Ornamental & Tree Diseases

Plant Disease Dispatch Sheets
December 18, 2000

M-416 Annosum Root Rot

Annosum root rot, caused by the basidiomycetous fungus Heterobasidiion annosum, is found throughout the northern hemisphere and Australia and locally in South Africa and South America. It is considered by many to be the most destructive fungal pathogen of managed coniferous forests in the northern hemisphere. Annosum root rot was first identified as a potential problem in the South in planted stands in the late 1920's and early 1930's. Economically significant losses began to occur in the late 1950's when thinning of pine plantations became widespread. Annosum root rot can reach prohibitive levels on high hazard sites in the absence of preventative disease management tactics.

The annosum root rot fungus has a wide host range including all conifers (especially pine, fir, Douglas fir, spruce, hemlock, larch, and juniper) and some hardwoods (especially when growing with pines). All southern pines are susceptible, but loblolly and slash pine are most severely affected.

Heterobasision annosum overwinters as mycelium in infected roots and stumps and as basidiocarps. Tremendous numbers of basidiospores are produced when temperatures are between 70 and 80 F and the soil is not too dry. Despite basidiocarps being partially buried, basidiospores are distributed effectively into the air. Wind-disseminated basidiospores, carried to freshly cut stumps and other wounds, account for long distance spread of the fungus. Basidiospores are also carried down into the soil by rain. Tree-to-tree transmission occurs by growth of the fungus through root grafts. Basidiospores germinate readily on fresh wounds, especially stumps provided during thinning operations. Mycelium from germinated basidiospores grows rapidly through stumps into root systems. The annosum root rot fungus secretes several wood degrading enzymes, which causes roots to decay and die.

Decay in roots is characteristically stringy. In most conifers (such as, fir, Douglas fir, spruce, hemlock, larch, and juniper) the annosum root rot fungus grows from roots into above ground parts causing heart rot. Trees may survive for many years, perhaps decades, without any additional symptoms produced. In other conifers, especially pines, H. annosum colonizes only roots, causing excessive resin exudation and rapid death of trees. Death is normally preceded by yellowing and subsequent blighting of needles, defoliation, and heavier than normal cone production. As time passes, pockets of dead and dying trees (infection centers) become scattered throughout affected stands. Infection centers enlarge at the rate of 1 to 6 feet per year, reaching diameters of 60 to 100 feet. Rate of infection center expansion is much faster on former agricultural sites and in soils with a high pH.

Since several symptoms produced by annosum root rot infected trees are nonspecific and can be brought on by other irritations, including other root rot fungi, positive field diagnosis depends on locating H. annosum basidiocarps. Basidiocarps are perennial and often located under litter at bases of dead or dying trees, under roots, and in stump cavities. Upper surfaces are tan to brown with distinct white margins, and lower surfaces are white. Basidiocarps are rubbery and difficult to tear. Smaller (1/8 inch diameter) basidiocarps occur on surfaces of infected roots and occasionally on slash. Even though basidiocarps are most common from December through March, their spores are produced throughout the growing season in the South.

Management recommendations include the following:

1. Evasion of the pathogen by planting on low-hazard sites. In low hazard sites depth to clay is less than 12 inches. Low hazard sites have poor internal drainage and high seasonal water tables. A high hazard site has 12 inches of sand or sandy loam above a clay subsoil with good internal drainage. In the absence of proactive, preventative disease management tactics, annosum root rot losses on high hazard sites will normally exceed 1 cord/acre (10-20 trees) 5 to 7 years after thinning.

2. Eradication or reduction of the pathogen using prescribed burns. Two pre-thin burns (one at least six months before thinning) and one or more post-thin burns. Post-thin burns alone are not recommended. Burning destroys basidiocarps and eliminates litter (mulch) which provides a favorable environment for basidiocarp development.

3. Eradication or reduction of the pathogen by mechanically removing and burning stumps and attached roots on infested sites.

4. Plant more disease resistant longleaf pine on high hazard sites.

5. Reduce number of thinning operations per rotation on high hazard sites by

(a) planting trees at wider spacing,

(b) thinning heavier than normal (below 70 sq. ft./acre)

(c) eliminating thinning operations on short pulpwood rotations.

6. Carry out thinning operations carefully to reduce incidence of wounds.

7. Thin when basidiospore population in the air is lowest (winter months in the north; dry, hot months in the south).

8. On disease free sites, protect the host by treating stumps with Sporax immediately after trees are cut. Sporax is inexpensive, effective, safe, and easy to apply. Tree fellers should treat at a rate of approximately 5 pounds per 60 sq. ft. of basal area per acre (225 7-inch trees). Sporax may aggravate the problem by preventing natural competitors from entering stumps if applied to stump surfaces in diseased plantations.

9. In annosum root rot infected stands, it is recommended that stumps be treated with Phlebiopsis gigantea, an aggressive, highly competitive fungus that colonizes stumps to the exclusion of the annosum root rot fungus. Spores from one culture plate of P. gigantea in 4 gallons of water will threat approximately 50 10-inch stumps (5 cc/stump). Spore suspension should be prepared fresh daily.

Vernon Ammon, Ph.D., Professor and Plant Pathologist
Mukund V. Patel, M.S., Extension Plant Pathologist
December 2000

Information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended or other products that may also be suitable and that these products have maintained label clearance.