Dents on the Leaves of a Phalaenopsis

Cold Damage

• Your moth orchid can handle a few rare cool nights, but if your plant spends many nights below 60 degrees Fahrenheit, its thick leaves can incur considerable damage. Indoor moth orchids aren't immune to cold damage, especially if they're kept close to a window on cold nights or near an air-conditioning vent in the heat of the summer. Cold-damaged orchid leaves may develop surface lesions or pits that become water-soaked before wilting and browning.

  There's no treatment for leaves that are already injured, but you can leave them on the plant if there isn't extensive damage to tissues. You should remove any badly damaged or collapsing leaves, though, to minimize the risk of secondary fungal or bacterial disease.

Heat Stress

• Moth orchids that become heat stressed exhibit the same symptoms as other plants, including wilt and decline, but recognizing wilt in a moth orchid can be trickier than in other species. Withered leaves on a moth orchid may become leathery or wrinkled, sometimes so deeply they appear pitted -- the more deeply withered the leaves become, the more pronounced the ridges and pits appear.

  Wilting happens when the leaf tissues transpire more moisture than they're taking in, so step up your watering efforts and check the environmental conditions. Keep your moth orchids at temperatures below 80 degrees Fahrenheit whenever possible and water as soon as your plant's growing medium dries out. Wait to fertilize a dehydrated plant until it resumes normal growth, excessive fertilizer when the plant is stressed can burn the roots, thus compounding the plant's problems.

Mesophyll Cell Collapse

• Similar to what happens when exposed to too much cold, in mesophyll cell collapse, the green mesophyll cells inside moth orchid leaves may self-destruct when they are held at temperatures below 45-degrees Fahrenheit for as few as two hours. Unlike cold damage, though, mesophyll cell collapse occurs any time these sensitive tissues get too cold, including during contact with cold water during misting.

  Young leaves are the most susceptible to this problem, especially in late fall and early spring when the environment is changing abruptly, but symptoms may not develop for six to eight weeks. Prevent future problems by misting and watering your orchids with lukewarm water and keeping them in an area with stable temperatures.

Leaf Spots

• Leaf spots are not unusual problems for moth orchids. They're caused by a variety of fungal organisms, including Cerospoora spp., Phyllosticta spp. and Septoria spp. Leaf spots may begin as yellow or purple lesions and then spread into other spots or widen on their own as they darken to a purple, black or brown color.

  Pick off diseased leaves and dispose of them away from your other orchids. In the future, water your plants more carefully to prevent standing water on the leaves -- leaf spot fungal spores generally require standing water to germinate. For especially widespread cases, you can apply neem oil to your plants, mixed at a rate of 1/2 tablespoon per quart in a small spray bottle. Apply neem once a week until the leaf spots stop spreading.