Orchids shed older leaves, and it's normal for them to turn yellow and drop off. However, if your orchid has young leaves that are wilting and turning yellow, it means there's a problem. Knowing why orchid leaves turn yellow can help you identify what that problem is.
Orchid leaves turn yellowish-green when they're exposed to too much light, because highlight degrades chlorophyll. In addition, exposure to direct sunlight can burn leaves, causing scorched yellow blotches to appear. Scorched areas turn brown or black as the leaves dry. Provide 12 to 16 hours of light per day, and don't expose orchids to direct sunlight between 11 a.m. and 2 p.m.
Aphids are small, greenish-white or black, soft-bodied insects that feed on plant sap. They can often be found on the undersides of leaves. Since they seldom grow more than 1/8 inch long, it may be difficult to see them. Look for a pile of shed white skins, which aphids leave on the upper leaf surfaces as they're developing. Symptoms of aphid infestation include wilting, yellowing leaves; deformed growth; mottled buds; and the appearance of honeydew on leaves and stems. Honeydew is a sticky material excreted by aphids. It attracts the sooty mould fungus, which makes leaves look black.
Thrips are tiny insects that feed on plant juices, causing yellow areas to appear on the leaves. In addition, the leaves wilt and drop. Flowers become spotted, discoloured and turn brown prematurely. Plants can be stunted, and severe infestations can even kill orchids. Thrips only grow to be 1/16 to 3/8 inch long, so they're hard to see, but if you softly blow into an open flower, you may be able to spot them crawling around inside. Thrips are very difficult to control. Infested plants should be segregated and all weeds and plant debris should be removed from the area. Insecticidal soaps can be used for control.
Fusarium wilt is a fungus that causes stunted plant growth, yellowing and wilting. Leaves may appear shrivelled and water-soaked. The fungus begins in the roots and moves up the plant. A pinkish-purple discolouration develops in the stem and the rhizome, and the roots rot. This disease can be spread by soil, wind, water and fungus gnats, and is controlled with the application of several fungicides, such as chlorothalonil and azoxystrobin.
Several leaf spots diseases are caused by different fungi and begin with the appearance of yellow areas on the undersides of leaves. The yellow spots caused by cercospora eventually cover the whole leaf. Old spots turn purplish-black and the leaves fall off. Septoria spots can appear on either side of the leaf. They look like small, sunken lesions that grow into irregular yellow patches. Rust fungi form orange, yellow or rust brown blisters on leaves. Plants with this disease should be isolated.
Orchids are prone to a number of viral diseases that can resemble insect damage, fungal diseases or the effects of air pollution. Symptoms vary depending on the virus, but stunted growth, tissue death and yellowing leaves are all symptoms. Viral diseases can't be cured, and infected orchids should be destroyed. To prevent viral infections, purchase virus-free plants and wash your hands before handling them. Control insects and disinfect cutting tools with a 1:10 bleach solution to prevent diseases from spreading.