How to Treat Yellow Leaves on an Avocado Tree

Native to Mexico as well as Central and South America, avocado trees (Persea americana) are tropical broadleaf evergreens that are hardy in U.S. Department of Agriculture plant hardiness zones 10 to 12. The leaves of a healthy avocado tree are unmistakably green. Avocado tree leaves turning yellow, a condition known as chlorosis, is an indication that the tree is stressed. It is important to address the cause of chlorosis, which can diminish fruit production.

Photosynthesis, the process by which plants convert sunlight into sugars they can use as energy, takes place in the leaves. Fruit trees, such as avocado trees, use that energy to produce fruits. Healthy foliage is therefore key to a healthy avocado crop.

On a healthy avocado tree, the leaves are glossy, dark green and 4 to 8 inches in length. The leaves of an avocado tree will start to turn yellow as a result of a decline in chlorophyll, the substance that enables the leaves to perform photosynthesis. Leaf drop is also a major problem for avocado trees since robust, well-distributed foliage helps prevent the sun from damaging the tree's prized fruit.

Yellow avocado leaves are usually a sign of nutrient deficiency. The location of the yellowing on the leaves offers clues as to which nutrient the tree is lacking. For example, avocado trees grown in soils that have a high pH can easily become iron deficient, which can cause yellowing between the veins of the leaves. You can correct this by applying iron chelates twice a year in late spring and late summer.

Interveinal yellowing can also be a sign of a zinc deficiency, which you can remedy by applying a zinc spray to the soil or directly to the leaves. Spraying the soil can be more effective. If you decide to spray the leaves, do so in June or July. Nitrogen deficiency, on the other hand, manifests as small leaves that are pale green or yellow and have yellow veins. In this case, apply nitrogen monthly when you water for the first time.

Avocado trees are susceptible to diseases and insect infestations that can result in leaf discoloration. Persea mites, or avocado mites, which are a nuisance in California, extract chlorophyll from the foliage when they feed, which can result in yellow areas on the underside of avocado leaves. If you notice that the yellowing is on the upper side of leaves, another species of mite, the avocado brown, is likely to blame.

A potentially fatal fungal disease known as Armillaria root rot can also cause the leaves of an avocado tree to grow small and yellow and fall off the tree prematurely. You may also notice mushrooms growing at the base of the infected tree after it rains. It is very difficult to save an avocado tree that has fallen victim to Armillaria root rot, and the disease can spread to nearby trees. When Armillaria root rot is in advanced stages, the avocado tree becomes so weak that it can fall over, making it a hazard.