There are a number of different conditions that can cause plant leaves to become mottled yellow, from insect activity to nutritional deficiencies and bacterial infection or mold, mildew and other fungi. Diagnosis of the cause of yellow mottling in plant leaves requires knowledge of the plant's species and common name for research purposes. Overall, it is difficult to pinpoint a single, universal cause of this condition in plant leaves, and plant owners will need to diagnose the condition based on specific symptoms.
One way to potentially narrow down the cause of yellowing leaves on a plant is to determine whether the leaf falls off the plant quickly after turning yellow or whether it stays on the plant for a while after its color starts to change. The leaves that stay on the plant after yellowing may be the result of chlorosis, which is a loss of chlorophyll that itself has several different causes. The leaves that yellow and subsequently fall off the plant quickly may be the result of factors such as weather conditions, insect infestation or the simple process of aging.