Multilocus sequence typing analysis of Italian Xanthomonas campestris pv. campestris strains suggests the evolution of local endemic populations of the pathogen and does not correlate with race distribution

Xanthomonas campestris pv. campestris (Xcc) is the causal agent of black rot in Brassicaceae. It is widespread in Italy and severe outbreaks occur under conditions that favour disease development. In this study a multilocus sequence typing approach (MLST) based on the partial sequence of seven loci was applied to a selection of strains representative of the main areas of cultivation and hosts. The aim was to investigate whether the long tradition of brassica crops in Italy has influenced the evolution of different Xcc populations. All loci were polymorphic; 14 allelic profiles were identified of which 13 were unique to Italian strains. Based on the seven loci, the most common genotype within the Italian Xcc strains (AP1) was also the most representative genotype found in worldwide Xcc strains. This genotype was included in a new clonal complex in addition to three other clonal complexes already identified in Xcc populations. The phylogenetic reconstruction using a concatenated dataset of four conserved protein-coding genes, dnaK, fuyA, gyrB and rpoD, showed that the Italian strains belonged to two genetic groups. Physiological races were also investigated for the first time in Italy. The race structure of Xcc was determined by inoculating eight differential Brassica lines belonging to five species and showed that, in Italy, race 4 is the most widespread, followed by races 1 and 6. No correlation was found between allelic profiles, host of isolation, geographical origin and races, although a prevalent race was identified within the same clonal complex.     

Zinc phosphate protects tomato plants against Pseudomonas syringae pv. tomato

Journal of Plant Diseases and Protection - The purpose of this study was to determine whether zinc phosphate treatments of tomato plants (Solanum lycopersicum L.) can attenuate bacterial speck...     

Molecular characterization of Diplodia seriata,a new pathogen of Prunus laurocerasus in Italy

Twig cankers and diebacks were observed on cherry laurel (Prunus laurocerasus L.) hedges located in a private garden of Perugia (Central Italy). Four fungal isolates, forming gray colonies and aerial mycelium, were obtained from cankers. For each isolate, Koch’s postulates were fulfilled by inoculating cherry laurel plants. Based on cultural characteristics, rep-PCR and phylogenetic analyses on ITS and EF1-α nucleotides sequences, the isolates were identified as Diplodia seriata De Not. To the best of our knowledge, this is the first report of D. seriata as a new pathogen of P. laurocerasus. The severe symptoms and the wide distribution of the host plant make the pathogen potentially dangerous for this ornamental species.     

Induction of Systemic Acquired Resistance in Pepper Plants by Acibenzolar-S-methyl against Bacterial Spot Disease

The ability of acibenzolar-S-methyl to induce resistance in pepper plants against Xanthomonas campestris pv. vesicatoria was investigated in both growth chamber and open field conditions. Growth chamber experiments showed that acibenzolar-S-methyl (300M) treatment protects pepper plants systemically and locally against X. campestris pv. vesicatoria. Evidence for this was a reduction in the number and diameter of bacterial spots and bacterial growth in planta. Systemic protection was also exerted by the acibenzolar-S-methyl acid derivative, CGA 210007, which may be produced by hydrolysis in the plant. The efficacy of acibenzolar-S-methyl was also found in open field conditions, where both leaves and fruit were protected from the disease. The highest efficacy (about 67%) was obtained by spraying the plants 6–7 times every 8–12 days with a mixture of acibenzolar-S-methyl and copper hydroxide (2.5 + 40ghl^–1 active ingredient). Persistence and translocation data obtained from the growth chamber experiments revealed a persistence of acibenzolar-S-methyl lasting five days after treatment with rapid translocation and negligible levels of acid derivative formation. Since the protection exerted by acibenzolar-S-methyl against bacterial spot disease was observed when the inducer was completely degraded, it would appear to be due to SAR activation.     

Ultrastructural Observations and DNA Degradation Analysis of Pepper Leaves Undergoing a Hypersensitive Reaction to Xanthomonas campestris pv. Vesicatoria

Ultrastructural details of the hypersensitive reaction induced by infiltration with avirulent race 2 Xanthomonas campestris pv. vesicatoria in pepper Early Calwonder-10R leaves (incompatible interaction) are reported. Affected cells displayed plasmalemma undulations and disruption, lysis of the chloroplast membrane, degeneration of other organelles, general cytoplasm disorganisation and, often, protoplast shrinkage. The nuclei contained large masses of electron-dense material, apparently formed by chromatin aggregation. In many cases a single chromatin-like layer was deposited on the inner side of the nuclear envelope leaving a finely granular matrix in the centre of the nucleus; the nucleolus usually disappeared. The nuclear envelope was sometimes ruptured and the internal matrix leaked into the cytoplasm. The content of many affected cells eventually coagulated and became very electron-dense. The walls often collapsed. All these alterations were especially visible in spongy mesophyll cells at sites where bacteria occurred in the intercellular spaces. Although some of the nuclear and cytoplasmic alterations recall certain aspects of apoptotic cell death, molecular determinations did not reveal any DNA degradation in hypersensitively reacting tissues. The first cell alterations in leaves infected with the virulent bacterial race 1 (compatible interaction) were observed only 27h after inoculation, when the cytoplasm of some cells showed limited internal disorganisation and plasmolysis at sites where bacterial colonies developed.     

