How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Armillaria root rot, also known as oak root fungus, can occasionally damage and kill citrus trees. Symptoms may not develop until after the disease is well established. The first symptoms of Armillaria root rot are poor growth or dieback of shoots, small yellowing leaves, and premature leaf drop. The fungus spreads by root contact or through rhizomorphs (black strings of fungal mycelia), which can grow short distances through the soil and contact and penetrate citrus roots. The pathogen invades the roots and crown, eventually girdling the crown region and destroying the entire root system. From the infection site the fungus invades lateral roots and the crown region, where it spreads as white mycelial plaques in the cambium region between the bark and wood. This distinguishes Armillaria from other wood-rotting fungi, which grow on the outer surface of the bark.

In late fall and winter, Armillaria often forms clusters of mushrooms at the base of infected trees a few days after a rain.

COMMENTS ON THE DISEASE

Armillaria root rot fungus is native to California's woody plants and affects many tree crops planted on hillsides, in former riverbeds, floodplains, and on other areas subject to overflow. The fungus can survive for many years in dead or living roots of fruit and nut trees, and on ornamental and native tree species. The disease often occurs along former streambeds and near creeks where the soil is moist and where dead roots or stumps harboring the fungus may be buried in the soil. The fungus requires cool, moist soil conditions for development and spread; it is therefore rarely a problem in desert areas.

MANAGEMENT

Management of Armillaria root rot relies primarily on preventing infection of new trees. Once infection is apparent, it is very difficult to save a tree. Avoid planting in a site likely to be infested with Armillaria. If there are infected trees in your orchard, remove them completely, including the roots, and let the trees dry thoroughly before disposing of them. Also remove the neighboring, apparently healthy trees; once symptoms appear on a tree, the disease has probably already spread to the roots of the surrounding trees.

To prepare infested sites for replanting, remove stumps and roots of the diseased tree. Destroy roots larger than 0.5 to 1 inch (1.2–2.5 cm) in diameter and fumigate the site.

IMPORTANT LINKS

• Photos of diseases and disorders of limbs, trunks, and roots

PUBLICATION

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441
Diseases
J. E. Adaskaveg, Plant Pathology, UC Riverside
Acknowledgment for contributions to Diseases:
J. A. Menge, Plant Pathology, UC Riverside
H. D. Ohr, Plant Pathology, UC Riverside

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