Several Florida orange tree diseases affect orange crops and orange trees grown in the Florida homeowner's landscaping. Some diseases can be controlled with chemicals, while others, such as Huanglongbing, require the complete removal and destruction of the affected orange trees. According to the University of Florida, citrus growers are trying to keep the disease out of Florida by checking supplies imported from China and by destroying infected trees.
Huanglongbing (HLB) was discovered in Florida in 2005. It is caused by a non-cultured, phloem-limited bacterium named Candidatus Liberibacter. The disease affects all cultivars--there is not a HLB-resistant cultivar. It destroys groves in a rapid fashion, as mature trees become non-productive, and young trees never mature into production mode. Symptoms include leaf yellowing or a mottled appearance on the leaves. The early symptoms can start on one branch, but quickly move through the entire tree. Small, sparse fruit is another symptom. HLB is transmitted by Trioza erytreae and Diaphorina citri. Both psyllids enjoy Florida's climate.
Citrus scab presents itself as scabs or warts on the twigs, leaves and fruit of Florida's orange trees. It spreads fast, and overwinters well on leaves and stems. Citrus scab usually first occurs in March and April. In the early stages, the scabs are well-defined growths on the leaves. The growths may be single or grouped. The scabby part of the growth ranges from pale to dark in color. If you notice citrus scab, it is best to remove the afflicted leaves. If the disease spreads before you notice it, use copper sprays on the trees about two to three weeks after the petals fall, and again in another two to three weeks to control the scab.
Greasy spot shows up as greasy-looking spots under the surface of the orange tree's leaves. The spots are leaf tissues that died from the infection of greasy spot. If the trees are heavily infected, it might suffer premature leaf drop. This disease weakens the trees, allowing other diseases and pests to take over. Greasy spot also affects the fruit, and you will see black spots on the rinds. Cleaning up fallen leaves under the trees every year can control this disease. The infected leaves must be discarded, so do not use them as mulch and do not add them to the compost pile. Copper sprays also help control greasy spot.