<Emphasis Type="Italic">Vicia faba</Emphasis> plants respond to oviposition by invasive <Emphasis Type="Italic">Halyomorpha halys</Emphasis> activating direct defences against offspring

The invasive stink bug Halyomorpha halys is established in many European and American agro-ecosystems, where it causes severe crop losses. Potential control measures might include enhancement of plant defences. When attacked by herbivorous insects that oviposit on it, the plant may respond by priming direct defences, which might affect the development of future brood. Halyomorpha halys attacks numerous plant species in the invaded areas. Here, we investigated whether Vicia faba plants challenged by H. halys females can impair the development of its offspring through the activation of induced direct defences. We measured the weight and dimension of nymphs that developed on oviposition-experienced plants after 7 and 17 days from hatching. Nymphs that developed on oviposition-experienced plants weighed less compared to those that developed either on control plants or on plants solely subjected to H. halys feeding, and third instars showed shorter dimensions (tibia length). In addition, when oviposition-experienced plants were attacked by nymphs, higher and more rapid expression of two jasmonic acid-dependent genes (cysteine proteinase inhibitor gene and NAI1) was detected, possibly due to a mechanism of priming. Increased expression of the salicylic acid-responsive PR1 gene was also detected in egg-experienced plants, although the response was delayed compared to JA-dependent genes. Our results suggest that V. faba plants recognize H. halys oviposition as a warning signal and pre-activate defences against future nymphal herbivory.     

Genetic and phenotypic diversity of Mediterranean populations of the olive knot pathogen,Pseudomonas savastanoi pv. savastanoi

Pseudomonas savastanoi pv. savastanoi (Psav) is a member of P. syringae sensu lato, and causes olive knot disease, a disease first reported over 2000 years ago. Analysing 124 isolates of Psav from 15 countries by rep‐PCR, the population genetic structure of Psav was investigated. A total of 113 distinct fingerprints were detected. Cluster analysis revealed the existence of two clusters and four subclusters. These clusters were associated with the geographic origin of isolates, which in turn correspond to historic human migration events and trade routes across the Mediterranean Sea. In contrast, multilocus sequence typing (MLST) of 2788 bp of the gapA, gltA, gyrB and rpoD genes found only one variable site among 77 representative isolates. Virulence variation was observed within the Psav population, with the most virulent strains generating knots that had a weight that was 10‐fold greater than those generated by the least virulent strains. Taken together, these data suggest that today's Psav population is the result of clonal expansion of a single strain, that moderate migration of the pathogen occurred between countries, and that changes in virulence arose during its evolution.     

Persistence and translocation of a benzothiadiazole derivative in tomato plants in relation to systemic acquired resistance against Pseudomonas syringae pv tomato

A reproducible and accurate procedure, based on HPLC analysis, has been developed to determine simultaneously acibenzolar‐S‐methyl (CGA 245 704) and its acid derivative (CGA 210 007) in tomato leaves. The limit of detection and quantification of the method are 0.015 and 0.15 mg litre^?1 for CGA 245 704 and 0.030 and 0.30 mg litre^?1 for CGA 210 007. In tomato plants treated with 250 µM CGA 245 704, it was found that the inducer rapidly translocates from treated leaves (cotyledons, 1st and 2nd) to untreated leaves (3rd to 5th), with the maximum translocation (40% of the total quantity found) occurring 8 h after the treatment. CGA 245 704 residues decreased as time elapsed in both treated and untreated tomato leaves, reaching negligible values 72 h after treatment. The acid derivative, CGA 210 007, was formed in tomato plants as early as 2 h after CGA 245 704 treatment, albeit only in the treated leaves. CGA 210 007 residues decreased in treated tomato leaves with a trend similar to that observed for CGA 245 704. Treatment of tomato plants with CGA 245 704 or CGA 210 007 at 250 µM systemically protected the plants against Pseudomonas syringae pv tomato attacks, the causal agent of bacterial speak disease. Evidence of this were reductions in the degree of infection, the bacterial lesion diameter and the bacterial growth in planta. Since neither CGA 245 704 nor CGA 210 007 inhibited bacterial growth in vitro and the protection against bacterial speak of tomato was observed when the two compounds were completely degraded, the protection must be due to the activation of the plant's defence mechanisms. © 2001 Society of Chemical Industry