What Causes Yellowing in Plants?

When a plant turns yellow, a number of different causes may be at hand. Without knowing what to look for, solving the issue proves difficult. Plants sometimes yellow due to various growth and environmental factors, as well as potential injuries and various diseases.

Overwatering a plant contributes to the yellowing of the plant. An excess of water in the soil depletes the oxygen within the soil. Without adequate oxygen, the root system of a plant dies out, which causes the plant to turn yellow. Too much fertilizer can also harm the root system, which leads to the plant turning yellow. Alternately, too little fertilizer can cause leaves to turn yellow, which can be remedied by using a high-nitrogen fertilizer. Yellowed leaves on an azalea plant, according to Texas A&M University, means the soil lacks the acidity level needed for the plant.

Exposure to gases can cause plants to turn yellow, particularly tomato plants. So a plant turning yellow might indicate a gas leak in the home. A sudden drop in temperature might bring about yellowing of a plant. Too much sunlight might also turn a plant yellow. Alternately, if a plant doesn't receive enough light, it may turn yellow. Plants need light in order to maintain their green color, as light helps chloroplast production.

If a plant receives physical injury to an area such as a leaf, that area might turn yellow. If a plant has been transported recently and then turns yellow, the plant likely experienced transplant shock. Plants can also receive injury from a number of pests that feed on them. An infestation of spider mites, aphids or any other pests will damage the plant. This damage causes yellowing and death, in some cases. A yellow plant that has stunted growth often has root knot nematodes invading the roots.

Several different plant diseases lead to various parts of plants turning yellow, most commonly the leaves. Viral diseases such as yellow mosaic cause plant leaves or flowers to turn yellow. Some fungal diseases may also cause yellowing. Rust, for instance, might cause yellow powder to form on the leaves. Early blight, commonly found on tomatoes, stems from a fungal infection that causes leaves to turn yellow with dark spots. Bacterial diseases can also lead to plants having yellowed parts and experience wilting.