Why do citrus trees have, in general, an unhealthy appearance?
Some citrus trees look unhealthy because of a common fungal disease called gummosis. Such trees are sparsely foliated with much twig dieback. Trees become infected when fungal spores on the ground splash onto the trunk. If the trunk remains wet for many hours, whether from rain droplets or irrigation, infection takes place. The fungus attacks and kills the bark but will not penetrate into the wood. If these lesions are discovered early, the fungus can be stopped and the tree saved. But once the infection spreads and kills more than a third of the bark tissue around the trunk, the tree will either die or produce very poorly.
Citrus trees will produce an amber color gum as a defense against the invasion. The gum exudes from the point of infection, trying to wall off the fungus. In a rainy climate, the globes of gum readily dissolve in the rain. Due to fluctuating temperatures, the fungus may die out on its own. When this happens, the infected bark will dry up and crack. New bark will eventually begin to grow around the wound. There is no sense in treating if the bark appears dried up and cracked; the fungus is gone.
The best control for this disease is prevention. Nothing can be done about the rain, but if trees are irrigated, the water must not hit the trunk of the tree. Keeping the trunk dry is essential to the health of the tree. There are fungicides registered to control citrus gummosis (Phytophthora species). But for a practical control measure for the homeowner, simply cutting out the diseased bark and painting the area with a registered copper fungicide will rectify the problem. In removing the diseased tissue, only the bark should be removed; there is no need to cut into the wood. The diseased bark will be near the gumming and will be discolored (brown). Healthy tissue is light in color. Along with cutting out the discolored bark, it is recommended to cut about an inch into the healthy bark. Furthermore, applying a pruning compound to the diseased area is of no value.