Arboricola Pv Research Articles

Evolution of the Polyphenols, Flavonoids, and Tannins Content in Walnut Leaves and Green Walnut Husk during Growing Season

The walnut leaves and the green walnut husk are been widely used in folk medicine. These vegetal materials have been reported as a source of bioactive molecules such as the phenolic compounds. In this research, there were analysed different quality parameters afferent to several walnut varieties, harvested from the germplasm collection located at S.C.D.P. Vâlcea, Romania. Romanian cultivars ‘Jupâneşti’, ‘Valrex’ and ‘Valmit’ and French one ‘Franquette’ were analyzed. The variation in bioactive compounds content from the walnut leaves and walnut husk at different developmental stages (from June to September) was investigated. The results showed that the content in polyphenols, flavonoids and tannins has significantly increased until the ripening stage, after that a decrease of the values of these parameters follows. Greater values of the three parameters were also observed at walnut leaves and green walnut husk of cultivars attacked by walnut blight, caused by the bacterium Xanthomonas arboricola pv. juglandis (Xaj). The findings of this study reveal the mode of variation in the content of polyphenols, flavonoids and tannins during the growing season of the walnut leaves and green walnut husk, emphasizing also the influence of Xaj. The results obtained demonstrated the potential of the walnut leaves and the green walnut husk as an economical source of antioxidant and antimicrobial agents.
 
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 In press - Online First. Article has been peer reviewed, accepted for publication and published online without pagination. It will receive pagination when the issue will be ready for publishing as a complete number (Volume 47, Issue 4, 2019). The article is searchable and citable by Digital Object Identifier (DOI). DOI link will become active after the article will be included in the complete issue.
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PP 1/317 (1) Xanthomonas arboricola pv. pruni on stone fruit

Specific scopeThis Standard describes the conduct of trials for the efficacy evaluation of plant protection products against Xanthomonas arboricola pv. pruni causing bacterial spot and canker on stone fruit.Specific approval and amendmentFirst approved in 2019‐09.

Characterization of the bacteria associated with alder angular leaf spot in Iran

Alnus subcordata (Caucasian Alder, Betulaceae) is known as one of the most important tree species in wood industry with high nitrogen fixation capability and high rate of biomass production. In recent years, a disease symptom with the characteristics of alder bacterial leaf spot has been reported from the forests in Mazandaran province, Iran. To characterize the leaf spot causal agent, samplings were conducted from infected trees in different regions of Mazandaran forests. Characterization of the bacteria isolated from symptomatic tissues was performed based on classical and molecular methods. Twenty colonies were purified and tested by PCR using primers X1 and X2 specific for the genus Xanthomonas and biochemical analyses as well. Based on molecular and phenotypic identifications, the isolates were identified as genus Xanthomonas. Phylogenetic analysis was carried out based on PCR amplification and sequencing of gyrB genes for 14 selected isolates. Consequently, according to the molecular data all recovered isolates were determined as X. arboricola. Furthermore, multilucus sequence analysis using gyrB, fyuA, dnaK and rpoD for various pathovars of X. arboricola showed that our isolates clustered in a single clade sister to X. arboricola pv. juglandis. The results of pathogenicity tests on alder leaves seedlings confirmed that all the isolates were pathogenic and caused leaf spot on A. subcordata. To the best of our knowledge, this is the first comprehensive study on the molecular identification of the alder bacterial leaf spot disease in Iran.

The accumulation of phenolic compounds and increased activities of related enzymes contribute to early defense against walnut blight

Xanthomonas arboricola pv. juglandis (Xaj) is the causal pathogen of walnut blight. This work evaluates the correlations between selected biochemical responses to Xaj and the degree of blight sensitivity in ten genotypes. Activity of superoxide dismutase (SOD), peroxidase (POX), l-phenylalanine ammonia-lyase (PAL) and polyphenol oxidase (PPO), and content of phenolic compounds and malondialdehyde (MDA) significantly increased in all walnut genotypes after inoculation by Xaj while Catalase (CAT) activity varied between genotypes. Genotype's blight sensitivity was correlated with relative increases in phenolic compound content and PAL, PPO and POX activity at the initial stage of inoculation. The results show both similarities and distinct features in defense response in genotypes, suggesting that the metabolism of phenolic compounds may play a more important role in initial stages of inoculation than originally thought.

