Lemon trees are easy to grow, although there are occasional problems with bacterial and viral diseases. The best preventative measures are to keep the ground beneath the trees swept clean of dead leaves and debris, and to spray trees in early spring with an insecticidal soap to prevent insects that spread diseases.
Citrus Canker is a bacterial disease that causes premature leaf and fruit drop. It is highly contagious and spreads rapidly during heavy rainstorms that splatter the tree and when debris comes in contact with the tree as a result of mowing. Leaves turn brown and have raised bumps surrounded by yellow circles or oily margins. Infected trees will eventually stop producing fruit.
Greasy Spot is a fungal disease cause by spores that breed in dead leaves beneath the tree, especially in warm, humid conditions. The spores are airborne and penetrate the lower side of the leaves. After an incubation period of several months, the leaves develop brown spots and eventually drop from the tree. According to Texas Citrus and Subtropical Fruits, the best preventive measure is to keep the ground under the tree free of dead leaves and debris.
Sooty Mold is a fungus that causes the leaves to blacken. The mold develops where insects such as aphids and whiteflies deposit honeydew on the leaves. As a preventive measure, spray the trees with insecticidal soap in early spring to prevent aphid and whitefly infestations. Be sure to coat the underside of the leaves where whiteflies are likely to congregate.
Citrus Tristeza is a viral disease that is spread by aphids. Infected trees have wilting, curling and yellowing leaves. A severe infection will block the tree’s circulatory system causing dieback and the death of the tree. As a preventive measure, spray the tree with insecticidal soap in early spring. Another good preventive measure, according to Purdue University Center for New Crops & Plant Products, is to use lacewings as beneficial insects to prevent an aphid infestation.
According to the University of Arizona Cooperative Extension, nutritional deficiencies are first noticed on trees between their third and fifth year. Iron and manganese deficiencies cause yellow leaves with dark green veins. It is difficult to tell the difference between these two deficiencies by appearance alone, but it doesn’t hurt to treat for both. Zinc deficiency causes a mottled yellow and green leaf pattern between the veins, and if the deficiency is severe the leaves will be small and stunted.