Heterobasidion annosum sensu lato (Heterobasidion root rot)

H. annosum sensu lato causes root decay in all hosts, but the extent of stem decay varies according to the host species (Greig, 1998). In resinous conifers, like most pine species, infection causes profuse resin excretion on the roots and at the base of the stem. This excretion blocks the flow of fluids in the stem. The tree dies relatively soon afterwards; the green crown turns brown and dies simultaneously.In coniferous genera such as Picea, Abies, Larix, Pseudotsuga and Tsuga, decay rises up the stem, usually as a heart rot. The tree may be asymptomatic for decades after infection. External symptoms only appear at an advanced stage of decay and include reduced growth, defoliation of the crown, and resin exudations on the stem. Wind-thrown trees with decayed roots or open gaps in the stand often indicate the presence of Heterobasidion root rot in the forest. In the field, Heterobasidion root rot can only be identified with assurance by the presence of basidiocarps, which usually hide under decayed roots, moss cover at the stem base of dead trees, in hollow stumps, or under the fallen trunks of diseased trees.Stem decay caused by Heterobasidion remains relatively hard for long time and is usually light brown in colour. The first stage of decay often has a violet coloration. Advanced decay contains small, white cellulose pockets ('white pocket rot'), often with black spots.

In healthy and slightly diseased stands in risky areas it is important to prevent infection by spores. This is done by performing cuttings during the seasons (cold winter or hot summer) when the risk of spore infection is low or absent. For cuttings carried out during the more risky warm season, the cut surface of fresh stumps can be treated with a protectant to prevent infection. The most commonly used protectants are urea and borax (Pratt et al., 1998) and the competitive fungus Phlebia gigantea (Holdenrieder and Greig, 1998). Care should be taken to avoid wounding the roots during logging operations, especially when cuttings are carried out during the warm season. Wide spacing and minimizing the number of thinnings during rotation will reduce the risk of infection.Little can be done to control the fungus in diseased stands. The only real possibility is to shorten the rotation time in heavily diseased stands.The extraction of infected stumps before regeneration essentially reduces the risk of infection in the next generation of susceptible conifers. The spread of the disease from stumps can also be reduced by favouring the regeneration of resistant tree species close to decayed stumps. Heterobasidion infection can be cleared from a site by cultivating a rotation of a resistant tree species. Current knowledge suggests that pure broadleaved tree stands are practically resistant. Susceptible conifers should be avoided or cultivated in mixture with resistant trees in afforestations on former agricultural soils (Korhonen et al., 1998b).