Phytophthora Root Rot Can Kill Mature Trees

Left: Phytophthora infected pine trees on August 29, 2003. Right: Same trees on Sept 10, 2003, after heavy irrigation. Note progress of disease due to irrigation. If trees were not irrigated they might have been saved.

Signs of Phytophthora Root Rot

· Affects mature broadleaf and pine trees

· Leaves begin to die on top of trees in mid summer, turning yellow and brown before dropping

· Trees continue to lose leaves even if trees are watered

· Entire tree can die within days in mid summer

· Extra watering causes trees to decline faster

· Roots near trunk base turn black, inner root wood is reddish brown rather than white

Phytophthora root rot has killed many mature shade trees in our region this summer. Although the disease is not uncommon the extent of the destruction in 2003 was particularly severe in Phoenix and Las Vegas. One problem is that trees with Phytophthora root rot are frequently misdiagnosed as being drought stressed. As a result, most people begin irrigating them more and this is the worst thing that can happen. Why? Excess water promotes the swimming zoospore stage of fungus and this infects most of the root system. The outcome is that trees die more rapidly with water than if they were not watered at all.

Identification of Phytophthora

Trees infected with Phytophthora are commonly found in over-watered sites or on trees that have been planted too deeply with elevated soil at their trunk base. Heavy soils that stay wet for extended periods and are not dried between waterings are most prone. The following trees are commonly infested: citrus, pines, Ficus, carob, Brazilian pepper.

Trees affected with Phytophthora root rot usually develop bronze or reddish color on leaves during late summer or early fall.

Above-ground symptoms vary between species, but generally include yellowing or chlorosis of leaves, leaf drop, reduced tree vigor and growth, and eventual wilting and death of the whole tree. Infected trees tend to die from the top down. Tree decline may occur slowly over one or more years, or they may collapse and die rapidly in high summer temperatures. Rapid death of trees usually occurs following excessively wet periods. On trees that decline gradually, leaves often exhibit a reddish or purple coloration, compared to green leaves on healthy trees.

Below-ground root symptoms need to be inspected by removing several inches of soil around the base of the declining tree. Diseased roots show bark that is reddish-brown to black that can easily sloughs off. The diseased wood below is water soaked with a reddish-brown color compared to healthy wood, which is white and not water soaked. The discoloration distinguishes Phytophthora root and crown rot from other root rots such as Texas root rot, which causes roots to be dry, light and corky. Roots on trees killed by standing, excess water are usually completely black and often have an unpleasant sulfur smell. Tree roots killed by drought conditions are usually withered and dehydrated.

Causal Organism and Disease Cycle

The genus Phytophthora (Oomycetes) contains about 40 species many of which are plant pathogens. Phytophthora means plant destroyer in Greek (phyton, plant + phtheiro, destroyer). P. infestans the best known, was responsible for the Irish potato famine in the 1850's. P. cactorum the first studied attacks madrone and other trees. Phytophthora are soil borne fungi common in all soils. Some species may be introduced on contaminated nursery stock or soil. While some forms are more destructive than others, depending on the tree, they all require extremely wet or saturated soils for infection. The fungi over winter and persist in soil as mycelium in infected wood or as thick-walled oospores. The oospores are viable in soil for several years. When soils become wet, oospores germinate to form thread-like fungal filaments or mycelia that can infect root tissues and produce reproductive structures called a sporangia. The sporangia fill up with infective spores called zoospores that can are released only when in soil that is completely wet. The zoospores can swim to susceptible plant tissue and infect otherwise healthy trees. They can also swim to the soil surface and move long distances in runoff water. The longer the soil stays wet, the greater the risk of infection. Some roots are more susceptible in spring, autumn or summer when soil temperatures are most productive for fungal growth and zoospore production. Roots are less susceptible in winter when fungal and root activity are low.

All the spore types require free water for germination. The spores exist in the soil or in killed plant material. The inoculum can be spread by sporangia being dispersed by wind and rain, and can be carried by splashing or running water. Susceptible weed hosts are another source. Also, inoculum can be carried by insects, animals such as snails and rodents, and humans carrying infested soil or plant residue on shoes, clothing, vehicles, and implements. They grow as free-living saprophytes for a short period until they find a suitable host.

Some microorganisms have been shown to parasitize oospores of P. cactorum. These include the oomycete Pythium spp., the hyphomycetes Dactylella spermatophaga, Diheterospora chlamydosporia, Humicola fuscoatra, Fusarium oxysporum, Cephalosporium spp., and Alternaria alternata, and bacteria in the genus Pseudomonas.

Control

Control of Phytophthora root rots is most successful with proper cultural practices and when necessary, chemical controls.

Avoid sites that drain slowly or poorly or are subject to periodic flooding. Marginal sites should be modified (install drain tiles, create diversion ditches, rip underlying pan layers) to provide the additional drainage recommended for growing tree fruit crops. Planting trees on ridges or berms will raise their crowns above the primary zone of zoospore activity and provide an important margin of safety, especially in a wet year.
Soil fumigation is generally considered ineffective because it never completely eradicates the fungus from orchard soils, and the Phytophthora fungi are easily reintroduced into fumigated soil.
Fungicides are most effective when used in combination with the cultural practices described above. Aliette® and Ridomil®, which are available for use on citrus in Arizona, are both effective fungicides for control of Phytophthora diseases.
Inoculation of soils with compost teas containing beneficial organisms appear to be particularly helpful in preventing and overcoming Phytophthora infections. Use of ectomycorrhizal inoculants such as MycorTree™ Ecto-Injectable™ are recommended for pine trees.

Above: Root zone excavation reveals Phytophthora infection as reddish coloration in roots.