Sequence and simple-sequence repeat analyses of the fungal pathogen Seiridium cardinale indicate California is the most likely source of the cypress canker epidemic for the Mediterranean region

Ozone risk assessment of common cypress (Cupressus sempervirens L.) clones and effects of Seiridium cardinale infection

Cupressus sempervirens is a relevant species in the Mediterranean for its cultural, economic and landscape value. This species is threatened by Seiridium cardinale, the causal agent of the cypress canker disease (CCD). The effects of biotic stressors on O3 risk assessment are unknown and a comprehensive O3 risk assessment in C. sempervirens is missing. To fill these gaps, two clones of C. sempervirens, one resistant (Clone R) and one susceptible to CCD (Clone S), were subjected to three levels of O3 (Ambient Air - AA; 1.5 × AA; 2.0 × AA) for two consecutive years in an O3-free-air controlled exposure facility and artificially inoculated with S. cardinale. Both the exposure- (AOT40) and flux-based (PODy) indices were tested. We found that PODy performed better than AOT40 to assess O3 effects on biomass and the critical level for a 4% biomass loss was 2.51 mmol/m2 POD2. However, significant O3 dose-response relationships were not found for the inoculated cypresses because the combination of middle level O3 (1.5 × AA) and inoculation stimulated a biomass growth in Clone S as hormetic response. Moreover, we found a different inter-clonal response to both stressors with a statistically significant reduction of total and belowground biomass following O3, and lower root biomass in Clone S than in Clone R following pathogen infection. In summary, Clone R was more resistant to O3, and inoculation altered O3 risk via an hormetic effect on biomass. These results warrant further studies on how biotic stressors affect O3 responses and risk assessment.

Ultrastructure of Terpene and Polyphenol Synthesis in the Bark of Cupressus sempervirens After Seiridium cardinale Infection

Cypress Canker Disease (CCD) pandemic caused by Seiridium cardinale is the major constraint of many Cupressaceae worldwide. One of the main symptoms of the disease is the flow of resin from the cankered barks. While inducible phloem axial resin duct-like structures (PARDs) have recently been characterized from an anatomical point of view, their actual resin production is still being debated and has never been demonstrated. Although the involvement of polyphenolic parenchyma cells (PP cells) in the bark of Cupressus sempervirens after S. cardinale infection was revealed in one of our previous studies using light microscopy, their evolution from the phloem parenchyma cells is yet to be clarified. This study investigated functional and ultrastructural aspects of both PARD-like structures and PP cells by means of more in-depth light (LM) and fluorescence microscopy (FM) combined with histochemical staining (using Sudan red, Fluorol Yellow, NADI Aniline blue black, and Toluidine blue staining), in addition to Transmission Electron Microscope (TEM). Two-year-old stem sections of a C. sempervirens canker-resistant clone (var. “Bolgheri”), artificially inoculated with S. cardinale, were sampled 5, 7, 14, 21, and 45 days after inoculation, for time-course observations. FM observation using Fluorol yellow dye clearly showed the presence of lipid material in PARD-like structures lining cells of the cavity and during their secretion into the duct space/cavity. The same tissues were also positive for NADI staining, revealing the presence of terpenoids. The cytoplasm of the ducts' lining cells was also positive for Sudan red. TEM observation highlighted the involvement of plastids and endoplasmic reticulum in the production of terpenoids and the consequent secretion of terpenoids directly through the plasma membrane, without exhibiting vesicle formation. The presence of a high number of mitochondria around the area of terpenoid production suggests that this process is active and consumes ATP. The LM observations showed that PP cells originated from the phloem parenchyma cells (and possibly albuminous cells) through the accumulation of phenolic substances in the vacuole. Here, plastids were again involved in their production. Thus, the findings of this work suggest that the PARD-like structures can actually be considered PARDs or even bark traumatic resin ducts (BTRD).

