Paolo Capretti

Biogeographical patterns and determinants of invasion by forest pathogens in Europe

A large database of invasive forest pathogens (IFPs) was developed to investigate the patterns and determinants of invasion in Europe. Detailed taxonomic and biological information on the invasive species was combined with country-specific data on land use, climate, and the time since invasion to identify the determinants of invasiveness, and to differentiate the class of environments which share territorial and climate features associated with a susceptibility to invasion. IFPs increased exponentially in the last four decades. Until 1919, IFPs already present moved across Europe. Then, new IFPs were introduced mainly from North America, and recently from Asia. Hybrid pathogens also appeared. Countries with a wider range of environments, higher human impact or international trade hosted more IFPs. Rainfall influenced the diffusion rates. Environmental conditions of the new and original ranges and systematic and ecological attributes affected invasiveness. Further spread of established IFPs is expected in countries that have experienced commercial isolation in the recent past. Densely populated countries with high environmental diversity may be the weakest links in attempts to prevent new arrivals. Tight coordination of actions against new arrivals is needed. Eradication seems impossible, and prevention seems the only reliable measure, although this will be difficult in the face of global mobility.

Role of social wasps in Saccharomyces cerevisiae ecology and evolution

Saccharomyces cerevisiae is one of the most important model organisms and has been a valuable asset to human civilization. However, despite its extensive use in the last 9,000 y, the existence of a seasonal cycle outside human-made environments has not yet been described. We demonstrate the role of social wasps as vector and natural reservoir of S. cerevisiae during all seasons. We provide experimental evidence that queens of social wasps overwintering as adults (Vespa crabro and Polistes spp.) can harbor yeast cells from autumn to spring and transmit them to their progeny. This result is mirrored by field surveys of the genetic variability of natural strains of yeast. Microsatellites and sequences of a selected set of loci able to recapitulate the yeast strain’s evolutionary history were used to compare 17 environmental wasp isolates with a collection of strains from grapes from the same region and more than 230 strains representing worldwide yeast variation. The wasp isolates fall into subclusters representing the overall ecological and industrial yeast diversity of their geographic origin. Our findings indicate that wasps are a key environmental niche for the evolution of natural S. cerevisiae populations, the dispersion of yeast cells in the environment, and the maintenance of their diversity. The close relatedness of several wasp isolates with grape and wine isolates reflects the crucial role of human activities on yeast population structure, through clonal expansion and selection of specific strains during the biotransformation of fermented foods, followed by dispersal mediated by insects and other animals.

Forest pathogens with higher damage potential due to climate change in Europe.

Abstract Most atmospheric scientists agree that climate changes are going to increase the mean temperature in Europe with increased frequency of climatic extremes, such as drought, floods, and storms. Under such conditions, there is high probability that forests will be subject to increased frequency and intensity of stress due to climatic extremes. Therefore, impacts of climate change on forest health should be carefully evaluated. Given these assumptions, several fungal diseases on trees may become more devastating because of the following factors: (i) abiotic stresses, such as drought and flooding, are known to predispose trees to several pathogens; (ii) temperature and moisture affect pathogen sporulation and dispersal, and changes in climatic conditions are likely to favour certain pathogens; (iii) migration of pathogens triggered by climatic change may increase disease incidence or geographical range, when pathogens encounter new hosts and (or) new potential vectors; and (iv) new threats may appear either because of a change in tree species composition or because of invasive species. If infection success is dependent on temperature, higher mean temperatures may lead to more attacks. Pathogens that have been of importance in southern Europe may spread northward and also upward to mountains. Pathogens with evolutionary potential for greater damage should be identified to estimate the magnitude of the threat and to prepare for the changing conditions. A review of the above-mentioned cases is presented. Some priorities to improve the ability to predict impacts of climate change on tree diseases are discussed.

Genetic variation in eastern North American and putatively introduced populations of Ceratocystis fimbriata f. platani

The plant pathogenic fungus Ceratocystis fimbriata f. platani attacks Platanus species (London plane, oriental plane and American sycamore) and has killed tens of thousands of plantation trees and street trees in the eastern United States, southern Europe and Modesto, California. Nuclear and mitochondrial DNA fingerprints and alleles of eight polymorphic microsatellite markers of isolates of C. fimbriata from these regions delineated major differences in gene diversities. The 33 isolates from the eastern United States had a moderate degree of gene diversity, and unique genotypes were found at each of seven collection sites. Fingerprints of 27 isolates from 21 collection sites in southern Europe were identical with each other; microsatellite markers were monomorphic within the European population, except that three isolates differed at one locus each, due perhaps to recent mutations. The genetic variability of C. fimbriata f. platani in the eastern United States suggests that the fungus is indigenous to this region. The genetic homogeneity of the fungus in Europe suggests that this population has gone through a recent genetic bottleneck, perhaps from the introduction of a single genotype. This supports the hypothesis that the pathogen was introduced to Europe through Naples, Italy during World War II on infected crating material from the eastern United States. The Californian population may also have resulted from introduction of one or a few related genotypes because it, too, had a single nuclear and mitochondrial genotype and limited variation in microsatellite alleles.

