Première expérimentation écologique sur les Entomophthorales (Phycomycètes) parasites de pucerons en vergers de pêchers

Résumé

Un essai infructueux d'implantation de l'entomophthorose dans une population deMyzus persicae en verger de Pêcher a révélé les principales difficultés inhérentes à ce genre d'expérimentation écologique où l'apport d'inoculum est accompagné d'une tentative de modification du milieu par des aspersions contrôlées. L'épizootiologie de l'entomophthorose dans les vergers de la vallée du Rhône est discutée en fonction de la fréquence des jours pendant lesquels l'humidité relative demeure élevée pendant plus de 12 heures et la température maximale excède 20°C.

Summary

A preliminary experiment has been undertaken in May 1970 in the Rhone Valley in order to implant focuses ofEntomophthora aphidis Hoffm. andE. thaxteriana Petch. in colonies ofMyzus persicae Sulz. and to study the effect of sprinkling.

The aim of the sprinkling is to provide the presence of a continuous film of water on the leaves of the trees during the delay (8 to 14 hours) required by the critical phase of the cycle of the pathogen (sporulation-infection).

Four plots were considered: The check-plot without inoculum or sprinkling, a plot with inoculum but without sprinkling, a plot with sprinkling but without inoculum and the fourth plot with inoculum and sprinkling.

By an extra infestation, an homogenous population has been established on the 6 trees of each plot. The inoculum consists partly in cultures of the fungous in Petri dishes, partly in dead aphids in the phase of projection of conidia.

The failure of the experiment is attributed to the very quick destruction of the inoculum by sorching winds, to the working-failure of the sprinkling system and specially to the adversity of the climatic conditions during the last decade of May (no diurnal rains, high frequency of windy days).

The epizootiology of Entomophthorales in orchards of the Rhone Valley is discussed at light of the frequence of the days with high relative humidity during more than 12 hours and that of the days with maximal temperatures exceeding 20°C.