California citrus trees are susceptible to a wide array of tree diseases. While many of California's citrus tree diseases are common in other areas, some diseases are more prominent in the state. Although many of these diseases do not cause permanent injury, several are detrimental to the citrus and can even kill the tree, if left untreated.
Citrus Greening Disease
HLB, which stands for Huanglongbing, is a bacterial disease that is transported by the Asian citrus psyllid, a tree hopping insect. All citrus tree variations are susceptible to this disease. Citrus trees that are infected with HLB often show delayed symptoms that can appear up to a year after the infection was introduced. Infected trees will develop yellowing foliage and shoots, odd-shaped fruit, premature foliage and fruit drop, growth stunt, dieback, and complete weakening and destruction of branches and twigs. Chemical controls can be used to prevent the Asian citrus psyllid and the HLB disease. Chemical control of this disease can only administered by entities that have been authorized by the California Department of Food and Agriculture. California residents are advised to contact the California Department of Food and Agriculture if you suspect that your tree is infected with HLB.
Armillaria Root Rot
Armillaria root rot is a fungal disease that originates and thrives in the soil. This soil-borne fungal disease can thrive in the soil for many years without a hosting tree. The armillaria fungi enter the citrus trees through their root systems and cause the decay and death of the trees' root systems. As the roots are damaged and killed, the citrus tree becomes unable to transport water and nutrients throughout its entire system. As a result, infected trees experience wilting, growth stunt, dieback and premature defoliation. Once symptoms appear, the infection has successfully decayed the roots of the citrus. Infected citrus trees, along with their root systems, cannot be saved, and should be removed and destroyed.
Citrus Septoria Spot
Citrus septoria spot is most commonly seen on orange, lemon and grapefruit trees. This disease is transported by fungal spores during windy and rainy periods. These spores infect developed fruit that has become injured from weather damage or mechanical equipment. Entering through the wounds of the fruit, this fungal disease causes small, tan- to brown-colored pits on the skin of the fruit. As the disease progresses, the small spots develop into large, dark-colored lesions. Although the lesions are sunken into the skin of the fruit, these lesions do not pass through the skin into the pulp of the fruit. Citrus septoria spot can be treated, controlled and prevented with a copper-based fungicidal spray.