The whitening of plants' leaves can be a cause for concern. Causes can include but are not limited to: over fertilization, too much or too little sunlight, over watering and improper soil. Knowing the preferred growing requirements of the affected plants can help in making a diagnosis.
Without light a leaf cannot properly develop. This can create a bleaching of normal colors and eventually cause the foliage of a plant to wither and drop off. A solution to this problem is gradually giving the plant more light until its optimum light requirements are met. Taking ill plants from shady locations and placing them in full sun too quickly may cause shock and the possible death of the specimen.
When a plant exhibits whitening/yellowing of leaves, it can be iron chlorosis. This condition is due to a lack of iron available to the plant. At times only part of the plant may be affected while those around them have fully bleached or have no symptoms. The more alkaline your soil is, the greater chance of your plants suffering from this disorder. Trunk injections of iron sulfate, ferric ammonium citrate and applications of iron chelate to the soil are used to treat this problem.
Pests such as white flies, aphids, leaf miners and others will cause damage to a plant and its leaves. Even those attacking the roots and stems of a plant can cause the leaves to lighten and eventually die. Aphids and whiteflies can be found at the base of leaves where they join the stem, on the underside of leaves as well as on the stem. Leaf miners are identified by the small "trails" they leave behind as they munch through a plants' leaf. A quick jet of water will knock some pests from plants or pesticides can be applied. Removing the affected leaves is common practice in the case of leaf miners. Additional leaf miner and aphid control can include parasitic wasps and chemical applications of pyrethrins, paraffinic oil, carbaryl and acephate.
Improper Placement or Soil
These issues can also contribute to bleached leaves. When a plant is placed in an unsuitable area it will look sickly, its colors will fade, it will lose its foliage and in some cases die. The best way to prevent problems like this is to plant only what will naturally thrive in the chosen area. Though soils can be amended and plants doused with fertilizers and various medications and pesticides, you will have less headaches if plants hardy to your area are initially chosen.