Role of an opportunistic pathogen in the decline of stressed oak trees

1 The importance of opportunistic pathogens, in particular Armillaria species, in forest decline has often been open to debate. 2 In order to assess the role of Armillaria gallica in the decline of oak trees, 60 Quercus robur trees with high (HIP trees) or low (LIP trees) levels of A. gallica inoculum, as measured by the density of epiphytic rhizomorphs on the root collar, were artificially defoliated for 2 years. Half of the HIP trees were treated when first defoliated with boric acid to reduce the A. gallica inoculum potential (BHIP trees). The ability of in situ rhizomorphs to colonize plant material was similar for LIP and BHIP, but was lower than in HIP trees, indicating that the boric acid treatment reduced the level of A. gallica inoculum. 3 Tree growth was similar between treatments as determined by dendrochronological comparisons. Although defoliation greatly reduced both tree growth and sapwood starch reserves at the beginning of autumn, growth response to defoliation and sapwood starch concentration at the beginning of autumn were similar for LIP, BHIP and HIP trees. 4 HIP trees suffered considerably greater crown deterioration and mortality following defoliation than either BHIP or LIP trees (62%, 32% and 5% mortality rates, respectively). The trees that died had very low sapwood starch concentrations. In addition, at similar levels of sapwood starch, HIP trees were much more likely to die than LIP or BHIP trees. 5 Two other factors influenced tree mortality. Past stress that reduced the tree growth a few years prior to the start of the experiment was shown to alter the tree's ability to cope with defoliation. Oak mildew selectively infected the defoliated trees and increased the severity of the defoliation stress. 6 Thus, trees subjected to high level of A. gallica inoculum had a lower ability to overcome the defoliation stress. These findings support the forest decline models developed by Manion in 1991 and show that it is important to take into account the role of opportunistic pathogens in tree mortality processes.