Lemon trees are a popular citrus trees for home growing because they have bright fruits, lovely foliage and will bloom and bear fruit nearly year-round if kept in the proper environment. Without some special attention, however, there are a few diseases that can attack your lemon tree. Knowing the signs and symptoms of diseases on lemon trees will help you keep your tree healthy for many years to come.
Citrus cankers cause scabby, brown lesions on the fruit, leaves, stems and branches of lemon trees. These cankers are caused by bacterial infections that usually enter the tree through a small, open wound. They can be easily spread by birds, air currents and non-sterile pruning equipment. Remove small affected areas, such as fruits, leaves and small branches, using sterile pruning shears. Larger cankers on the trunk of the tree should be removed during cool weather when the bacteria are dormant and will not simply reinfect the tree. You can also use copper pesticides to treat citrus canker on lemon trees, but do not use them after the tree has flowered.
Root rot, also called "brown girdle," is caused when the phytophthora fungus invades the roots of your lemon tree. It causes wilting, discolored and wrinkled fruit, as well as leaf and limb death. You may also spot hardened, cracked, oozing places on the trunk of the tree. Watering will not help because the tree cannot use its roots to draw water due to the infection. Prune the tree's branches 2 feet off the ground, and remove all affected fruit, leaves and limbs from the area. While most trees cannot withstand root rot, lemon trees may survive if these measures are taken quickly. You can also treat the soil around the tree with a fungicide to help eradicate the infection. If the problem persists, however, you will need to remove the tree to prevent the infection from spreading to other plants and trees.
Sooty mold is a secondary infection caused by an infestation of aphids or mealybugs, which ooze a sweet substance as they feed on the tree. Sooty mold, as its name implies, is black and velvety. It appears mainly on the leaves of lemon trees, but can spread to branches if left unchecked. Remove all affected leaves and fruits and dispose of them in plastic bags. You can treat the mold itself with a copper fungicide, but you will also need to eradicate the insect invasion in order to permanently eliminate the mold.