Native to southern Asia, key lime tree, also called West Indian lime and Mexican lime is a shrub-like tree that grows up to 7 feet in height and features slender branches with fragrant leaves and small-sized fruit used in baking, limeade and seasoning. Like other citrus trees, the key lime tree is susceptible to a variety of fungal diseases that adversely affects its health, fruit production ability and quality.
Key lime tree is susceptible to several fungal diseases including Diplodia dieback, Alternaria brown spot, sooty mold, greasy spot and Phytophthora root rot. Each disease damages a different part of the tree, and the extent of damage depends on the size of infestation.
According to the University of Florida, Diplodia dieback is a serious fungal disease of key lime trees, causing infected branches to wilt or droop, and young twigs or branches to die back. Leaves infected with Alternaria brown spot feature brown spots or blotches with yellow margins and black veins, while infected fruit bears sunken spots or pockmarks. This fungal disease also infects young branches, causing complete defoliation in severe cases. The fungal disease sooty mold occurs due to the honeydew excretion of pests including aphids, scales and mealybugs that feed on the tree. It forms a black covering on infected leaves, branches, twigs and even fruit. Symptoms of greasy spot include brown oily or water-soaked spots on the undersides of leaves with light brown or yellow spots on the upper surface. Phytophthora root rot or foot rot is a soil-borne fungal disease that causes the bark in crown roots and the portion of the trunk at soil level to peel, accompanied by a brown resin exudation from infected areas above the ground.
Diplodia dieback damages larger fruit-bearing branches of the tree by causing them to die back if left untreated. Sooty mold impedes the process of photosynthesis--the process by which plants make food--and delays fruit coloring in severe cases. It signifies the presence of damaging numbers of pests on the tree. Large infestations of greasy spot causes defoliation of a key lime tree, making the tree susceptible to damage by severe cold and pests. Phytophthora root rot causes complete or partial girdling of the key lime trunk and tissue damage, that eventually leads to decline, leaf and fruit drop and death, if untreated.
Most fungal diseases on key lime trees can be prevented by practicing good irrigation and sanitation habits. Plant disease-free key lime trees in well-drained soil and always water at soil level instead of overhead. Collect and dispose of fallen plant debris from around the tree, as it may contain fungal spores.
In addition to prevention measures, inspect the key lime tree frequently for symptoms of disease, and immediately adopt relevant control measures to prevent spread. Spray infected areas with copper-containing fungicides every two to four weeks. Control aphid and other pest population to prevent the honeydew secretion that causes sooty mold.