Infection by Armillaria mellea and some Factors Affecting Host Resistance and the Severity of Disease

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Abstract
Infection by A. mellea in Britain occurs by means of rhizomorphs; the transfer of mycelium at root contacts is probably not important as a means of spread. Following the initial phase of rhizomorph penetration, established infection in the cambium is preceded by ectotrophic spread of mycelium in the bark scales. Field investigation confirmed reports in the literature that in susceptible conifers the greatest number of deaths generally occurs approximately 6–8 years after planting. However, attacks can also occur in much older crops and the reasons for this are discussed. Inoculation experiments showed that suppressed trees and those experiencing conditions of reduced light intensity are more susceptible to infection than dominant trees and trees in full light. Species susceptibility in a variety of soils was studied by inoculation of trees 20–30 cm tall. Of the conifers tested Pseudotsuga menziesii and Abies grandis were more resistant than Picea abies, P. sitchensis, Larix kaempferi and Pinus sylvestris. Quercus robur and Fagus sylvatica were more resistant than all the conifers except A. grandis . Soil type influenced disease development and the proportion of trees infected was higher in an acidic sand soil than in an alkaline clay soil.

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