The diseases that attack a lemon tree will attack all citrus trees. Careful pruning will take care of most of the problems, as long as you catch it early. There are fungicides that are helpful and your local garden center will know what is best and legal to use in your area.
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Citrus tristeza is a virus that causes a young seedling to turn yellow, the stems to become pitted or the rootstock to go rapidly downhill. In the most severe cases, the trees will be lost.
Citrus canker is a contagious bacterial infection that affects the leaves by causing a yellow lesion in the shape of a halo and/or scabs. It can also affect the fruit and twigs. Be sure to disinfect pruning equipment after use .
Brown rot is more commonly known as root rot. It will appear on the trunk as dark brown oozing patches. Root rot is normally caused by too much watering or planting in poorly drained soil.
Citrus Whitefly Infection
Citrus whiteflies infect the lemon tree by eating on the underside of the leaves. The leaves will curl up and become sticky. Remove and destroy any infected leaves because that is also where they lay their eggs.
Brown Soft Scale
Brown soft scale is a tiny insect that lives mostly on the wood and leaves, but can attack the fruit as well. It will suck the sap out of the tree until the leaves become yellow and drop off. The insect looks like a natural bump on the tree.