Phytophthora Root Rot of Conifers
Click for larger image Fraser fir (Abies fraseri) killed by phytophtora root rot in poorly drained portion of Christmas tree plantation. L. Haugen, USFS, Bugwood.org

Phytophthora root rot of conifers is caused by fungi in the genus Phytophthora. Many species of fir, true cedars, white-cedar (arborvitae), larch, pine, spruce, yew and Douglas-fir are affected. It produces a swimming spore called a zoospore.

Symptoms and Diagnosis

Symptoms include wilting, stunted foliage, chlorosis and eventual death of seedlings especially in low, wet areas. Infected seedlings have cambium which is red-brown or butterscotch-colored particularly at the root collar. Feeder roots are dead, black and fine. Trees with root rot turn yellow-red and die, sometimes suddenly. Roots from trees infected with Phytophthora cinnamomi will show gummosis (heavy pitch bleeding). These infected trees have roots which are minimal, often brown and rotting. The deterioration of the roots causes the trees to suddenly wilt, turn yellow, lose needles, collapse and die. Stress, such as drought, heat, under watering or over watering, can also weaken roots and make them susceptible to infection. Phytophthora produces no easily recognizable fruiting bodies or spores on infected tissue, and diagnosis can be confirmed only by isolating the organism in pure culture medium. Phytophthora species are often difficult to isolate because they are frequently overgrown by other fungi, especially Pythium species.

Life Cycle

All species of Phytophthora spp. survive and live indefinitely in soil. Thick-walled, microscopic survival structures, oospores and chlamydospores, are produced in infected host tissue. There is variation between species as to the presence or absence of these spores and also to their size and shape. The most reliable method of identifying Phytophthora spp. is the finding of the sporangial stage. They are commonly produced on infected tissue and can be seen on microscopic examination of isolations made on specific media. Zoospores swim through water and contact roots or the lower trunk tissue of potential hosts where they germinate and infect these tissues. The fungus grows into host tissue, girdling the infected root or trunk and eventually killing the tree. As the host dies the fungus produces the survival structures in decaying tissue. If the tree is removed these spores are returned via rotted host tissue to the soil where they enable the fungus to survive in the absence of a host.

Integrated Pest Management Strategies

1. Choose a tree wisely. Plant a resistant species if possible. Nordmann, Turkish Fir, and to a lesser extent Douglas Fir, appear to confer resistance. Choose trees from a reputable nursery and choose bigger (older) seedling trees with stronger root systems.

2. Planting and care. Plant in sites with well-drained soils. Also, avoid over watering trees-especially when more mature. Younger trees need more water than trees 5 years (or so) and older. In a mixed planting, use of drip irrigation will allow plugging of the irrigation on trees that don’t need the water while enabling moisture to be delivered to the young trees when needed. Increase soil organic matter and tilth. Phytophthora thrives in heavy soils, like clay soils. Consider planting on a raised bed. Results with susceptible species are not conclusive; however, a raised bed will provide more aeration to roots.

3. Don’t stress trees. Anything that stresses trees: heavy fertilizer on young tender roots, herbicide damage to roots, too much or too little water, can make them susceptible to infection.

4. Avoid compaction of soil. Avoid heavy equipment, such as ATV or truck driving, or even walking paths, in areas where trees are being planted.

5. Cool the soil. Consider cooling the soil with a drought tolerant cover in between tree rows such as Sheep Fescue.

6. Avoid moving soil from root rot areas to new areas. Phytophthora lives in the soil and can be moved with equipment-shovels, planting augers, even boots. Also, disinfect tools after working in a root rot area with a solution such as 50% Clorox. Don’t allow people to walk in root rot areas-keep these cordoned off.

7. Chemical controls. Chemical controls are not practical for the home gardener but Subdue Maxx, Syngenta (mefenoxam) is currently registered for use on Christmas trees in California.

Organic Strategies

Strategies 1, 2, 3, 4, 5, and 6 are strictly organic approaches.

More images:

Click for larger image
Bareroot sand pine seedlings (Pinus clausa) killed by Phytophtora root rot. E. L. Barnard, FDACS, Bugwood.org
Click for larger image
Mortality from phytophthora root rot in a stand of 4-year-old sand pines (Pinus clausa). E. L. Barnard, FDACS, Bugwood.org
Pests and Problems

Click a link in the site map below to see other "Pests and Problems" pages