Yellowing leaves on a Phalaenopsis are not necessarily a cause for alarm. It's normal and natural for older leaves to yellow and gradually drop off. Older phalaenopsis often have somewhat elongated stems where old leaves have dropped away. On healthy plants, new roots will continuously emerge from the stem, eventually forming a mass of roots.
If you discover the yellowing leaf is located on the bottom of the plant, don’t worry. This is a natural process of the plant to discard the mature leaf in order to produce a new leaf. However, if the leaves are yellowing from the top of the plant, there is a problem.
If the leaves on your Phalaenopsis orchid are yellow, it could be an indication that something is wrong. There are a number of factors that can cause the leaves of an orchid to become discolored, including direct sunlight, low temperatures, and root rot.
Too Much Light
If the leaves are still plump and firm, but they're yellowing, the plant is most likely receiving too much light, and it's washing out the color. The leaves of a Phalaenopsis orchid can burn and turn yellow if they are exposed to direct sunlight. Try putting your orchid in a place that receives sufficient indirect sunlight. If you're placing the plant on a windowsill, try to make it a north- or west-facing window.
Next, ask yourself if the temperature is right. Too low temperatures can also cause orchid leaves to turn yellow. Make sure the temperatures around your orchid are between 65 and 80 degrees Fahrenheit during the day and 60 and 70 degrees Fahrenheit at night. Keep the orchid away from open windows, fans or air conditioning vents, as it prefers warmer temperatures and fairly high humidity.
If the leaves are wrinkled and listless, the plant is most likely dehydrated. If not, check the roots. Overwatering can lead to root rot, which can, in turn, cause its leaves to turn yellow. You should only water the plant when the top 1 inch of the potting medium is dry and the roots are white. Make sure there are enough holes in the pot to allow proper drainage. If your orchid is suffering from root rot but you see your plant still has some healthy green roots, trim the rotted roots and repot the plant in new media. Mist the leaves the first week in place of watering.
To avoid overwatering, water your Phalaenopsis orchid with three ice cubes once a week, so that the roots will soak up water slowly.
Fungal or Bacterial Diseases
Yellowing leaves could be caused by a fungal infection that starts as yellowing areas on the bottom of the leaves. Eventually, it will turn black and affect both sides of the orchid leaves.
If the leaves are yellow and you notice a foul smell, it could be a bacterial infection. In either case, separate the orchid from other plants to prevent the disease from spreading, then use a sterile pair of scissors to remove the affected area. Spray the plant with fungicide to finish it up.