Answer: There are two fungal diseases that can cause the symptoms you describe; black rot and bacterial soft rot. Black rot (caused by a fungus, Phytophthora) attacks the leaves, pseudobulbs, and roots of orchids, causing them to turn black. You can cut out the damaged portion of the pseudobulb and remove the affected leaves to stop the spread of the disease. Be sure to isolate the sick plants so the disease doesn't spread to the healthy ones.
Bacterial soft rot symptoms begin with amber-colored spots on the bases of the affected leaves. These turn brown and spread very rapidly, eventually becoming a chestnut brown and then black as the disease progresses. Sometimes it's best to discard diseased plants rather than risk the spread to other plants.
Orchids thrive in average household temperatures, bright light (10 to 15 hours each day), and moist soil. They require moist air, so put the pots on a tray filled with pebbles in which you keep a half-inch of water. Mist the leaves occasionally, too. Orchids don't like cold drafts or direct sunlight. If you're meeting all of these cultural requirements and the plants continue to show signs of disease, you may want to toss out the affected plants and start all over with new plants, sterilized pots and fresh potting soil.
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