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Phlebiopsis by Michelle Cram, USDA-USFS, 320 Green St., Athens, GA 30602 Phlebiopsis gigantea is a common saprophytic fungus that causes white rot of conifer logs and stumps. It is used as a biological control of annosum root rot, caused by Heterobasidion spp., in western Europe. In the United States, P. gigantea was used as a biological control for annosum root rot from the 1970s to 1995. Currently, the Environmental Protection Agency is reviewing the status of P. gigantea as a pesticide. Annosum root rot is primarily a problem in conifer plantations that have been partially cut (e.g. thinned). It is a pathogenic white-rot fungus that colonizes freshly cut stumps by air-borne spores. The fungus then progress from the stump down the stump's root system and infects adjacent trees through root graphs or root contacts. A tree can be infected by Heterobasidion and show no aboveground symptoms until at least half the root system or the root collar is infected. Above-ground symptoms include declining tree (often in groups), windthrown trees, and resin-soaking or white-string rot at the root collar. Bark beetles often occurs in infected trees. Below-ground symptoms of annosum root rot progress from resin-soaking to white-stringy rot of tree roots. The fruiting body of Heterobasidion spp.forms irregular masses of white fungus between bark scales that develop into a leathery conk. The conks are brown with a white margin and a cream colored lower surface. These fruiting bodies usually form on the base of the tree in the duff layer.
The fruiting body of P. gigantea grows flat (1mm thick or less) along the wood surface. The surface has minute hairs (pilose) and appears pinkish-buff or marble-gray in color.
P.gigantea occurs throughout North America, Central America, Europe, East Africa, and southern Asia. The fungus plays an important role in the decomposition of conifer debris.
P. gigantea is found on the wood and bark of gymnosperms (notably on species of Pinus, Picea, Abies, and Tsuga). There have been reports of P. gigantea causing sap rot of both gymnosperm and angiosperm pulpwood in storage and of wood products. The annual fruiting body of P. gigantea produces sexual spores (basidiospores) that become airborne and spread to woody debris via the wind and rain. P. gigantea also has an asexual stage that produces spores (oidia) from mycelial fragmentation. The asexual spores are easily produced on artificial media in the laboratory. However, their production and role in nature is unclear. P. gigantea has an interesting mode of action as a biological control agent. The hyphae of this fungus antagonize the hyphae of Heterobasidion spp.(and some other fungi) on contact - a phenomenon termed hyphal interference. Any hypha of Heterobasidion hypha that makes contact with a hypha of P. gigantea shows rapid, localized disruption: the protoplasm becomes disorganized and its membrane integrity is affected. This is shown in the figure on the right, where the dye neutral red was added to an agar plate: several annosum hyphae have grown up to a hypha of P. gigantea and have locally taken up the dye whereas their normal membrane integrity would exclude this dye.
Artificial inoculation of pine stumps with P. gigantea has been shown to significantly decrease the presence of annosum root rot in several studies in England (Rishbeth 1963, Tubby et al. 2008) and in the southern United States (Hodges 1964, Ross and Hodges 1981). P. gigantea is also effective in colonizing spruce stumps. Although P. gigantea is common in the environment, natural levels of inoculum were found to be too low and sporadic to effectively control Heterobasidion spp. without artificial inoculation (Rishbeth 1963, Ross 1973).
Commercial Availability At this time, P. gigantea is only commercially available in England, Sweden, Norway, Switzerland, and Finland. The English product is a spore suspension and is sold by the Omex International under the name Pg Suspension. The Finnish product is a dry formula that is sold by Kemia Oy under the name Rotstop. In the United States, P. gigantea was commercially available until 1995 when the Environmental Protection Agency notified the Forest Service that P. gigantea was a biological pesticide and would need registration as such.
References Hajek, A. 2004. Natural Enemies: An Introduction to Biological Control. New York: Cambridge University Press. 378 pp. Hodges, C.S. 1964. The effect of competition by Peniophora gigantea on the growth of Fomes annosus in stumps and roots. Phytopathology 54:623. Rishbeth, J. 1963. Stump protection against Fomes annosus. III. Inoculation with Peniophora gigantea. Ann. App. Bio. 52:63-77. Ross, E. W. 1973. Fomes annosus in the southeastern United States: Relation of environmental and biotic factors to stump colonization and losses in the residual stand. Washington DC: U.S. Department of Agriculture Forest Service Technical Bulletin No. 1459. Ross, E.W. and C.S. Hodges, Jr. 1981. Control of Heterobasidion annosum colonization in mechanically sheared slash pine stumps treated with Peniophora gigantea. Asheville, NC: U.S. Department of Agriculture Southeast For. Expt. Stn. Res. Paper SE-229. Tubby, K.V., D. Scott, and J.F. Webber. 2008. Relationship between stump treatment coverage using the biological control product PG Suspension, and control of Heterobasidion annosum on Corsican pine, Pinus nigra ssp. laricio. For. Path. 38:37-46.
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