Le chancre du châtaigner dans le Massif des Maures : une hypovirulence spontanée solidement implantée

A study was carried out in the Maures mountains in chestnut plots where hypovirulent strains had been introduced as biocontrol agents (10 coppices and 1 orchard) and in 3 adjacent coppices where no treatment had been performed. The incidence of chestnut blight, assessed by estimating of the percentage of trees infected by Chryphonectria parasitica, was high (between 91 and 70 %). But the severity of the cankers caused by C. parasitica was low : the percentage of healing cankers varied between 41 and 95 %. This low severity could be accounted for by the high incidence of hypovirulence in all the studied plots : hypovirulent isolate were obtained from healing and also active cankers. Molecular analysis of the hypovirus responsible for hypovirulence in C. parasitica was performed for hypovirulent isolates obtained in 3 plots : the hypovirus used for biocontrol was never detected. All the hypovirus, except one, were similar to the hypovirus highly disseminated in Italy and Switzerland. All these results show that hypovirulence is well established in the Maures mountains, but it is not the consequence of the biocontrol performed in this area for more than 25 years. / Le chancre du châtaignier causé par Cryphnectria parasitica, introduit en France après la seconde guerre mondiale, est considéré comme le principal problème sanitaire de cette essence dans les domaines frrestier et frutier. Depuis les années 1970, une méthode biologique de lutte existe contre cette maladie et est couramment utilisée en vergers en France. Elle tire parti du phénomène de l'hypovirulence et consite à inoculer, tout autour des chancres jugés actifs, des souches virosées qui vont contaminer les souches ayant causé ces chancres. Ainsi les chancres traités par cette méthode voient leur progression rapidement stopée et cicatrisent.