Although numerous diseases affect citrus trees in Florida, many can be prevented or controlled with regular applications of copper fungicide. Diseases that cause defoliation also affect the production and quality of fruit and should be treated as soon as possible. Citrus trees need full sun, well-drained soil, regular fertilization and good air circulation for optimum health and production. Examine citrus trees frequently for signs of diseases, pests or nutritional deficiencies.
Brown spot caused by the fungus Alternaria infects young leaves and shoots, as well as the fruit of citrus trees. The symptoms include brown spots or blotches with yellow margins and blackened leaf veins on the foliage. Affected stems die back and defoliate. The fruit develops brown spots with yellow halos and drops prematurely from the trees. Tangerine and tangelo trees are particularly susceptible to brown spot. It can be treated with copper fungicide applied according to the manufacturer's directions.
Pink to light brown rounded pustules on the fruit are the first symptoms of citrus scab on citrus trees. The pustules turn yellowish-brown and corky, then crack and eventually turn gray. Citrus scab infects grapefruit, orange, tangerine and tangelo trees.
Copper fungicide applied at the rate recommended by the manufacturer can help control citrus scab. The fruit develop a resistance to the disease two to three months after the petals drop.
Yellow spots on the upper leaf surfaces and brown blisters on the lower leaf surfaces are symptomatic of greasy spot on citrus trees caused by the fungus Mycosphaerella citrii. The spots on both sides of the foliage turn greasy-looking and the foliage drops, resulting in a decrease in fruit production.
Greasy spot affects grapefruit, orange, tangerine and tangelo trees. It can be treated with a combination of copper fungicide and horticultural oil applied according to the manufacturer's directions.
The fungus Diaporthe citrii causes melanose on the foliage of citrus trees. Minute circular dark depressions with yellow margins appear on infected leaves. The margins disappear as the spots become rough and raised. Young leaves are often distorted by melanose. There is no effective preventative or treatment for melanose.
Phytophthora Root Rot
Phytophthora root rot affects citrus trees planted in areas with poor drainage. The soil-borne fungus causes the roots to decay, the bark to peel on the trunk at soil level and reddish brown sap to ooze from lesions above soil level. Partial or complete girdling of the affected tree results in leaf chlorosis and drop, and fruit drop, as well as the decline, and eventual death, of the tree. There is no chemical preventative or treatment for Phytophthora root rot.