Gnomoniopsis castanea is an emerging fungal pathogen causing nut rot of sweet chestnut, Castanea sativa. This study was aimed at testing and modelling the effects of climate on disease incidence. Up to 120 ripe nuts were collected in 2011 from trees in each of 12 sites located in the northwest of Italy. The incidence of G. castanea in each site was expressed as the number of infected nuts out of the total number of nuts sampled (%), determined by combining the results of morphological identification of isolates obtained from nuts, and their typing through a newly developed taxon-specific molecular assay. Disease incidence ranged from 20 to 93%, depending on site. Geostatistical analyses revealed that, despite the clustering of sites (P < 0Á05), disease incidence was not spatially autocorrelated (P > 0Á05). This finding suggests that the disease is influenced by site-dependent factors whose scale (c. 7Á5-15Á6 km) is consistent with the climate variability throughout the sampling region. Multivariate analyses on maximum, mean and minimum temperatures and on rainfall showed that warmer temperatures were associated with higher levels of disease incidence. The temperatures of months before nut harvesting were selected as predictors for partial least squares regression (PLSR) models (GnoMods) of G. castanea incidence. External validation on data collected either on sites or in years not used for model fitting showed the good predictive abilities of the GnoMods (Spearman's q obs/pred > 0Á72, P < 0Á05). The above findings support a relationship between climate and incidence of G. castanea, providing statistical tools to forecast disease incidence at site level.

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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.

Hybridization between species is being recognized as a major force in the rapid adaptive evolution of fungal plant pathogens. The first stages of interspecific hybridization necessarily involve nuclear-mitochondrial chimeras. In their 2001 publication, Olson and Stenlid reported that mitochondria control the virulence of first generation hybrids between the North American fungal pathogen Heterobasidion irregulare and its congeneric H. occidentale. By assessing saprobic ability and gene expression of H. irregulare × H. annosum sensu stricto hybrids and of their parental genotypes, we demonstrate that mitochondria also influence saprobic growth of hybrids. Moreover, gene expression data suggest that fungal fitness is modulated by an intimate interplay between nuclear genes and mitochondrial type, and is dependent on the specific mitonuclear combination.

The Eurasian spruce bark beetle Ips typographus and their fungal associates can cause severe damages to Norway spruce forests. In this paper, by using both molecular and cultural methods, we compared fungal assemblages on bark beetles from different locations, characterized by different beetle population levels. Ips typographus was trapped in the western Alps in two areas with outbreaks and in two control areas. By sequencing of clone libraries of Internal Transcribed Spacer (ITS) 31 fungal Operational Taxonomic Units (OTUs) were identified, while fungal isolations yielded 55 OTUs; only three OTUs were detected by both methods. Culturing and PCRbased assays provide different non-overlapping results. Fungal assemblages on insects varied from stand to stand in response to varying ecological conditions and to the short distance spreading ability of I. typographus, and this was supported by data from additional 12 stands.Ophiostomatoid fungi showed higher diversity in outbreak areas; the pathogenic Ophiostoma polonicum was relatively uncommon, while O. bicolor was the most abundant species, supporting the well-known hypothesis of a temporal succession. Ubiquitous endophytes of trees or common airborne fungi, like Penicillium spp., Alternaria alternata, Epicoccum nigrum and Trichoderma spp., were present both in outbreak and in control areas. Wood decaying basidiomycetes were almost never detected on beetles. Yeast assemblage, detected only by molecular analysis, was consistent with those reported elsewhere in Europe, suggesting a very long association between some yeasts and bark beetles.