Assessment of Xanthomonas arboricola pv. juglandis Bacterial Load in Infected Walnut Fruits by Quantitative PCR.

Xanthomonas arboricola pv. juglandis is the etiologic agent of important walnut (Juglans regia L.) diseases, causing severe fruit drop and high economic losses in walnut production regions. Rapid diagnostics and knowledge of bacterial virulence fitness are key to hinder disease progression and apply timely phytosanitary measures. This work describes an X. arboricola pv. juglandis-specific real-time quantitative PCR (qPCR) using X. arboricola pv. juglandis-specific DNA markers to quantify the bacterial load in infected walnut plant tissues. Method validation was achieved using calibration curves obtained with serial dilutions of X. arboricola pv. juglandis chromosomal DNA and standard curves obtained from walnut samples spiked with X. arboricola pv. juglandis cells. High correlations (R2 > 0.990 and > 0.995) and low limits of detection (35 chromosomes/qPCR reaction and 2.7 CFU/qPCR reaction) were obtained for both markers considering the calibration and standard curves, respectively. Assessment of qPCR repeatability, reproducibility, and specificity allowed us to demonstrate the reliability and consistency of the method. Furthermore, in planta quantification of X. arboricola pv. juglandis bacterial load using infected walnut fruit samples showed a higher detection resolution compared with standard PCR detection. By allowing quantification of virulence fitness of distinct X. arboricola pv. juglandis strains in planta, the proposed qPCR method may contribute to assertive risk assessment of walnut diseases caused by X. arboricola pv. juglandis and ultimately help to improve phytosanitary practices.

Xanthomonas citri subsp. citri and Xanthomonas arboricola pv. pruni: Comparative analysis of two pathogens producing similar symptoms in different host plants.

Comparative studies in Xanthomonas have provided a vast amount of data that enabled to deepen in the knowledge of those factors associated with virulence and Xanthomonas plant interaction. The species of this genus present a wide range of host plants and a large number of studies have been focused to elucidate which mechanism are involved in this characteristic. In this study, comparative genomic and phenotypic analysis were performed between X. citri subsp. citri (Xcc), one of the most studied pathogens within Xanthomonas, and X. arboricola pv. pruni (Xap), a pathogen which has aroused great interest in recent time. The work was aimed to find those elements that contribute to their host divergence despite the convergence in the symptoms that each species cause on Citrus spp. and Prunus spp., respectively. This study reveals a set of genes that could be putatively associated with the adaptation of these pathogens to their hosts, being the most remarkable those involved in environmental sensing systems such as the case of the TonB-dependent transporters, the sensors of the two-component system and the methyl accepting chemotaxis proteins. Other important variants were found in processes related to the decomposition of the cell wall as could be appreciated by their dissimilar set of cell-wall degrading enzymes. Type three effectors, as one of the most important factors in delineating the host specificity in Xanthomonas, also showed a different array when comparing both species, being some of them unique to each pathogen. On the other hand, only small variations could be connected to other features such as the motility appendages and surface adhesion proteins, but these differences were accompanied by a dissimilar capacity to attach on host and non-host leaf surface. The molecular factors found in this work provide the basis to perform a more in-depth functional analyses that unveil those actual factors associated with pathogenesis and host specificity in Xcc and Xap.

