Leonardo Varvaro

A framework to gauge the epidemic potential of plant pathogens in environmental reservoirs: the example of kiwifruit canker

New economically important diseases on crops and forest trees emerge recurrently. An understanding of where new pathogenic lines come from and how they evolve is fundamental for the deployment of accurate surveillance methods. We used kiwifruit bacterial canker as a model to assess the importance of potential reservoirs of new pathogenic lineages. The current kiwifruit canker epidemic is at least the fourth outbreak of the disease on kiwifruit caused by Pseudomonas syringae in the mere 50 years in which this crop has been cultivated worldwide, with each outbreak being caused by different genetic lines of the bacterium. Here, we ask whether strains in natural (non-agricultural) environments could cause future epidemics of canker on kiwifruit. To answer this question, we evaluated the pathogenicity, endophytic colonization capacity and competitiveness on kiwifruit of P. syringae strains genetically similar to epidemic strains and originally isolated from aquatic and subalpine habitats. All environmental strains possessing an operon involved in the degradation of aromatic compounds via the catechol pathway grew endophytically and caused symptoms in kiwifruit vascular tissue. Environmental and epidemic strains showed a wide host range, revealing their potential as future pathogens of a variety of hosts. Environmental strains co-existed endophytically with CFBP 7286, an epidemic strain, and shared about 20 virulence genes, but were missing six virulence genes found in all epidemic strains. By identifying the specific gene content in genetic backgrounds similar to known epidemic strains, we developed criteria to assess the epidemic potential and to survey for such strains as a means of forecasting and managing disease emergence.

Disease and Frost Damage of Woody Plants Caused by Pseudomonas syringae: Seeing the Forest for the Trees

Abstract Pseudomonas syringae is a phytopathogenic bacterium that causes diseases of monocots, herbaceous dicots, and woody dicots, worldwide. On woody plants, reports of disease due to P. syringae have markedly increased in the last years and the diseases have been recognized as a major threat to the primary products of agroforestry practices. Detection in Italy of a new highly aggressive population of P. syringae in 2008 on kiwifruit, which caused severe epidemics in the following years throughout the kiwifruit-growing areas of Asia, Europe, Oceania, and South America, rendered the entire kiwifruit industry vulnerable to the disease. Similarly, occurrence of an aggressive population of P. syringae on horse chestnut in 2002 in the Netherlands has rapidly established itself as a major threat to horse chestnut throughout Northwest Europe. To better understand the origin of such disease epidemics, a thorough knowledge of the pathogen is needed in sensu lato . Here, we report the most important features of the pathogen and its hosts in an attempt to clarify some key aspects. In particular, the diseases and the economic losses they cause, disease epidemiology, pathogen diversity, and the possible means of disease control have been discussed throughout the manuscript. In addition to the ability to cause the disease, the damage caused to woody plants through the ice nucleation activity of this bacterium is discussed.

Bacterial Populations Related to Gerbera ("Gerbera jamesonii" L.) Stem Break

Bacterial distribution, both external (epiphytic) and internal (endophytic), on Gerbera jamesonii L. cv. Provence and its relationship to gerbera stem break and ethylene production were investigated. The greatest number of epiphytic bacteria was found at capitulum level and 20 cm below. Three genera of bacteria were identified: Acinetobacter, Bacillus and Pantoea. A silver-nitrate solution greatly reduced ethylene production in cut flowers. The use of acid fuchsin solution revealed an occlusion of the xylem vessels, probably due to bacterial cells. The bacteria Acinetobacter, Pantoea and Bacillus appeared to be involved in stem break once their populations reached 105 cfu g-1 of stem tissue.

The Pseudomonas viridiflava phylogroups in the P. syringae species complex are characterized by genetic variability and phenotypic plasticity of pathogenicity-related traits.

As a species complex, Pseudomonas syringae exists in both agriculture and natural aquatic habitats. P.viridiflava, a member of this complex, has been reported to be phenotypically largely homogenous. We characterized strains from different habitats, selected based on their genetic similarity to previously described P.viridiflava strains. We revealed two distinct phylogroups and two different kinds of variability in phenotypic traits and genomic content. The strains exhibited phase variation in phenotypes including pathogenicity and soft rot on potato. We showed that the presence of two configurations of the Type III Secretion System [single (S-PAI) and tripartite (T-PAI) pathogenicity islands] are not correlated with pathogenicity or with the capacity to induce soft rot in contrast to previous reports. The presence/absence of the avrE effector gene was the only trait we found to be correlated with pathogenicity of P.viridiflava. Other Type III secretion effector genes were not correlated with pathogenicity. A genomic region resembling an exchangeable effector locus (EEL) was found in S-PAI strains, and a probable recombination between the two PAIs is described. The ensemble of the variability observed in these phylogroups of P.syringae likely contributes to their adaptability to alternating opportunities for pathogenicity or saprophytic survival.

Epidemiological study of hazelnut bacterial blight in central Italy by using laboratory analysis and geostatistics.

