If your plant's leaves are turning white, it probably looks pretty ghastly. However, before you go on the hunt for a plant vampire or toss your plant in the garbage, try troubleshooting a few basic plant problems that could be causing your plant's paleness. Many of the causes behind white leaves are actually extremely easy to fix and your plant can recover in no time.
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If your plant's leaves appear to be coated in a chalky, dusty, white powder, then your plant probably has powdery mildew. Remove all affected leaves and dispose of them in a garbage bag to prevent reinfection. Move the plant to an area with plenty of light and good air circulation. Water deeply but seldom so that any lingering spores will starve rather than germinate. Continue to monitor the plant for further signs of infection. You may have to prune off problem leaves a few more times. However, powdery mildew seldom actually kills a plant as long as you are careful to improve the lighting and minimize watering habits--like watering repeatedly throughout the day--that could create a humid, optimal environment for the mildew to grow.
Spider Mites and Mealy Bugs
These insects suck the juices out of the leaves of plants, causing the leaves to appear washed out or even to fall off. Mealy bugs look like fuzzy balls covered with webbing, while spider mites are tiny white spots on the leaves. Both of these insects can drain a plant of color and liquids, causing the white leaves. Generally, these bugs will first create small white areas on the leaves, then the areas will spread to turn the entire leave white. If you have a spider mite or a mealy bug problem, the discoloration will likely extend to the stems of the plant as well. Because spider mites are not insects, they do not react to most insecticides. Make sure that you are using a mite-targeted treatment. Mealy bugs can be treated with diazinon. Some people just wash their plants in a mild soap to get rid of both types of infestation.
Iron chlorosis is a vitamin deficiency that washes out plants' leaves and can leave them pale or entirely bleached if the problem is serious enough. This problem occurs in alkaline soils more frequently than under other conditions. You will need an iron-supplemented fertilizer, either ferrous sulfate or iron chelates. If the plant in question is actually a tree, then you may need to give it injections of iron sulfate or ferric ammonium citrate to restore the leaves to a normal color.