Diseases of Ornamental Orange Trees

Viral Diseases

• Several viral diseases threaten oranges and other citrus trees growing in the United States. The exocortis virus, which infects both orange and lime species, harms the host’s bark, causing it to fall off or become distorted. According to the Texas A&M Extension, the exocortis virus kills adult trees in as little as 2 or 3 years. The psorosis virus causes scaling of orange tree bark and serious, long-term health problems. The tristeza virus causes significant cosmetic damage by deforming new growth and causing foliage, buds and stems to wilt and die.

Bacterial Canker

• The Xanthomonas axonopodis bacteria is a variation of the X. campestris species, which is a common pathogen affecting hundreds of trees and shrubs in North America. This bacteria attack the leaves and fruit of its host, causing unsightly discoloration and deformity. It also forms cankers, which are deformed, bulbous growths that disrupt the circulatory tissues in the plant's stems and shoots. According to the Smithsonian Marine Station, rotting and discolored legions appear on infected fruit and damaged leaves may fall from the tree prematurely.

Soil-Borne Fungi

• Several fungal pathogens, including those responsible for root and foot rots of orange trees, infiltrate the citrus trees’ roots through the soil. The Phytophthora gummosis fungus causes a root rot disease that destroys the leaves and bark of its host. According to the Arizona Cooperative Extension, an advanced infection of this pathogen deteriorates the tree trunk’s structural integrity, causing it to eventually collapse and die. Symptoms of other root diseases, like dry root rot caused by the Citrus macrophylla fungus, are similar to those of an infestation of Phytophthora. Infections of soil-borne fungi are difficult to diagnose until serious symptoms emerge because most of the damage occurs on wood and roots below the soil line.

Fungal Leaf and Fruit Diseases

• A number of pathogenic fungi directly attack the leaves and fruit of orange trees. While these problems are more devastating on fruit-producing trees, they also mar oranges grown for their ornamental value. Alternaria fruit rot, also called black rot, decays the ripening fruit of orange trees, leading to severe discoloration on the infected fruit’s stem end. Other fungi, like Cercrospora ciri-grisea, cause leaf spot diseases on citrus trees. These ailments cause black or brown spots on the surface of leaves, buds and stems. Infected foliage begins to dry out and wilt as the infection progresses.