Seasonal patterns of spore deposition of Heterobasidion species in four forests of the western Alps

Patterns of sporulation by Heterobasidion species were studied between the spring of 1998 and December 2000 in four forests in the Western Alps. The maximum inoculum concentration, defined as Deposition Rate (DR), ranged from 169 spores*m-2*h-1 to 1,550 spores*m-2*h-1. Although spores were captured, using woody traps, from February to October at most sites, inoculum concentration consistently peaked in the late summer or early fall. In one of the four study sites, similar fluctuations of DR were recorded in two years of sampling. A significant correlation was found between DR and the average minimum air temperature of the four weeks before sampling. Approximately 1,200 spores were isolated and identified at the species level by PCR-based methods. No significant variations of basidiospore frequencies were detected for either H. abietinum or H. annosum among sampling periods. However, the frequency of H. parviporum spores was always significantly higher in the summer. These findings suggest different patterns of sporulation among Heterobasidion species.