Dematophora root rot, or Rosellinia root rot—Rosellinia (=Dematophora) necatrix
Rosellinia species fungi, most commonly R. necatrix, cause Dematophora or Rosellinia root rot, also called white root rot. Although relatively uncommon in landscapes, when it occurs it quickly kills hosts.
Foliage symptoms as described below are generally the first clue that a root disease is present. Rosellinia also produces cobwebby mycelia that may be visible on the lower trunk, in soil over infected roots, growing from infected roots, or beneath bark of the root crown and major roots. These whitish growths of Rosellinia are much smaller and more scattered in comparison with the fanlike mycelia of Armillaria root disease found under bark. Mycelia of Rosellinia also lack the characteristic mushroomlike odor produced by fresh mycelia of Armillaria. In comparison, Phytophthora root rot produces no whitish mycelia.
When Rosellinia root rot is the cause of disease, exposing roots and removing the cortex (outer layer) of infected roots will reveal a whitish, fluffy layer of fungal mycelia. Older mycelia of Rosellinia can become blackish, brown, or tan and may occur crustlike on dead roots or the root crown. Dark cankers may be present in the lower trunk and main roots of infected hosts.
When Rosellinia-infected bark or root pieces are sealed in a moist chamber such as a plastic bag or jar for a few days, abundant, white mycelia are produced. Tiny, stalked spore-forming structures (coremia) with fluffy white growths on the end may also be produced and visible with a hand lens.
If Rosellinia root rot is suspected, it may be best to quickly seek an expert to confirm its presence. Prompt removal of the infested host may be warranted before the fungus spreads to nearby hosts.
The fungus lives as mycelia in roots of infected plants. Rosellinia can also persist in soil for several years in dried pieces of infected bark and wood. When the soil is moist and especially if soggy and of moderate temperature, the mycelia grow from infected roots and spread through the soil. If mycelia contact healthy roots of a host, they penetrate them and cause infection. Once inside a root the fungus spreads throughout the entire root system. Infected roots die rapidly. If plants of the same species grow near each other they may naturally form root grafts through which R. necatrix can spread from plant to plant.
Over 300 species of woody plants can be infected by Rosellinia species. Hosts include apple, avocado, ceanothus, cotoneaster, holly, pear, poplar, privet, and viburnum. Note that cause of Rosellinia root rot in avocado is Rosellinia bunodes. See Fungal Databases from the U.S. Department of Agriculture for a complete list of Rosellinia hosts.
Aboveground symptoms of Rosellinia root rot are the same as those of various other root and crown rots including Armillaria root disease and Phytophthora root rot. Root rot initially causes foliage yellowing and wilting throughout the entire plant or in just a portion of the canopy. Leaves are undersized and sparse. Branches that are killed often retain dry foliage. Hosts generally die relatively quickly when infected with R. necatrix.
There are no effective fungicides or other cure for hosts infected with R. necatrix. Prevention is the only available management strategy.
Obtain high-quality nursery stock that is pathogen-free from a reputable source. Minimize the likelihood of Rosellinia root rot by preparing the site well before planting. Good soil drainage and appropriate irrigation are especially important. For example, if soil drains poorly, plant on soil mounds or raised beds. Provide established plants a good growing environment and proper cultural care. Especially avoid excess irrigation and aeration deficit, or waterlogging (soggy soil).
If a plant died from Rosellinia root rot, uproot and dispose of it. If the same species grows nearby, trench around the infested site to break any root grafts that can naturally form between the same species growing near each other. Before replanting, remove as many root pieces from soil as possible. Establish a dry (unirrigated) zone where the Rosellinia-killed plant grew and prevent soil movement or water runoff from the infested site. Do not replant the site with hosts of R. necatrix.
Adapted from Integrated Pest Management for Avocados and Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Scattered, whitish mycelia of Rosellinia necatrix on soil at the base of an infected tree.
Rosellinia canker and mycelia in trunk base exposed beneath bark.
Rosellinia spore-forming structures (magnified) can develop when infected root pieces are sealed in a moist chamber.