EFFICACY OF BIOCHEMICAL PREPARATIONS AND EXTRACT FROM Hypericum perforatum AGAINST BACTERIAL DISEASES

The biotechnical preparations: Biosept Active (based on a grapefruit extract) and BioZell (based on thyme oil) as well as Hypericum perforatum extract, streptomycin solution and fungicide Champion 50WP (active ingredient substance – e.i. 50% copper hydroxide) were investigated for antimicrobial effects against plant pathogenic bacteria: Agrobacterium tumefaciens, Pseudomonas syringae pv. syringae and Xanthomonas ar- boricola pv. corylina. The screening was carried out in vitro on three media: Nutrient Agar (NA Difco), Pseudomonas Agar F (Merck) – analogue of King B and 523. In the experiments, the agar plate method was applied. There were no statistically significant differences in the effect of streptomycin and Champion 50WP on the growth inhibition of three bacteria strains for medium 523 and Nutrient Agar and of P. syringae pv. syringae and X. arboricola pv. corylina for medium King B. It was determined that the antibacterial activity of Biosept Active and BioZell biopreparations and H. perforatum extract against Agrobacterium tumefaciens (strain C58), Pseudomonas syringae pv. syringae (strain 760) and Xanthomonas arboricola pv. corylina (strain RIPF-x13) were dependent on the strain of pathogen as well as the growth medium used. According to the research results obtained, the Biosept Active preparation and H. perforatum extract demonstrated high bacteriostatic activity against three bacterial strains grown on the Nutrient Agar medium.

Macrolactin a is the key antibacterial substance of Bacillus amyloliquefaciens D2WM against the pathogen Dickeya chrysanthemi

B. amyloliquefaciens D2WM, a plant growth-promoting rhizobacteria from the Camellia oleifera rhizosphere, exhibits strong antimicrobial activity and has the capacity to promote plant growth, and this study aimed to determine its antibacterial mechanism against the soft rot disease pathogen D. chrysanthemi HBEU-9. The metabolites from D2WM were also identified and purified. The crude extract (CE) of D2WM effectively inhibited the mycelial growth of five fungal species and the growth of five bacterial species. In particular, CE was well able to inhibit walnut blight caused by Xanthomonas arboricola pv. Juglandis, which is an effect that has rarely been reported. Compared with the control, the addition of 0.2%–0.3% CE to the growth medium notably decreased the cell biomass of HBEU-9 and significantly increased the membrane permeability, soluble proteins and reducing sugars of HBEU-9. SEM and TEM analyses revealed that CE could destroy the cell wall and distort cells and decreased the levels of cell-degrading enzymes (Cx, PG and PMG) by 23.4%, 53.8% and 27.5%, respectively. Importantly, CE retained nearly 80% activity after either storage for 7 months at 4 °C or exposure to 121 °C for 30 min. In addition, the most essential antibacterial compound, macrolactin A, was purified and identified from D2WM cultures through column chromatography, semi-preparative HPLC and HPLC-MS. Taken together, our results highlight the potential of B. amyloliquefaciens D2WM as a biocontrol agent against bacterial diseases and the utility of macrolactin A as an antimicrobial compound.

First Report of Bacterial Spot Caused by Xanthomonas arboricola pv. pruni on Almond in Italy

First Report of Bacterial Spot Caused by <i>Xanthomonas arboricola</i> pv. <i>pruni</i> on Almond in Italy

Etiology of leaf spot and fruit canker symptoms on stone fruits and nut trees in Iran

During July 2015 to June 2017, 63 walnut and 94 stone fruit orchards were surveyed in Iran to identify and characterize the causal agent of a disease inducing leaf spot and fruit canker symptoms. Altogether, 189 Prunus spp. and 141 walnut samples (i.e. leaves, branches and fruits) were collected from 10 Provinces in northern, northwestern, central and southern Iran. Forty-three yellow-pigmented Gram-negative bacterial isolates were recovered from symptomatic plants. Only seven bacterial strains isolated from walnut in Mazandaran and Golestan Provinces were identified as X. arboricola pv. juglandis based on pathogenicity assays, specific PCRs, and multilocus sequence analysis. Based on the phenotypic features and phylogenetic analysis of gyrB gene sequences, strains isolated from stone fruit trees were identified as multiple Pantoea species (i.e. P. agglomerans, P. ananatis, P. vagans and P. pleuroti). None of these strains caused symptoms on Prunus spp. plantlets tested under greenhouse conditions. Although the occurrence of bacterial blight of walnut in Iran was ascertained using molecular-phylogenetic analysis, no evidence was obtained that the observed disease was the same as the bacterial spot and canker of stone fruits caused by X. arboricola pv. pruni in the country.

