Oranges and other citrus fruits are susceptible to a number of diseases. Some of these have not yet been found in Florida, but others are very common and affect leaves, twigs, fruit and the trunk of the tree. Good cultural practice is important, as is early detection and choosing healthy, vigorous rootstock.
Scab appears as a rash of scabs or warts on leaves, twigs and fruit. Scab is aggressive and can spread rapidly. According to the University of Florida Extension, water--from rain, overhead irrigation and dew--is the most important factor affecting disease development. Early symptoms of citrus scab include well-defined, cone-shaped growths on one side of the leaf, with a corresponding depression on the opposite side. The lesions may be seen individually or in groupings. Affected areas usually become covered with scab-like, corky growths and infected leaves can become stunted. Immature fruit on infected trees become misshapen with warty growths, but the interior fruit quality is usually not affected. The best way to control scab is to remove and destroy infected leaves, fruit and twigs before it can spread. Avoid introducing disease in the landscape by carefully examining citrus trees for disease lesions and insect damage and eggs before purchase. Use of drip irrigation is recommended to reduce the spread of disease. Copper sprays are also available to control scab---carefully follow label precautions.
Small, dark brown, raised lesions on leaves that have a sandpaper texture are the most recognizable symptoms of melanose. A "tear-streaking" symptom appears when fungal spores stream down the fruit's surface. Small lesions combine together and form a large "mudcake" lesion. Melanose lesions are scar tissue and have a rough texture. Usually, melanose is most severe in older trees that have been neglected and trees that are cold-damaged, with numerous dead twigs and wood. Leaf drop does not occur with melanose disease and fruit is only damaged superficially. Copper sprays are an option for malanose control, but the best practice is removal of small, dead twigs and avoiding use of overhead irrigation.
Many citrus trees are "budded." This means the upper portion of a tree, the scion, has been grafted onto hardier rootstock. In the case of foot rot, a fungus usually causes lesions that begin near the bud union (where the trees have been joined together). Foot rot lesions will either grow upward on downward on the trunk, and they appear as patches of water on the bark and oozing of gum. Progress of the disease may be limited if the tree creates a callous or scar tissue around the affected region. Citrus tree canopies will show nutrient deficiencies, leaf and fruit size reduction, dropping of leaves and wood die-back and general poor growth. The organisms responsible for foot rot do well under soil with high moisture content. Symptoms usually occur after long periods of rain and excessive irrigation. Ways to control foot rot include planting trees higher in poor drainage areas (create mounds or raised areas when planting), encouraging circulation around the tree base and removal of plant debris and decaying material on the ground. Fungicides are also available, as are resistant rootstock, such as trifoliate orange, Citrus macrophylla and Swingle citrumelo.
Greasy spot fungus causes spots on citrus leaves that look like greasy spots just below the surface of the leaf. The leaves function for two to three years but eventually drop. In severe cases, leaf drop is substantial. Trees weakened by greasy spot are more susceptible to other diseases and pests and are less protected during frosts. Sweet oranges are not as impacted as grapefruit, lemons and tangelos, but fruit damage will appear as pinpoint black specks on the rind. The best control of greasy spot is to remove and destroy fallen citrus leaves near the infected tree before summer rains occur. Oil and copper sprays are also available for control.