G. Lione

The American forest pathogen Heterobasidion irregulare colonizes unexpected habitats after its introduction in Italy

Habitat preference of an invasive fungal tree pathogen is here compared with that of a sympatric and native closely related congener to test the hypothesis that the invasive ability of the exotic organism may be linked to its capacity to colonize substrates unavailable to the indigenous relative. We compared the distribution of infectious airspora of the North American Heterobasidion irregulare introduced into Italy with that of the native H. annosum, both regarded to be able to establish only in the presence of conifers. Geostatistical and statistical analyses were employed to test for association between both species and five Mediterranean vegetation types. Results show that H. annosum is positively associated with pines and negatively associated with deciduous oaks. The probability of finding its spores decreases to almost 0 at distances over 500 m from pines, and this species is virtually absent in pure oak forests. Spores of H. irregulare are present irrespective of vegetation type, and this species can be found not only where pines are present, but also in pure oak forests. This knowledge implies that spread of H. irregulare is not limited by the fragmented distribution of pine woodlands in central Italy and is essential to both predict and hinder its progress in Europe.

The saprobic and fruiting abilities of the exotic forest pathogen Heterobasidion irregulare may explain its invasiveness

Abstract The North American fungal pathogen Heterobasidion irregulare is invading and threatening pine stands in Italy and is freely hybridizing with the native species Heterobasidion annosum. Susceptibility of native hosts has been excluded as a factor driving H. irregulare invasion. Here we tested whether H. irregulare and H. annosum differ in their ability to saprobically colonize pine wood, and whether saprobic growth is correlated to fruiting bodies production. When inoculated in pine logs, H. irregulare genotypes colonized a volume of wood significantly larger than H. annosum genotypes. Heterobasidion irregulare significantly exceeded H. annosum in all parameters used as metrics of fruiting body production, including number and size of fruiting bodies, and pores perimeter index (PPI), an index summarizing the amount of surface available for spore production. Number of fruiting bodies and PPI were significantly correlated with volume of wood colonized by Heterobasidion genotypes. Results may explain why H. irregulare has been reported to sporulate more abundantly than H. annosum and provide explanations for its high transmission potential in Italy. This knowledge implies that approaches to control the spread of H. irregulare should be aimed at limiting saprobic establishment of the fungus rather than focusing on identifying more tolerant tree species.

HRM analysis provides insights on the reproduction mode and the population structure of Gnomoniopsis castaneae in Europe

Gnomoniopsis castaneae is an emergent nut rot agent of chestnut in southern Europe. To elucidate its population genetics, three simple sequence repeat (SSR) and two hypervariable markers were developed and assessed through highresolution melting (HRM) analysis on 132 isolates collected from 10 sites in Italy, France and Switzerland. High allele diversity (ranging from 0.23 to 0.40 depending on site) and number of haplotypes (49) were observed. More than 70% of the molecular variance could be accounted among isolates within sites. Multilocus analysis showed absence of linkage disequilibrium, suggesting a predominant role played by sexual reproduction and random mating. Data analyses indicated the presence of at least two putative distinct subpopulations and this was confirmed by several approaches, including analysis of shared haplotypes, multivariate and Bayesian analyses. Based on data of allelic diversity, the possibility that the pathogen could have been introduced is discussed. This work assessed the genetic variability and the sexual strategies of G. castaneae in Europe, adding useful information on the epidemiology of this fungal plant pathogen.