Evaluating Walnut (Juglans spp.) for resistance to walnut blight and comparisons between artificial inoculation assays and field studies

The host resistance of walnut (Juglans spp.) against Xanthomonas arboricola pv. juglandis (Xaj) was evaluated for 18 walnut genotypes in 2016 and 2017 at Chengdu, Sichuan, China, using three assays: one under natural conditions (field investigations) and two under controlled conditions (detached leaflet assay and whole plant assay). Field investigations were used as a benchmark for comparing the other two assays. Five genotypes, ‘JS 91’, ‘JS 92’, ‘JS 86’, ‘Qingxiang’ and ‘JS 71’, displayed partial resistance in all assays, suggesting that they may be good candidates for further evaluation. The rankings for resistance among the 18 genotypes showed significant correlations between field investigations and both the detached leaflet assay and whole plant assay. However, low determination coefficients indicated that assays under controlled conditions should be considered as complementary methods and not as a substitute for field analysis when evaluating resistance to walnut blight.

Inheritance of susceptibility to bacterial spot (Xanthomonas arboricola pv. pruni) in peach offspring populations derived from Brazilian and Japanese cultivars/selections.

Bacterial spot (caused by Xanthomonas arboricola pv. pruni) is a serious disease and difficult to control in peach cultivation, and inheritance manner of susceptibility is unclear. Five hundred and fourteen offspring and their parents from 27 peach full-sib families were evaluated for susceptibility to bacterial spot by evaluating lesion length value (LLV) after artificial inoculation to shoots from trees growing in the field. Brazilian cultivars including ‘Chimarrita’ and selections derived from them had notably lower LLVs ranging from 0.302 to 0.490 than those from Japanese cultivars/selections ranging from 0.514 to 1.295. Family means in offspring crossed between Brazilian cultivars/selections (low LLVs) and other cultivars/selections (high LLVs) showed rather low LLVs, whose values were close to Brazilian cultivar/selection parents. These results suggested that the susceptibility was controlled by single major gene and that the Brazilian and Japanese cultivar/selection parents showed dominant and recessive homozygotes, respectively. In contrast, the LLVs of family means were very high ranging from 0.719 to 1.194 in offspring population derived from crosses among Japanese cultivars. Repeated backcrosses of Brazilian cultivars/selections with Japanese cultivars/selections having high fruit quality are proposed as an effective method for developing new cultivars combining bacterial spot resistance and fruit quality in Japan.

THE ACTIVITY OF ESSENTIAL OILS OBTAINED FROM SPECIES AND INTERSPECIES HYBRIDS OF THE Mentha GENUS AGAINST SELECTED PLANT PATHOGENIC BACTERIA

