Resistance of spruce to Heterobasidion: a realistic proposition?

Heterobasidion spp. cause the largest timber losses of all pathogens in European forestry. Various techniques can be used to reduce losses, although in long-managed forests, options are very limited. The most effective way of controlling any plant disease is through the use of resistant hosts, but whether such an approach would be useful against long-lived and persistent root diseases of trees is debatable. Work in the UK over the last 25 years has focused on understanding chemical and physical defence responses of Sitka spruce to Heterobasidion annosum. Inoculation of clonal materials has demonstrated that variation in susceptibility to the pathogen is present in Sitka spruce. The tree, therefore, deploys a range of defences in order to reduce the chance of pathogen ingress.
In this paper, the constitutive defences in bark tissues of Picea species are contrasted with the induced defences that occur following mechanical damage and/or pathogen attack. The potential roles of terpenoid and phenolic compounds in defence are discussed, in terms of utility as markers for relative host susceptibility to H. annosum. Concurrent with the deployment of chemical defences, a cascade of events is initiated leading initially to production of a ligno-suberized boundary zone (LSZ) in cells extant at the time of wounding and/or pathogen attack. The LSZ enables de-differentiation of bark cortex and secondary phloem to produce a trauma-induced phellogen (wound phellogen) tissue; this generative tissue re-establishes the integrity of the periderm, producing wound phelloderm to the interior and wound phellem to the exterior. The application of metabolomics and genomics are increasing our understanding of the deployment of defences in clones of Sitka spruce showing different levels of susceptibility to Heterobasidion.