How to Manage Pests

UC Pest Management Guidelines

Damage usually starts in larger roots and spreads into the crown. Patches or large areas of bark on the underground portion of the crown show a moist, dark decay, which later dries and adheres to the wood. In some cases, dry bark may also be seen aboveground. The wood below the dead bark is hard, dry, and stained grayish brown to purple. Unlike Phytophthora gummosis, dry root rot does not produce gumming, and the lesion extends deep into the wood. The initial infection may occur at the time of planting or at any time during the life of the tree, but aboveground symptoms may only appear several years after the initial infection when the crown region has been girdled. Once the crown region is girdled, the tree collapses.

COMMENTS ON THE DISEASE
Although the disease is normally a chronic problem affecting only a few scattered trees in a grove, it can develop into an epidemic in some orchards. The exact cause of dry root rot has not been established, but a Fusarium solani is most often isolated from diseased wood. Dry rot organisms often infect a tree through the crown or larger roots that have been injured by Phytophthora fungi, mechanical injury, gophers, or root burn caused by a large overdose of fertilizers, herbicides, or nematicides. All common rootstock are susceptible to dry root rot.

MANAGEMENT
Good orchard management, especially careful irrigation, is essential for preventing dry rot. If the soil around the tree crowns and roots is saturated for long periods of time, the chances for injury and subsequent fungal infection increase. When establishing furrows, provide berms along the trees so that the crowns are protected from the water. Adjust sprinklers so that water does not hit the trunks. During cultural operations, avoid injury to the underground portions of the crown, especially during the cool and wet season. Follow label instructions for applying fertilizers, herbicides, and nematicides to avoid overdosing the trees and burning root tissue. Before fertilizing young trees, wait at least 6 weeks after planting or until the trees show new growth.

Check regularly for signs of Phytophthora root rot or vertebrate damage that may provide entry sites for dry root rot. If you suspect a dry root rot infection, dig all the way around the tree because the decay may be underneath the crown roots or on one or more of the main lateral roots. You may be able to slow the spread of the disease by exposing the crown region and allowing it to dry. Prune the tree skirts and remove the soil from the crown region. Correct any adverse soil conditions, such as poor drainage. Remove trees that have become unproductive because of severe infection. No effective chemical treatments are available.

PUBLICATION

UC IPM Pest Management Guidelines: Citrus
UC ANR Publication 3441
Diseases
H. D. Ohr, Plant Pathology, UC Riverside
J. A. Menge, Plant Pathology, UC Riverside

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