Plant essential oils of six aromatic herb species and interspecies hybrids of the family Lamiaceae – chocolate mint (Mentha piperita × ‘Chocolate’), pineapple mint (Mentha suaveolens ‘Variegata’), apple mint (Mentha × rotundifolia), spearmint (Mentha spicata), orange mint (Mentha × piperita ‘Granada’) and strawberry mint (Mentha × villosa ‘Strawberry’) – were investigated for antimicrobial effects against plant pathogenic bacteria: Agrobacterium tumefaciens, Pseudomonas syringae pv. syringae and Xanthomonas arboricola pv. corylina. The screening was carried out in vitro on agar plates filled with the target organism. All essential oils screened exhibited a higher level of antibacterial activity against A. tumefaciens and X. arboricola pv. corylina than streptomycin used as a standard in all tests. The antimicrobial effect of streptomycin and five mint oils was at the same level for P. syringae pv. syringae. There were no significant differences in the influence of the chocolate mint oil on the growth inhibition of all bacteria tested. Plant essential oils from pineapple mint, apple mint, spearmint and strawberry mint showed the weakest antimicrobial activity against P. syringae pv. syringae and the strongest towards A. tumefaciens and X. arboricola pv. corylina. The essential oils from strawberry mint, pineapple mint, spearmint and apple mint had the strongest effect on A. tumefaciens, and the lowest inhibitory activity was exhibited by the chocolate mint and orange mint essential oils. X. arboricola pv. corylina was the most sensitive to the strawberry mint, pineapple mint and spearmint oils. The chocolate mint oil showed the greatest activity against P. syringae pv. syringae.

First Report of Xanthomonas arboricola Causing Bacterial Blight on Pecan Trees in Portugal

Pecan (Carya illinoinensis) is an economically important nut-producing tree in North America and widely cultivated in many countries (Thompson and Conner 2012), with a smaller commercial relevance in Portugal. During field surveys performed in June 2014 and July 2016, necrotic lesions were observed on leaves of pecan trees in an orchard in Alcobaca, Portugal. The lesions were similar to those on walnut (Juglans regia) caused by Xanthomonas arboricola pv. juglandis. Symptoms were characterized by small brown spots that coalesced to form necrotic areas surrounded by chlorotic halos. No damage was observed on fruits under these field conditions. Lesions from leaves collected during the two surveys were plated on yeast extract dextrose calcium carbonate medium and incubated for 4 days at 26°C. Yellow, mucoid colonies typical of Xanthomonas spp. were then streaked on a fresh nutrient agar medium to obtain pure colonies. Isolates were able to induce a hypersensitivity response on Nicotiana benthamiana leaves 72 h after inoculation (Lelliot and Stead 1987). A portion of the gyrB gene from two isolates (CPBF 765, GenBank accession no. MG897454; CPBF 1494, no. MG897455) was sequenced with primers XgyrB1F and XgyrB1R (Young et al. 2008). Both isolates were assigned to X. arboricola, showing 98 to 99% identity with sequences from X. arboricola pathovars. Pathogenicity tests were performed on C. illinoinensis and J. regia ‘Hartley’, both members of the family Juglandaceae. For each of the two isolates, three plantlets from each species, with at least four fully expanded leaves, were inoculated by spraying with bacterial suspensions of 10⁸ CFU/ml, optical density of 0.1. Plants inoculated with X. arboricola pv. juglandis LMG 747ᵀ were used as positive controls and plants sprayed with sterile distilled water as negative controls. All plantlets were kept under a 16-h photoperiod (24°C day and 18°C night) and observed daily for 4 weeks. Spots on walnut and pecan leaves were first observed 1 week after inoculation with CPBF 765 and CPBF 1494. Disease developed progressively, forming small necrotic areas surrounded by chlorotic halos. Similar symptoms were produced on the positive controls but absent on the negative controls. Bacterial colonies showing characteristics similar to the original isolates were reisolated from both plant species tested. In addition, multiplex polymerase chain reaction for detection of X. arboricola pv. juglandis (Fernandes et al. 2017) was negative for both strains. The genetic relationship of CPBF 765 and CPBF 1494 was further compared with representative strains of X. arboricola by multilocus sequence analysis using partial sequences of seven genes (Fischer-Le Saux et al. 2015): atpD, GenBank accession nos. MG897444 to MG897445; dnaK, MG897446 to MG897447; efp, MG897448 to MG897449; fyuA, MG897450 to MG897451; glnA, MG897452 to MG897453; gyrB, MG897454 to MG897455; and rpoD, MG897456 to MG897457. X. arboricola CPBF 765 and CPBF 1494 were genetically distant from monophyletic clusters of Xanthomonas pathovars pruni, corylina, juglandis, and populi. The isolates did not cluster together, making allocation to pathovar level unlikely. CPBF 1494 was more closely related to X. arboricola CFBP 1022 isolated from J. regia, whereas CPBF 765 was more closely related to X. arboricola CFBP 4021 and X. arboricola pv. celebensis CFBP 7150 isolated from Magnolia spp. and Musa spp., respectively (Fischer-Le Saux et al. 2015). This first report of X. arboricola causing leaf blight symptoms on pecan trees is a warning to countries with important pecan-growing areas.