Incidence of Xanthomonas arboricola pv. corylina, the causal agent of hazelnut bacterial blight, was analyzed spatially in relation to the pedoclimatic factors. Hazelnut grown in twelve municipalities situated in the province of Viterbo, central Italy was studied. A consistent number of bacterial isolates were obtained from the infected tissues of hazelnut collected in three years (2010–2012). The isolates, characterized by phenotypic tests, did not show any difference among them. Spatial patterns of pedoclimatic data, analyzed by geostatistics showed a strong positive correlation of disease incidence with higher values of rainfall, thermal shock and soil nitrogen; a weak positive correlation with soil aluminium content and a strong negative correlation with the values of Mg/K ratio. No correlation of the disease incidence was found with soil pH. Disease incidence ranged from very low (<1%) to very high (almost 75%) across the orchards. Young plants (4-year old) were the most affected by the disease confirming a weak negative correlation of the disease incidence with plant age. Plant cultivars did not show any difference in susceptibility to the pathogen. Possible role of climate change on the epidemiology of the disease is discussed. Improved management practices are recommended for effective control of the disease.

Influence of Nitrogen Fertilization on the Colonization of Olive Phylloplane by Pseudomonas syringae subsp. savastanoi.

P. s. subsp. savastanoi (Smith) Young et al. is the causai agent of one of the most common disease of oiive (Olea europea L.) and oieander (Nerium oleander L.) plants. On olive plant the disease is known as “knot” or “tubercle”. Populations of the pathogen were tested to verify the influence of calcium nitrate on their ability to colonize olive phyiloplane. Olive plants, one-year young, cv. Nocellara del Belice, have been utilized in pot in greenhouse. Two different levels of nitrate were used: 0,65 g/l (low concentration) and 1,3 g/l (high concentration); as control, olive plants have been utilized without fertilization. The inoculum (108 cfu/ml) has been spreaded for each thesis, as follow; (a) 48 h after fertilization; (b) 48 h before fertilization; (c) at the same time with fertilization; (d) 7 days after fertilization. Nitrogen fertilization show a positive effect on the colonization of olive phylloplane by P. s. subsp. savastanoi. Indipendently by doses, calcium nitrate distributed before the inoculum is the most effective on colonization of olive phylioplane by P. s. subsp. savastanoi. The trend of inoculum without nitrogen fertilization, following an initial increment shows a decrease.

Mutability in Pseudomonas viridiflava as a programmed balance between antibiotic resistance and pathogenicity

Mutable bacterial cells are defective in their DNA repair system and often have a phenotype different from that of their wild-type counterparts. In human bacterial pathogens, the mutable and hypermutable phenotypes are often associated with general antibiotic resistance. Here, we quantified the occurrence of mutable cells in Pseudomonas viridiflava, a phytopathogenic bacterium in the P. syringae complex with a broad host range and capacity to live as a saprophyte. Two phenotypic variants (transparent and mucoid) were produced by this bacterium. The transparent variant had a mutator phenotype, showed general antibiotic resistance and could not induce disease on the plant species tested (bean). In contrast, the mucoid variant did not display mutability or resistance to antibiotics and was capable of inducing disease on bean. Both the transparent and mucoid variants were less fit when grown in vitro, whereas, in planta, both of the variants and wild-types attained similar population densities. Given the importance of the methyl-directed mismatch repair system (MMR) in the occurrence of mutable and hypermutable cells in human bacterial pathogens, we investigated whether mutations in mut genes were associated with mutator transparent cells in P. viridiflava. Our results showed no mutations in MMR genes in any of the P. viridiflava cells tested. Here, we report that a high mutation rate and antibiotic resistance are inversely correlated with pathogenicity in P. viridiflava, but are not associated with mutations in MMR. In addition, P. viridiflava variants differ from variants produced by other phytopathogenic bacteria in the absence of reversion to the wild-type phenotype.

Whole-Genome Sequencing of 10 Pseudomonas syringae Strains Representing Different Host Range Spectra.

ABSTRACT Pseudomonas syringae is a ubiquitous bacterium that readily persists in environmental habitats as a saprophyte and also is responsible for numerous diseases of crops. Here, we report the whole-genome sequences of 10 strains isolated from both woody and herbaceous plants that will contribute to the elucidation of the determinants of their host ranges.

Summer Heat and Low Soil Organic Matter Influence Severity of Hazelnut Cytospora Canker

Cytospora canker, caused by the fungus Cytospora corylicola, is present in hazelnut production areas worldwide. The disease is widespread throughout the main production areas of Italy. The causal agent is considered to be a secondary invader of damaged tissue that attacks mainly stressed plants. However, little is known of disease severity and stress factors that predispose plants to infection. In particular, the role of pedoclimatic factors was investigated. Direct survey indicated that disease severity varied across several study sites. Geostatistics showed a strong positive correlation between disease severity index and summer heat (r = 0.80 and 0.91 for July and August, respectively) and strong negative correlation between disease severity index and soil organic matter (r = -0.78). A moderate positive correlation between disease severity index and magnesium/potassium ratio (r = 0.58) and moderate negative correlations between disease severity index and total soil nitrogen (r = -0.53), thermal shock (r = -0.46), and rainfall (r = -0.53) were determined. No significant correlation between disease severity index and soil aluminum (r = -0.35), soil pH (r = -0.01), and plant age (r = -0.38) was found.

Correction for Bartoli et al., Whole-Genome Sequencing of 10 Pseudomonas syringae Strains Representing Different Host Range Spectra.

Volume 3, no. 2, [e00379-15][1], 2015. Page 1: [Table 1][2] should appear as shown below. View this table: TABLE 1 Genome characteristics [1]: /lookup/doi/10.1128/genomeA.00379-15 [2]: #T1