Diseases of Navel Orange Trees

Phytophthora Foot Rot

• Phytophthora foot rot is caused by the soil microorganism Phytophthora parasitica and P. citrophthora. The disease is also referred to as gummosis and is very common in citrus trees grown with the graft union near or below the soil surface. This exposes the scion tissue to the two fungi. Phytophthora foot rot is characterized by longitudinal cracks and gumming in the bark. As the disease progresses, the bark starts to become water-soaked, slimy and reddish brown. There is sparse and yellowing foliage and trees are likely to collapse entirely from the girdling caused by the infection. Preventive measures include avoiding standing water around the crown during irrigation and digging deep planting holes. Chemical control includes the use of systemic fungicides for phytophthora.

Alternaria Fruit Rot

• Alternaria fruit rot is a fungal disease of navel oranges and is also referred to as black rot. It is caused by the fungus Alternaria alternata and leads to premature fruit drop. The disease is characterized by the fruit color turning light several weeks before the normal color break in fruit. The infected fruit has a brown to black discoloration at the blossom end. Often the decay extends deeper into the fruit. Control strategies include proper fertilization and irrigation. There are no chemical control methods for alternaria fruit rot.

Stubborn Disease

• Stubborn disease is caused by the mycoplasma-like organism called Spiroplasma citri and is a common disease in navel orange trees. The disease can infect trees at any time during their growth and results in dull-flavored, malformed fruit; stunted growth and flat-topped trees; twig die-back; and small, stiff and upright foliage. The symptoms are worse during hot weather. Control options include the removal of infected trees and replacement with healthy cultivars. Propagation in the nursery should be done with stubborn-free budwood.