Symptoms and Signs
The general symptoms of dry root rot are similar to those caused by Phytophthora species and other agents that damage the roots or girdle the trunk. These include reduced vigor, dull green leaf color, poor new growth, and twig dieback. If extensive root damage occurs, the leaves suddenly wilt and dry on the tree.
The disease 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, 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 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 will eventually collapse.
Comments on the Disease
The dry root rot organism often infects a tree through the crown or larger roots that have been injured by Phytophthora spp., mechanical injury, gophers, or root burn caused by a large overdose of fertilizers, herbicides, or nematicides. All common rootstocks are susceptible to dry root rot.
Although the disease is normally a chronic problem and generally only affects a few scattered trees in a grove, it can develop into an epidemic in some orchards. It is caused by Fusarium solani infecting major roots and the root crown. Fusarium solani typically is a saprophytic fungus that colonizes dead and dying wood. The development of dry root rot is not well understood, but tree stress and other injuries are believed to predispose the tree, allowing Fusarium solani infection to develop into a pathogen that eventually kills infected trees.
Good orchard management, especially careful irrigation, is essential for preventing dry root 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.
- Irrigate carefully:
- Ensure that the application matches tree water requirements.
- Keep the trunk dry (adjust sprinklers so water does not hit the trunk).
- Check drainage: water should not be allowed to stand in contact with the tree crown for an extended period of time.
- Clean equipment thoroughly before moving it between orchards. Movement of equipment facilitates the spread of the pathogen.
- Avoid mechanical injury to the underground portions of the crown during cultural operations, especially during the cool and wet season.
- Follow label instructions for applying fertilizers, herbicides, and nematicides at recommended rates to avoid causing phytotoxicity and burning root tissues when excessive amounts of these materials are used. 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.
- 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 fungicides are available.
For more information on monitoring and management of dry root rot, see UC Ag Experts Talk: Citrus Dry Root Rot.