Diseases of Backyard Orange Trees

Healthy orange trees resist disease.

Orange trees make attractive specimen trees in a landscape with the added benefit of producing a fruit popular for eating and juicing. However, they are susceptible to a range of diseases, and homeowners must be vigilant in detecting problems and treating or removing diseased trees promptly. Healthy trees are less susceptible to disease so growers are advised to follow a fertilization and watering schedule specific to their area. A local cooperative extension service can provide details.

Citrus Scab

Citrus scab destroys the looks of the fruit.

Citrus scab disease transforms a round orange into a warty, misshapen mess. While the fruit inside is usually not damaged, the unpleasant appearance makes it difficult to eat. The leaves and stems display warts and the “scabs” that give the disease its name. The lesions are pale and others are dark. Leaves will often be distorted. The fungus Elsinoe fawcettii is responsible for the disease. Homeowners control citrus scab by removing infected leaves and stems as quickly as possible and avoid spreading it by watering by a drip method rather than a spray irrigation system. Copper sprays have also been effective in controlling citrus scab.

Stubborn Disease

Stubborn disease stunts the growth of the orange tree.

Stubborn disease of citrus is found in California and Arizona. It primarily attacks Washington navel oranges but has been found in Valencia and sweets. Mycoplasma, Spiroplasma citri, causes the disease. The orange tree may be stunted and appear flat on top. Branches may have “witches brooms” clump of shoots originating from the same spot on a branch. The leaves may have yellow spots and be slightly curled. The fruits become smaller each year and have little flavor. The tree may bloom out of season. There is no cure for stubborn disease. The infected tree should be removed and destroyed.

Rio Grande Gummosis

Oozing sap and clumps of gum are signs of Rio Grande Gummosis.

Sap or gum seeping from the trunk of an orange tree may signal Rio Grande gummosis, a fungal disease probably caused by Diplodia natalenris. This disease attacks trees not vigorous due to lack of fertilization, weakened by freeze damage, trees that stand in wet soil or soil with a high salt content. Blobs of sap form near cracks in the trunk. The wood beneath it has an orange tint. There is no cure for Rio Grande gummosis, so proper care of the orange tree to maintain vigor is important to prevent the disease.

Psorosis

Psorosis causes bark to flake and peel.

Older orange trees may develop psorosis or scaly bark. A virus that infected the tree as much as 10 years earlier probably causes the disease. The bark on the trunk and limbs of the tree flakes off leaving damaged areas. The tree declines and fruit production drops off as twigs and branches die. There is no cure for psorosis. California and Arizona have implemented programs to remove diseased trees and prevent the spread. Homeowners should buy young trees from a certified nursery to avoid purchasing an infected tree.

References

• University of Arizona Cooperative Extension: Diseases of Citrus in Arizona/stubborn
• Texas A&M University: Fungal Diseases and Nematodes
• Citrus Clonal Protection Program: Diseases

Resources

• University of Florida Extension: Your Florida Dooryard Citrus Guide