Biological control of bacterial plant diseases with Lactobacillus plantarum strains selected for their broad-spectrum activity.

The use of lactic acid bacteria (LAB) to control multiple pathogens that affect different crops was studied, namely, Pseudomonas syringae pv. actinidiae in kiwifruit, Xanthomonas arboricola pv. pruni in Prunus and Xanthomonas fragariae in strawberry. A screening procedure based on in vitro and in planta assays of the three bacterial pathogens was successful in selecting potential LAB strains as biological control agents. The antagonistic activity of 55 strains was first tested in vitro and the strains Lactobacillus plantarum CC100, PM411 and TC92, and Leuconostoc mesenteroides CM160 and CM209 were selected because of their broad‐spectrum activity. The biocontrol efficacy of the selected strains was assessed using a multiple‐pathosystem approach in greenhouse conditions. L. plantarum PM411 and TC92 prevented all three pathogens from infecting their corresponding plant hosts. In addition, the biocontrol performance of PM411 and TC92 was comparable to the reference products (Bacillus amyloliquefaciens D747, Bacillus subtilis QST713, chitosan, acibenzolar‐S‐methyl, copper and kasugamycin) in semi‐field and field experiments. The in vitro inhibitory mechanism of PM411 and TC92 is based, at least in part, on a pH lowering effect and the production of lactic acid. Moreover, both strains showed similar survival rates on leaf surfaces. PM411 and TC92 can easily be distinguished because of their different multilocus sequence typing and random amplified polymorphic DNA profiles.

High-Quality Draft Genome Sequence of Xanthomonas arboricola pv. juglandis CPBF 1521, Isolated from Leaves of a Symptomatic Walnut Tree in Portugal without a Past of Phytosanitary Treatment.

Here, we report the draft genome sequence of Xanthomonas arboricola pv. juglandis CPBF 1521, isolated from symptomatic leaves of an ornamental walnut in a public site in Portugal without any record of phytosanitary treatment. This isolate may constitute a genomic reference of a wild-type strain in comparative genomics studies.

First Report of Bacterial Leaf Spot Caused by the Quarantine Pathogen Xanthomonas arboricola pv. pruni on Peach in Hungary

First Report of Bacterial Leaf Spot Caused by the Quarantine Pathogen <i>Xanthomonas arboricola</i> pv. <i>pruni</i> on Peach in Hungary

Environmental and inoculum effects on epidemiology of bacterial spot disease of stone fruits and development of a disease forecasting system

Bacterial spot disease of stone fruits, caused by Xanthomonas arboricola pv. pruni, is of high economic importance in the major stone-fruit-producing areas worldwide. A better understanding of disease epidemiology can be valuable in developing disease management strategies. The effects of weather variables (temperature and wet/dry period) on epiphytic growth of X. arboricola pv. pruni on Prunus leaves were analyzed, and the relationship between inoculum density and temperature on disease development was determined and modeled. The information generated in this study, performed under controlled environmental conditions, will be useful to develop a forecasting system for X. arboricola pv. pruni. Optimal temperature for growth of epiphytic populations ranged from 20 to 30 °C under leaf wetness. In contrast, multiplication of epiphytic populations was not only interrupted under low relative humidity (RH) (< 40%) at 25 °C, but also resulted in cell inactivation, with only 0.001% initial cells recovered after 72 h incubation. A significant effect of inoculum density on disease severity was observed and 106 CFU/ml was determined as the minimal infective dose for X. arboricola pv. pruni on Prunus. Infections occurred at temperatures from 15 to 35 °C, but incubation at 25 and 30 °C gave the shortest incubation periods (7.7 and 5.9 days respectively). A model for predicting disease symptom development was generated and successfully evaluated, based on the relationship between disease severity and the accumulated heat expressed in cumulative degree day (CDD). Incubation periods of 150, 175 and 280 CDD were required for 5, 10 and 50% of disease severity, respectively.

