Pre-existing forests as sources of pathogens? The emergence of Armillaria ostoyae in a recently planted pine forest

Fungi are among the principal causal agents of emerging plant diseases, which are a matter of worldwide concern. Changes in land use, such as the expansion of cultivated areas, are implicated in the emergence of fungal diseases, but have been less often reported for native species plantations. In the maritime pine (Pinus pinaster) forest of the Landes de Gascogne (south-western France), pine mortality due to the root rot fungus Armillaria ostoyae (Basidiomycete) has been increasing over the last 30 years. The first cases of this disease occurred in 1920 only few years after a period of rapid major change to the landscape. During the second half of the 19th century the landscape was transformed from marshes to the largest monospecific maritime pine plantation forest in Europe. We carried out two surveys (0.24 and 1 million hectares) of Armillaria root rot disease in the Landes area, to investigate the spatial distribution of pathogen damage and to determine the role of historical factors in the establishment of this pattern. For the two surveys, spatial analyses and generalised linear models revealed a significant effect on the current geographical distribution of A. ostoyae disease of the proportion of pre-existing forest in the vicinity of afforested areas and a significant effect of the proximity of the first forests planted in the coastal dunes. These results suggest that A. ostoyae was commonly distributed in pre-existing forest areas, and that most of these fragments acted as source for the colonisation of newly planted forests. Better predictions on the risk of establishment of new disease foci in this forest area can be achieved from these results.