Longdistance transmission of fungal disease

Fungal spores and pieces of mycelium are known to be spread by a variety of animals, particularly insects. The felted beech coccus Cryptococcus fagisuga spreads the beech bark disease, now causing damage in English pole-stage stands, through the wounds it makes. Endemic on Fagus sylvatica in Europe, Nectria coccinea is also spreading on American beech F. grandifolia, the minute sap-sucking felted beech coccus having been accidentally introduced into Nova Scotia around 1890.

For long-distance spread, abiotic agents such as water and wind are also important. Spores, being so small and light, are readily carried high into the atmosphere and moved long distances. This undoubtedly helps explain why the same or similar species of fungi are found on different continents. Nevertheless, spread by spore is not always the main agent of infection. Fomes root rot, mentioned above, will readily infect new host trees by spores enabling it to spread over long distances between forests but honey fungus species rarely spread this way, relying instead on vegetative spread by rhizomorphs, concentrated mycelial strands looking like black bootlaces ramifying through the soil.

The long-distance transmission of pests and diseases hidden in timber and infected planting stock is a major concern of foresters, and quite rightly so given the catalogue of major diseases released into new areas: Dutch elm disease (see Section 5.4.5), chestnut blight (see Section 5.4.6) and the important root rot Rhizina undulata introduced from northern areas to the tropics and southern hemisphere (Wingfield et al., 2001) to mention a few. However, measures taken to prevent spread, particularly quarantine and specific regulations regarding treatment, have considerably improved, although there have been notable exceptions! The military have not always adopted equivalent standards for timber which they export, indeed a number of tree diseases that took a major toll in Europe during the twentieth century, including chestnut blight, seem often to have started near military bases. A particularly clear-cut case has recently been described from the Presidential Estate of Castelporziano 24 km south of Rome, long famous for its rich forest and exclusively Italian flora. The US Army was briefly encamped in this forest during its drive up Italy in 1944, a brief incident directly related to the death of large areas of stone pines Pinus pinea in the 1980s. This was caused by Fomes root rot Heterobasidion annosum which spreads from the root of one tree to the next by spores. When samples of the fungus were collected, all turned out to have DNA signatures typical of eastern North American variants of this worldwide species. The samples also differed amongst themselves, indicating that the fungus had gone through many generations since its introduction to the forest. The fungus may well have originated from untreated lumber brought in as crates or pallets by the army during 1944 and existed unnoticed for decades (Gonthier et al, 2004). Once spread, control is often almost impossible; prevention is infinitely preferable to cure.

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