Role of ascospores in the spread of Cryphonectria parasitica
In France, Cryphonectria parasitica (EPPO A2 quarantine pest) occurs in the south (south of a line going from Savoie to Charente-Maritime, including Corse). Affected chestnut trees can be found in forests and orchards. In recent years, the disease has progressed towards the north-west and new isolated foci have been detected in areas which were previously free from the disease (Bretagne, Normandie and Alsace). Disease spread is ensured by conidia (asexual reproduction phase) which are dispersed by water over short distances and ascospores (sexual reproduction phase) which are released in the air and dispersed over longer distances. In France so far, ascospores had rarely been observed and therefore poorly studied. Experiments were done in 6 chestnut stands between 1995 and 1999. It was observed that the production of ascospores was not a rare event. Using two types of traps, it was shown that peaks of ascospore release occurred in spring and summer (whereas in USA, peaks were observed at the end of summer and in autumn). For one month, numbers of ascospores released could be related to temperatures and it was noted that numbers increased with temperatures. Ascospores were also released during rainfall periods. It was observed that old cankers produced higher numbers of ascospores. Pathogenicity of ascospores and conidia (asexual reproduction) was compared in inoculation tests. In particular, ascospores induced larger necrosis on chestnut twigs than conidia. It was concluded that ascospores play a significant role in disseminating the disease. In addition, as they introduce genetic variation within populations of the fungus, their release could cause difficulties in the use of hypovirulent strains which must be vegetatively compatible with the population to control. To limit the spread of the disease, prophylactic measures should be applied as early as possible. Diseased trees or branches have to be destroyed to reduce the inoculum. It was recommended that tree destruction and pruning should be done during dry periods.
Sources
Guérin, L.; Bastien, S.; Dechavanne, R.; Poitevin, H. (2000) Le chancre du châtaignier. Rôle des ascospores dans la progression de la maladie.
Phytoma – La Défense des Végétaux, no. 532, 55-58.