms
The damage that most fungal and bacterial diseases leave behind are circular or oblong spots on the foliage or flowers.
Sometimes they also cause the center growing point, called the crown, to turn black or rot and may lead to the death of the plant. When this happens, the crown has been killed by the organism and the pattern of spots or rots is a symptom of the disease and is one of the ways that a disease is identified.
Crown rot on a phalaenopsis orchid
Fortunately, most of the controls that I mention are effective against a broad range of disease problems, so an exact disease diagnosis is seldom necessary to remedy the problem.
Viruses are dreaded by orchid growers because there is no practical cure for them. The most obvious symptom of a virus infection is streaking or color breaks in the flower.
For more info on virus, check out this good info from the American Orchid Society.
If you suspect virus you can obtain a virus test kit. For sure you want to isolate this plant from others. I f you suspect virus, it is usually wise to discard this plant rather than running the risk of the virus spreading to other healthy plants.
Prevention is Key
Here are a few other pointers that relate specifically to disease:
* Water your orchids during the day when the moisture will evaporate from the leaves before nightfall. Cool, damp leaves and water left in the crowns of the plants in the evening are an invitation to disease.
* Make sure your orchids have enough airflow. This reduces the time moisture stays on the leaves and removes stagnant air.
* Remove diseased leaves with a sharp sterile knife or scissors. (See below "When Surgery is Required")
* Sterilize your cutting tools each time you use them on another plant.
* When you find a disease problem treat it right away. Procrastinating could cost your orchid its life.
Here are the steps that I recommend:
1. If the plant is badly diseased, discard it. You probably won't be able to save it, and it could infect your other healthy plants.
2. If you find dark brown spots that look like disease and they are close to the end of the leaves, remove this section of the leaf. See "When surgery is Required."
3. As a general sanitation practice, after you've preformed surgery or if your orchid has a disease spot that cannot be removed surgically, spray the leaves with a mild fungicide/bactericide and hope for the best. Physan 20, Phyton 27, Natriphene, or RD-20 are all mild fungicides/bactericides that work for this purpose.
4. Reevaluate the area you're growing the orchid in to be sure it's getting enough air circulation and you're doing all you can to follow the recommended disease preventative measures.
All chemical pesticides are poisons that have some toxicity to humans. Read the precautions on the pesticide label and follow them carefully. Wear rubber gloves (the disposable ones work great) when mixing and spraying these materials.
When surgery is Required
Performing surgery to cut out the infection is the simplest and most effective method of stopping the spread of disease. A single-edge razor blade is ideal for the job because it's extremely sharp and sterile and can be disposed of after the operation. The sharpness is important so that as little as possible of the healthy tissue is damaged in the process.
Remove all the damaged or diseased leaf by cutting the leaf off about 1/2 inch to 1 inch into healthy leaf tissue that shows no signs of the disease. Be careful not to cut into the diseased tissue and then into healthy tissue, or you'll spread the disease.
Some people dress the edge of the cut with a simple fungicidal material like sulfur or cinnamon, but this usually isn't necessary.
So provide the right growing environment, follow a good prevention program, and be vigilant to catch any disease problems early.