Seiridium (Sporocadaceae): an important genus of plant pathogenic fungi

The genus Seiridium includes multiple plant pathogenic fungi well-known as causal organisms of cankers on Cupressaceae. Taxonomically, the status of several species has been a topic of debate, as the phylogeny of the genus remains unresolved and authentic ex-type cultures are mostly absent. In the present study, a large collection of Seiridium cultures and specimens from the CBS and IMI collections was investigated morphologically and phylogenetically to resolve the taxonomy of the genus. These investigations included the type material of the most important Cupressaceae pathogens, Seiridium cardinale, S. cupressi and S. unicorne. We constructed a phylogeny of Seiridium based on four loci, namely the ITS rDNA region, and partial translation elongation factor 1-alpha (TEF), β-tubulin (TUB) and RNA polymerase II core subunit (RPB2). Based on these results we were able to confirm that S. unicorne and S. cupressi represent different species. In addition, five new Seiridium species were described, S. cupressi was lectotypified and epitypes were selected for S. cupressi and S. eucalypti.

The Epidemic Spread of Seiridium cardinale on Leyland Cypress Severely Limits Its Use in the Mediterranean

Leyland cypress (× Hesperotropsis leylandii) is a fast-growing conifer used in most temperate regions as an ornamental tree for hedges and screens, and is one of the most commercially important trees in Europe. In recent years, severe diebacks and mortality due to cypress canker have been observed on Leyland cypress plantations in Southern Europe. This study was conducted to evaluate (i) the spread and impact of cypress canker caused by Seiridium cardinale in plantations of a sample area of 1,250 km² in central Italy, (ii) the response of the most commonly grown Leyland cypress varieties to artificial inoculation with to S. cardinal, and (iii) the pathogenicity of S. cardinale isolates obtained from Leyland cypress. Of the 1,411 surveyed trees, 11.4% had been killed by cypress canker and 43.9% of the living trees were affected by the disease. The number of diseased or dead trees and the percentage of cankered trunks was significantly correlated with the mean trunk diameter of the plantations. Six months after inoculation, the size of developed cankers was significantly different among the inoculated Leyland cypress cultivars but all of them showed markedly larger cankers than the C. sempervirens canker-resistant control clone. All of the tested S. cardinale isolates obtained from Leyland cypress also caused cankers on Cupressus sempervirens when inoculated as conidial suspensions or mycelia. Leyland cypress is highly prone to contract cypress canker in the Mediterranean due to its high susceptibility to S. cardinale infections, low genetic variability among the grown cultivars, and cracks which form on fast-growing trunks, favoring entry of the fungus into the inner bark and the occurrence of infections.

Population structure of the invasive forest pathogen Hymenoscyphus pseudoalbidus

Understanding the genetic diversity and structure of invasive pathogens in source and in introduced areas is crucial to the revelation of hidden biological features of an organism, to the reconstruction of the course of invasions and to the establishment of effective control measures. Hymenoscyphus pseudoalbidus (anamorph: Chalara fraxinea) is an invasive and highly destructive fungal pathogen found on common ash Fraxinus excelsior in Europe and is native to East Asia. To gain insights into its dispersal mechanisms and history of invasion, we used microsatellite markers and characterized the genetic structure and diversity of H. pseudoalbidus populations at three spatial levels: (i) between Europe and Japan, (ii) in Europe and (iii) at the epidemic's front in Switzerland. Phylogenetic and network analysis demonstrated that individuals from both regions are conspecific. However, populations from Japan harboured a higher genetic diversity and were genetically differentiated from European ones. No evident population structure was found among the 1208 European strains using Bayesian and multivariate clustering analysis. Only the distribution of genetic diversity in space, pairwise population differentiation (GST) and the spatial analysis of principal components revealed a faint geographical pattern around Europe. A significant allele deficiency in most European populations pointed to a recent genetic bottleneck, whereas no pattern of isolation by distance was found. Our data suggest that H. pseudoalbidus was introduced just once by at least two individuals. The potential source region of H. pseudoalbidus is vast, and further investigations are required for a more accurate localization of the source population.

