Yellowing leaves on a Phalaenopsis orchid or moth orchid are not necessarily a cause for alarm. It's normal and natural for older leaves to yellow and gradually drop off. But if all the leaves on your moth orchid are yellow, it could indicate that something is wrong. Several factors can cause the leaves of an orchid to become discolored, including direct sunlight, low temperatures, and root rot.
Natural Aging Process
Older Phalaenopsis often have somewhat elongated stems where old leaves have dropped away. New roots will continuously emerge from the stem of healthy plants, eventually forming a mass of roots.
If you discover a yellowing leaf on the bottom of the plant, don’t worry. This yellowing is a natural process of the plant to discard the mature leaf to produce a new leaf. However, if the leaves are yellowing from the top of the plant, there is a problem.
Too Much Light
The leaves of a Phalaenopsis 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. 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.
Next, ask yourself if the temperature is right. 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. Too low temperatures can also cause orchid leaves to turn yellow. Keep the orchid away from open windows, fans, or air conditioning vents, as it prefers warmer temperatures and relatively 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 orchid potting mix. Mist the leaves the first week in place of watering.
To avoid overwatering, water your Phalaenopsis with three ice cubes once a week so that the roots will soak up water slowly.
Moth orchids need fertilizer during the summer, with a diluted orchid fertilizer usually every third to fourth week. If orchids do not get enough nitrogen and magnesium, they have trouble producing chlorophyll, turning the leaves yellow. If your orchids’ leaves begin to yellow around the tips and the sides, your plant is likely low in nitrogen, magnesium, or both. Feed regularly over the summer, but halt fertilizer during the fall and winter and the spring flowering season.
Fungal or Bacterial Diseases
Yellowing leaves could be caused by a fungal infection that starts as yellowing areas on the bottom of the leaves. If the leaves are yellow and you notice a foul smell, it could be a bacterial or fungal infection. In most cases, it's caused by too much watering and may eventually lead to crown, root, or stem rot. Eventually, it will turn black and affect both sides of the orchid leaves.
If you suspect it's a fungal or bacterial problem, separate the orchid from other plants to prevent the disease from spreading. Use a sterile pair of scissors to remove the affected area. Spray the plant with a fungicide to help it fight off fungal issues.
Spider mites, scale insects, thrips, and mealybugs are common invaders of moth orchids. They reproduce and spread fast. They are often hard to see unless you inspect them carefully with a magnifying glass or look carefully under leaves. Some common telltale signs of a potential pest invasion include webbing or discoloration spots where bugs feed off the foliage.
Mites are tiny greenish-yellow bugs about the size of a sharp pencil point. They are fast movers, barely visible to the naked eye. Thrips can get bigger than mites, about 1/8-inch long, with thin bodies and wings. Mealybugs can almost get about 1/4-inch long with a white, fuzzy appearance; Scale insects are about the same size as mealybugs but are darker, rounder, and have a glossy-looking sheen to their armor-like outer shell.
You can wash many of these little bugs away with a strong blast of water. Also, physically wipe the leaves with a water-moistened washcloth or paper towel. For a significant infestation, spray the plants with a horticultural oil like neem oil or insecticidal soap. The best time to spray them is in the morning. After 15 to 30 minutes, hose off the oil or soap before the sun's intense rays reach the plant. Keep infected plants away from your healthy plants until the infestation has been remedied.