Orchid Disease
There are many types of orchid disease. Orchid pests are fairly common, and have their own page. There are also fungi, molds, viruses, and weeds that can attack orchids, as well as various types of environmental damage such as sunburn, drought, or cold damage that can be prevented by better orchid care. I'll describe some of these orchid diseases below, and keep adding more as I think of them.Overwatering and Underwatering
Underwatering tends to result in plants losing leaves; most orchids won't wilt when dry. Pseudobulbs will shrivel, though. New growths may fail to expand properly and have accordion-like creases, either lengthwise or perpendicular to the leaf's central crease. Or the plant may simply be weak and fail to bloom. Overwatering produces similar symptoms to underwatering: it causes the potting mix to break down, smothering the roots, or may simply cause the roots to rot. In either case, the plant can't get enough water anymore! If a plant has underwatering symptoms but you're not sure whether you're overwatering or underwatering, inspect the root system to find out if it's rotten; try to lift the plant out of the pot with its potting mix to look at the roots. (Clear plastic pots make monitoring root health easier, so I like to use them.) If the root system is in bad shape, repot the orchid, removing all the old, dead roots. If it has few roots left, you may want to raise the humidity to help it recover.Orchid Weeds
If another plant is competing with your orchids, such as ferns, Oxalis, or anything else, it's a weed. It might be a nice plant, but it's not in the right place, so either move it to another pot or kill it viciously. You might have to repot the orchid to get rid of it. It's best to do this before the weed goes to seed, or the problem can multiply through an entire orchid collection.Orchid Viruses
There are several kinds of viruses that can affect orchids. These typically appear as stripes or streaks on leaves and/or flowers, usually of dead, brown tissue. Orchid viruses are incurable; either discard the plant or isolate it from the rest of your collection. Different viruses affect different types of orchids, and each has its own characteristic pattern of dead tissue. But identifying the virus is irrelevant since they are all incurable. To avoid spreading orchid viruses, sterilize any cutting tools you use on your orchids in between plants. You can do this with bleach, blowtorches, or in the oven. You can also use disposable razor blades that you throw away after each plant. This should be done any time you trim orchids, such as when removing old roots in repotting, or when dividing orchidsBacterial Infections and Fungal Infections
There are many possible bacterial and fungal infections of orchids. These are most common in plants that are being overwatered (e.g., root rot) or whose foliage is often wet. Automatic misting systems can cause the latter problem, as can letting water settle into the crown of a plant (to which Phalaenopsis are especially vulnerable, since the configuration of the leaves prevents the water from draining away). To help avoid all the various kinds of rot, it is best to avoid overwatering, and to water plants early in the day as they will tend to dry more quickly when there is still plenty of light and temperatures are warmer. Also make sure there's plenty of air circulation, such as by positioning fans near your orchids. For leaf rot, cut off the infected part of the leaf. Crown rot also requires cutting off the infected tissue, though you'll feel really bad about it since that's the plant's growth tip! For root rot, repot the plant, taking care to trim off all dead and dying roots. Though orchids don't like being pruned, pruning orchids to remove diseased tissue can save a plant's life. Sometimes fungal diseases will affect orchid flowers, producing various types of brown spotting. Remove affected flowers and hopefully it won't spread to the rest. Also be sure to discard the flower stem promptly when the plant goes out of bloom. You can also use a bactericide or fungicide such as Phyton 27 to help control most of these infections. Follow the manufacturer's instructions.Sunburn
If leaves are exposed to enough sunlight that they overheat, those parts of the leaf will die, leaving brown or black splotches. This damage is generally irreversible; the best thing to do is reduce the lighting. Plants can also adapt to higher lighting over time; sunburn is most common when a plant has just been taken outside for the summer and is getting too much sunlight!Cold Damage
If you've been growing orchids outdoors, you might occasionally see cold damage from unusually cold weather or as winter approaches. It's best to check the weather forecast and bring plants inside before this happens. Should a freeze affect your orchids despite your best efforts, many of your orchids will survive if you're lucky. There might be some damaged leaves, though; affected parts of the leaves tend to collapse and become thin, as frost crystals have punctured the cell walls. Generally, there's nothing to be done in response to this damage, though you should remove any large portions of dead tissue.Fertilizer Burn
If the tips of the leaves turn brown, this may be due to too much fertilizer, or to fertilizer salts having built up in the orchid's potting mix. In the latter case, simply repot and make sure to flush the plant with water regularly to prevent salt buildup in the future. Of course, if you're overfertilizing use less fertilizer! The general rule for orchids is to fertilize "weakly, weekly".Talk about YOUR experience with orchid disease!
What orchid diseases have you been experiencing? How have you dealt with them? Do you have a question about how to treat a disease? Post about it! Photos are especially helpful, particularly if you're trying to find out what's wrong with a plant. It also helps if you give the name of the affected orchid, as some diseases are specific to certain orchid types.
Other Visitors' Posts
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possible root rot? 
I noticed foul odor coming from water in phrag bessae dish--smells like rot. Then flower dropped off today. I thought they needed to be left with pot sitting …
Crown rot
My beautiful Phalaenopsis Orchid was growing. Beautiful roots new leaves. Then I noticed the leaves not looking quite right and turning yellow. I checked …
FUZZY WHITE GROWTH ON FLOWER STEMS AND POTTING MIX AS WELL s on the orchid itself.
I am trying to root a phalanopsis. It started to grow another leaf and more flower spikes on the old stems but no roots. I cut off the flower spikes …
white spots on leaves
please let me know what to do for 2 of my orchids.
they have white fizzy spots on them.
thank you
Bacteria or Virus??
I bought a phalaenopsis a few months ago from a Farmer's Market (in Ca). The blooms soon started to wilt, I repotted it to find a mangled smaller plastic …