This study examines the effects of climate warming on one of the most widely distributed and destructive forest pathogens, Phytophthora cinnamomi. In Europe, the winter survival of the pathogen is the dominant cue for the development of the disease it causes to oaks, especially Quercus robur and Q. rubra. The potential pathogen and disease geographic ranges were compared in France between two reference periods, 1968-1998 and 2070-2099. Simulations were obtained by combining a physiologically based approach predicting the pathogen winter survival in relation to microhabitat temperature (in the phloem of infected trees) with a regionalized climatic scenario derived from a global circulation model. Positive anomalies in winter temperatures calculated with this scenario were in the range 0.5-5 1C between the periods 2070-2099 and the 1968-1998, according to sites and months. As a consequence, higher annual rates of P. cinnamomi survival were predicted, resulting in a potential range expansion of the disease of one to a few hundred kilometers eastward from the Atlantic coast within one century. Based on this example, the study emphasizes the need of a better understanding of the impacts of global change on the biotic constraint constituted by plant pathogens.

Phytophthora cinnamomi is the causal agent of a perennial canker that develops on the lower bole on northern red oak and pedunculate oak. The disease has a limited range in Europe, being reported only in southwest France. This limited distribution is probably linked to the susceptibility of P. cinnamomi to frost. A model was developed in previous work to estimate the impact of temperatures of <0 degrees C on the winter survival of P. cinnamomi in trunk cortical tissues and on the subsequent development of cankers. In this article, we report the use of this model to simulate canker development in 503 locations across France during a 30-year period. The predicted canker extension decreased sharply when the median P. cinnamomi winter survival index decreased from 0.95 to 0.65, with cankers that poorly developed when the median survival index was lower than 0.5 to 0.6. The actual incidence of the disease in 192 stands located across southwest France was compared with that of the model outputs. Both presence of disease in stands and frequency of cankered trees in infected stands, but not canker size on infected trees, were strongly related to the median P. cinnamomi survival index. No disease was present in stands with median survival index lower than 0.65, and the frequency of cankered trees in infected stands remained very low in stands with a median survival index between 0.65 and 0.70. Aspect was an additional factor explaining disease incidence, while the effect of elevation was likely due to its effect on winter temperatures. Maps of winter suitability to P. cinnamomi-induced cankers on oaks in France are presented.

La maladie de l'encre, signalée sur les chênes rouges dès 1948 dans les Pyrénées-Atlantiques, est causée par un champignon appelé Phytophthora cinnamomi. Cette maladie se manifeste par la présence, à la partie inférieure du tronc, de plaies chancreuses boursouflées d'où s'écoule un liquide noirâtre. Le chêne pédonculé semblait jusqu'à présent relativement épargné par cette maladie. Mais, en 1996, des jeunes arbres présentant des symptômes ont été découverts dans quelques départements du Sud-Ouest. P. cinnamomi, originaire de la zone tropicale, est très sensible au gel. Cela pourrait expliquer la répartition actuelle de la maladie, limitée au Sud-Ouest de la France. L'Inra a développé et validé sur quatre sites du Sud-Ouest un modèle permettant de calculer une note de survie du champignon à l'intérieur des troncs, à partir de la température de l'air. L'analyse spatiotemporelle des notes de survie, calculées par le modèle sur une sélection de postes du territoire national, montre que les zones potentiellement à risque sont nettement plus étendues que les zones avec mention actuelle de maladie. Il est donc possible, dans les prochaines années, de voir la maladie se développer dans de nouveaux secteurs, en particulier en Bretagne. Mapping the impact of frost on ink disease in oak trees Ink disease, caused by Phytophthora cinnamomi, has been reported on red oaks since 1948 in SouthWestern France. Symptoms are expressed on the lower part of the trunk, as bleeding cankers with a black exudate. Until recently, pedunculate oak seemed to be less affected by the disease. However, the disease has been reported in several young plantations since 1996 in SouthWestern France. P. cinnamomi, which has a tropical origin, is very sensitive to freezing temperatures. This may explain the present distribution range of the disease, limited to the SouthWestern part of France. INRA has developed a model of the fungus survival inside trunks as a function of air temperature. The model was validated in four sites. We used this model to calculate the fungus survival over 30 years from meteorological data collected at a selection of stations across the nation. The space-time analysis of these data showed that zones with potential disease risk spread much beyond the present limit of the reported disease zone. The disease might therefore develop in new areas, particularly in Brittany, in the coming years. Le chêne est la première essence commerciale feuillue française. L'économie du chêne et de ses produits dérivés se caractérise par la très grande diversité des utilisations de cette essence, principalement dans l'ameublement, dans le bâtiment et dans divers emplois en emballage, calage, bois sous rails, fonds de wagons, etc. (Normandin, 1990).