Xanthomonas prunicola sp. nov., a novel pathogen that affects nectarine (Prunus persica var. nectarina) trees.

Three isolates obtained from symptomatic nectarine trees (Prunus persica var. nectarina) cultivated in Murcia, Spain, which showed yellow and mucoid colonies similar to Xanthomonas arboricola pv. pruni, were negative after serological and real-time PCR analyses for this pathogen. For that reason, these isolates were characterized following a polyphasic approach that included both phenotypic and genomic methods. By sequence analysis of the 16S rRNA gene, these novel strains were identified as members of the genus Xanthomonas, and by multilocus sequence analysis (MLSA) they were clustered together in a distinct group that showed similarity values below 95 % with the rest of the species of this genus. Whole-genome comparisons of the average nucleotide identity (ANI) of genomes of the strains showed less than 91 % average nucleotide identity with all other species of the genus Xanthomonas. Additionally, phenotypic characterization based on API 20 NE, API 50 CH and BIOLOG tests differentiated the strains from the species of the genus Xanthomonas described previously. Moreover, the three strains were confirmed to be pathogenic on peach (Prunus persica), causing necrotic lesions on leaves. On the basis of these results, the novel strains represent a novel species of the genus Xanthomonas, for which the name Xanthomonas prunicola is proposed. The type strain is CFBP 8353 (=CECT 9404=IVIA 3287.1).

Transcriptome reprogramming of resistant and susceptible peach genotypes during Xanthomonas arboricola pv. pruni early leaf infection.

Bacterial spot caused by Xanthomonas arboricola pv. pruni (Xap) is a major threat to Prunus species worldwide. The molecular mechanisms of peach resistance to Xap during early leaf infection were investigated by RNA-Seq analysis of two Prunus persica cultivars, ‘Redkist’ (resistant), and ‘JH Hale’ (susceptible) at 30 minutes, 1 and 3 hours-post-infection (hpi). Both cultivars exhibited extensive modulation of gene expression at 30 mpi, which reduced significantly at 1 hpi, increasing again at 3 hpi. Overall, 714 differentially expressed genes (DEGs) were detected in ‘Redkist’ (12% at 30 mpi and 1 hpi and 88% at 3 hpi). In ‘JH Hale’, 821 DEGs were identified (47% at 30 mpi and 1 hpi and 53% at 3 hpi). Highly up-regulated genes (fold change > 100) at 3 hpi exhibited higher fold change values in ‘Redkist’ than in ‘JH Hale’. RNA-Seq bioinformatics analyses were validated by RT-qPCR. In both cultivars, DEGs included genes with putative roles in perception, signal transduction, secondary metabolism, and transcription regulation, and there were defense responses in both cultivars, with enrichment for the gene ontology terms, ‘immune system process’, ‘defense response’, and ‘cell death’. There were particular differences between the cultivars in the intensity and kinetics of modulation of expression of genes with putative roles in transcriptional activity, secondary metabolism, photosynthesis, and receptor and signaling processes. Analysis of differential exon usage (DEU) revealed that both cultivars initiated remodeling their transcriptomes at 30 mpi; however, ‘Redkist’ exhibited alternative exon usage for a greater number of genes at every time point compared with ‘JH Hale’. Candidate resistance genes (WRKY-like, CRK-like, Copper amine oxidase-like, and TIR-NBS-LRR-like) are of interest for further functional characterization with the aim of elucidating their role in Prunus spp. resistance to Xap.