Ash dieback or chalarose - what do we know?
La Chalarose du Frêne : que sait-on ?
Résumé
Common ash ( Fraxinus excelsior) is threatened by a recently identified fungal pathogen Hymenoscyphus pseudoalbidus (asexual form Chalara fraxinea). This disease is called ash dieback or "Chalarose" in French. It appeared in north-eastern Europe in the mid 1990's, and has since spread across most of the natural range for common ash. The disease was first detected in France in 2008. The Forest Health Department (DSF) has implemented a monitoring scheme for fungus presence throughout France along with its progression over time. By the end of 2009, 13 French Departements were infected in north-eastern France, along with one isolated outbreak in the north of France. One year later in 2010, 18 Departements were infected, and their number grew to 29 in 2011, with fungus infections spanning the north-northeast of France, reaching the Massif Central in the centre of France. Ascospores may be the only mode of fungal spread. They develop during the summer from apothecia that grow on previous year's ash leaf petioles and rachises in the forest litter. A study is underway in France to assess ascospore dispersal. A wide range of symptoms characterizes the disease. These include leaf wilting, foliage necrosis, shoot dieback, orange bark necrosis, premature leaf fall, overall defoliation, and abnormal colour of the wood. These symptoms change with time. Special emphasis is placed on cortical necrosis that appears in the tree collar. A French team of scientists only very recently established the connection between this symptom and H. pseudoalbidus. Very severe forms of these necroses are found in populations that were the first to be affected by the disease in France. A study conducted in the first French Departement to be infected shows that necrotic foci are found everywhere, albeit small in size. From the Forest Health Department's findings, it appears that trees with foliage necrosis in the collar do not necessarily show signs of defoliation in the tree crown. The two mechanisms of infection (foliage/shoot necrosis and collar necrosis) thus seem distinct - a hypothetical new disease cycle for ash dieback is proposed. In 2010, the DSF put up a network of monitoring plots on adult trees in contaminated areas. This will help better measure the way symptoms progress in infected populations as well as give a better estimate of the overall impact of the fungus on ash. Preliminary results are discussed and compared with those from studies under-taken in other countries, showing that mortality is not apparent after two years and appears to progress very slowly. In France, the first adult trees died around three years after infection. The disease is often first detected in seedlings and appears in adults only later. Overall, all ages and landscapes are susceptible to infection. Current recommendations for forestry management in France are comparable to those coming from other infected European countries: no more investment in ash as a species, selective harvest of infected trees while apparently tolerant trees are kept in place where possible so that potential naturally occurring resistance in ash population is not destroyed. Log transport is allowable after removal of necrosis-bearing zones, since the fungus is absent from healthy wood. When infected parts are removed, the risk of infection disappears. The article also explains why different countries react differently to the disease.
Le Frêne commun, Fraxinus excelsior, est menacé par un champignon pathogène récemment identifié, Hymenoscyphus pseudoalbidus (forme asexuée, Chalara fraxinea ). Cette maladie, la chalarose, apparue au Nord-Est de l’Europe dans les années 1990, s’est disséminée dans une grande partie de l’aire naturelle du Frêne commun. Détectée en France en 2008, elle affecte fin 2011 les frênaies du Nord et de l’Est du territoire et s’est rapidement étendue jusque dans le Massif central. Les multiples symptômes de cette nouvelle maladie sont décrits. L’accent est mis sur les nécroses corticales qui apparaissent au collet des arbres. Ce n’est que très récemment que ce symptôme a été attribué à H. pseudoalbidus. La rapidité de l’évolution des symptômes dans les peuplements est abordée grâce aux premiers résultats d’un suivi symptomatologique réalisé sur un réseau de placettes. Cet article est l’occasion de dresser un bilan des connaissances sur la biologie de ce nouveau pathogène, de décrire la situation des pays atteints et d’aborder les recommandations de gestion.