Key lime (Citrus aurantifolia) has almost too many nicknames to list. Among the most common are Mexican lime, West Indian lime and bartender's lime. Famous for its pivotal role in authentic key lime pie, this small, tart fruit is easy to grow. The mature tree is only about 12 feet high, though thornless varieties are smaller. The white flowers are deliciously fragrant and are followed in early summer by small, 1.5- to 2-inch fruit. Key lime grows best in U.S. Department of Agriculture plant hardiness zones 9 and above. Several diseases and insect attacks can cause brown spots on key lime foliage.
Citrus red mites attack the upper portions of the leaves, causing brown spots. If the mite population is heavy, the mites feeding on the inner sap of foliage can cause the leaves to die and fall from the tree. Two-spotted spider mites hide in protected areas of the tree and feed mostly on the undersides of the leaves, causing mottling and brown spots. Try spraying citrus red mites with a miticide containing sulfur. For two-spotted red mites, keep the trees adequately watered to eliminate the dry, dusty conditions they prefer. A hard spray from the garden hose can knock most spider mites off the key lime tree. For a particularly heavy infestation, spray the tree with a commercial miticide formulated specifically to control spider mites.
Alternaria Brown Spot
If brown spots are appearing on young shoots as well as the key lime's leaves, Alternaria may be to blame. Check the leaf veins for blackening, a telltale symptom of Alternaria disease. The brown spots may have yellow margins, or "halos," and the disease may eventually jump to the fruit, causing sunken, black to brown spots. Prune the key lime tree, if necessary, to improve air circulation, as wet foliage is a highway for Alternaria fungal spores. Prune heavily infected areas using sterilized cutting tools, and dispose of the material away from the garden. Spray the tree with a fungicide containing copper, and follow label directions for proper application rates and timing of repeat sprays.
A fungal disease, greasy spot first appears on the underside of the leaves, causing blisters that penetrate the leaf to form yellow spots on the upper surface. The yellow spots and blisters darken to brown and, as the name suggests, look greasy. Infected leaves fall prematurely, reducing fruit production. Spray infected trees with a copper fungicide mixed with oil. This type of fungicide clings to the tree longer, providing more protection.
Although this disease, caused by Botryosphaeria ribis, is usually considered minor, left untreated it can turn entire trees brown and lifeless. The infection may appear in scattered areas of the tree, causing those areas to decline and die. The disease then spreads to the rest of the canopy. Sunken, grayish cankers and gum exudate indicate the presence of gummosis. Young trees are especially susceptible. Remove infected areas as soon as you find them, and destroy the material away from the yard. Avoid wounding the tree with gardening tools and lawn mowers, as this opens the door for gummosis infection. Keep the tree as healthy as possible to help it resist and recover from infection.
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