Real-time PCR detection of Biscogniauxia mediterranea in symptomless oak tissue

Aims: Real‐time PCR, based on TaqMan® chemistry, was used to detect Biscogniauxia mediterranea, a fungal pathogen that after a long endophytic phase may cause charcoal disease in oak trees.

Environmental factors related to damage by Heterobasidion abietinum in Abies alba forests in Southern Italy

Abstract A study was carried out in southern Italy to determine the role of some factors in favouring Heterobasidion annosum s. l. spread and damage in eight Silver-fir (Abies alba Mill.) forests. All the studied stands were colonised by H. abietinum. The presence of the fungus (as carpophores on colonised wood) was detected by means of transects and varied from 7.0 to 56.0% of trees. Stand history, precipitation and forest origin, analysed by the χ2-test, had a significant effect on rot severity. Damage was more severe in stands more xerophytic where the woods were planted on pasturelands.

New proteins orthologous to cerato-platanin in various Ceratocystis species and the purification and characterization of cerato-populin from Ceratocystis populicola

Natural variants of cerato-platanin (CP), a pathogen associated molecular pattern (PAMP) protein produced by Ceratocystis platani (the causal agent of the plane canker stain), have been found to be produced by other four species of the genus Ceratocystis, including five clones of Ceratocystis fimbriata isolated from different hosts. All these fungal strains were known to be pathogenic to plants with considerable importance in agriculture, forestry, and as ornamental plants. The putative premature proteins were deduced on the basis of the nucleotide sequence of genes orthologous to the cp gene of C. platani; the deduced premature proteins of Ceratocystis populicola and Ceratocystis variospora reduced the total identity of all the others from 87.3% to 60.3%. Cerato-populin (Pop1), the CP-orthologous protein produced by C. populicola, was purified and characterized. Pop1 was a well-structured α/β protein with a different percentage of the α-helix than CP, and it self-assembled in vitro in ordered aggregates. Moreover, Pop1 behaved as PAMP, since it stimulated poplar leaf tissues to activate defence responses able to reduce consistently the C. populicola growth.

The relatedness of the Italian F intersterility group of Heterobasidion annosum with the S group, as revealed by RAPD assay

The PCR technique based upon randomly amplified polymorphic DNA (RAPD) markers using 10-mer oligonucleotide primers was used to investigate the phylogenetic relationships among 33 isolates of Heterobasidion annosum belonging to the intersterility groups F and S. The F isolates originated from Italy, the S isolates from the Italian Alps, northern Europe and North America. The presence or absence of some bands was shown to be population-specific. The North American S group and the European S group differed significantly from each other, but within the latter group there were no significant differences between the Italian S and the northern European S populations, with as much similarity within these populations as between them. More DNA polymorphisms appeared within European F and S groups than in the North American S group. The North American S group appeared to be more closely related to the European S group than to the F group. The isolates of the Italian F. population seemed to be most closely related to the Italian S isolates and then, in decreasing order, to the northern European S and North American S isolates. The order of genetic similarity was inversely correlated with the mating frequency in vitro obtained in an earlier study.

Use of loop-mediated isothermal amplification for detection of Ophiostoma clavatum, the primary blue stain fungus associated with Ips acuminatus.

ABSTRACT Loop-mediated isothermal amplification (LAMP) is an alternative amplification technology which is highly sensitive and less time-consuming than conventional PCR-based methods. Three LAMP assays were developed, two for detection of species of symbiotic blue stain fungi associated with Ips acuminatus, a bark beetle infesting Scots pine (Pinus sylvestris), and an additional assay specific to I. acuminatus itself for use as a control. In common with most bark beetles, I. acuminatus is associated with phytopathogenic blue stain fungi involved in the process of exhausting tree defenses, which is a necessary step for the colonization of the plant by the insect. However, the identity of the main blue stain fungus vectored by I. acuminatus was still uncertain, as well as its frequency of association with I. acuminatus under outbreak and non-outbreak conditions. In this study, we employed LAMP technology to survey six populations of I. acuminatus sampled from the Southern Alps. Ophiostoma clavatum was detected at all sampling sites, while Ophiostoma brunneo-ciliatum, reported in part of the literature as the main blue stain fungus associated with I. acuminatus, was not detected on any of the samples. These results are consistent with the hypothesis that O. clavatum is the main blue stain fungus associated with I. acuminatus in the Southern Alps. The method developed in the course of this work provides a molecular tool by which it will be easy to screen populations and derive important data regarding the ecology of the species involved.

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.