Population genetic analyses provide insights on the introduction pathway and spread patterns of the North American forest pathogen Heterobasidion irregulare in Italy

A population genetics approach is used to identify the most likely introduction site and introduction pathway for the North American forest pathogen Heterobasidion irregulare using 101 isolates from six sites in Italy and 34 isolates from five sites in North America. Diversity indices based on sequences from ten loci indicate the highest diversity in Italy is found in Castelfusano/Castelporziano and that diversity progressively decreases with increasing distance from that site. AMOVA, Bayesian clustering and principal coordinates analyses based on 12 SSR loci indicate high levels of gene flow among sites, high frequency of admixing, and fail to identify groups of genotypes exclusive to single locations. Cumulatively, these analyses suggest the current infestation is the result of multiple genotypes expanding their range from a single site. Based on two sequenced loci, a single source site in North America could provide enough variability to explain the variability observed in Italy. These results support the notion that H. irregulare was introduced originally in Castelporziano: because Castelporziano has been sealed off from the rest of the world for centuries except for a camp set up by the US military in 1944, we conclude the fungus may have been transported in infected wood used by the military. Finally, spatial autocorrelation analyses using SSR data indicate a significant under-dispersion of alleles up to 0.5-10 km, while a significant overdispersion of alleles was detected at distances over 80 km: these ranges can be used to make predictions on the likely dispersal potential of the invasive pathogen.

History of the invasion of the anther smut pathogen on Silene latifolia in North America

Understanding the routes of pathogen introduction contributes greatly to efforts to protect against future disease emergence. Here, we investigated the history of the invasion in North America by the fungal pathogen Microbotryum lychnidis-dioicae, which causes the anther smut disease on the white campion Silene latifolia. This system is a well-studied model in evolutionary biology and ecology of infectious disease in natural systems. Analyses based on microsatellite markers show that the introduced American M. lychnidis-dioicae probably came from Scotland, from a single population, and thus suffered from a drastic bottleneck compared with genetic diversity in the native European range. The pattern in M. lychnidis-dioicae contrasts with that found by previous studies in its host plant species S. latifolia, also introduced in North America. In the plant, several European lineages have been introduced from across Europe. The smaller number of introductions for M. lychnidis-dioicae probably relates to its life history traits, as it is an obligate, specialized pathogen that is neither transmitted by the seeds nor persistent in the environment. The results show that even a nonagricultural, biotrophic, and insect-vectored pathogen suffering from a very strong bottleneck can successfully establish populations on its introduced host.

Phenotypic diversification is associated with host-induced transposon derepression in the sudden oak death pathogen Phytophthora ramorum

The oomycete pathogen Phytophthora ramorum is responsible for sudden oak death (SOD) in California coastal forests. P. ramorum is a generalist pathogen with over 100 known host species. Three or four closely related genotypes of P. ramorum (from a single lineage) were originally introduced in California forests and the pathogen reproduces clonally. Because of this the genetic diversity of P. ramorum is extremely low in Californian forests. However, P. ramorum shows diverse phenotypic variation in colony morphology, colony senescence, and virulence. In this study, we show that phenotypic variation among isolates is associated with the host species from which the microbe was originally cultured. Microarray global mRNA profiling detected derepression of transposable elements (TEs) and down-regulation of crinkler effector homologs (CRNs) in the majority of isolates originating from coast live oak (Quercus agrifolia), but this expression pattern was not observed in isolates from California bay laurel (Umbellularia californica). In some instances, oak and bay laurel isolates originating from the same geographic location had identical genotypes based on multilocus simples sequence repeat (SSR) marker analysis but had different phenotypes. Expression levels of the two marker genes analyzed by quantitative reverse transcription PCR were correlated with originating host species, but not with multilocus genotypes. Because oak is a nontransmissive dead-end host for P. ramorum, our observations are congruent with an epi-transposon hypothesis; that is, physiological stress is triggered on P. ramorum while colonizing oak stems and disrupts epigenetic silencing of TEs. This then results in TE reactivation and possibly genome diversification without significant epidemiological consequences. We propose the P. ramorum-oak host system in California forests as an ad hoc model for epi-transposon mediated diversification.