'Candidatus Liberibacter solanacearum' (Lso) haplotype D, transmitted by the carrot psyllid Bactericera trigonica, is a major threat to carrot production, particularly in the Mediterranean Basin and the Middle East. We investigated the impact of plant age at the time of inoculation on symptom development and yield under greenhouse and outdoor conditions. In greenhouse trials, 1- and 2-month-old plants were inoculated with psyllids harboring Lso. Both age groups showed reduced taproot weight compared with controls, but younger plants were more severely affected, exhibiting 90% yield reduction, while 2-month-old plants showed a 30% reduction. Witches' broom symptoms appeared earlier in plants inoculated at 1 month (5 weeks postinoculation); however, within 2 weeks, plants inoculated at 2 months reached comparable levels of symptom incidence and severity. Additional greenhouse experiments using Lso-free psyllids confirmed that yield losses and disease symptoms were specifically due to Lso infection and not psyllid feeding. In outdoor trials, we tested the effects of inoculation at 7, 11, 15, and 19 weeks postsowing (wps) on disease incidence and yield. Significant yield losses occurred only in plants inoculated at 7 and 11 wps, with up to 45% reduction by harvest time. Symptoms began to appear at 20 wps in plants inoculated at 7 and 11 wps. At 23 wps, symptoms also appeared in plants inoculated at 15 wps, with a similar incidence to that observed in the earlier inoculation groups. Molecular detection of Lso at harvest (23 wps) confirmed that all plants inoculated at 19 wps or earlier were infected. These results demonstrate that carrots are highly susceptible to Lso-induced yield losses at the earlier stages of growth and highlight the importance of managing early season vector pressure to minimize economic damage.

'Candidatus Liberibacter solanacearum' (Lso), haplotype D, is an insect-transmitted, phloem-limited bacterium that induces developmental abnormalities in carrots, including witches' broom and hairy root symptoms. We hypothesize that these symptoms result from Lso-induced hormonal imbalances. To investigate this, we analyzed the spatial and temporal distribution of Lso in carrot plants and assessed its effects on hormone-related gene expression and phytohormone levels. Our findings revealed that Lso first accumulates in the shoot apical meristem before spreading to root tissues, aligning with phloem flow dynamics. Transcriptomic and biochemical analyses indicated that cytokinin (CK) biosynthesis and response genes were upregulated, whereas gibberellin biosynthesis genes were downregulated. In contrast, no significant changes were observed in auxin biosynthesis or signaling. Hormone quantification further demonstrated increased CK levels in lateral roots and decreased CK levels in the root meristem of infected plants, with no detectable changes in auxin levels. Additionally, salicylic acid and jasmonic acid were significantly elevated following Lso infection, suggesting a persistent plant defense response. To validate the role of CK and auxin in symptom development, we applied synthetic growth regulators to infected and uninfected plants. CK treatment exacerbated witches' broom symptoms, whereas auxin application mitigated this phenotype but enhanced lateral root formation. These results suggest that Lso manipulates phytohormone homeostasis to induce disease symptoms, offering a potential avenue for symptom mitigation through targeted hormone applications. Our findings provide novel insights into the role of plant hormones in Lso pathogenesis and highlight new strategies for managing carrot yellows disease.

This research focused on studying the dynamics of the bacterial pathogen Xylella fastidiosa in almond trees across different developmental stages. The objective was to understand the seasonal distribution and concentration of X. fastidiosa within almond trees. Different tree organs, including leaves, shoots, branches, fruits, flowers, and roots, from 10 X. fastidiosa-infected almond trees were sampled over 2 years. The incidence and concentration of X. fastidiosa were determined using qPCR and isolation. Throughout the study, X. fastidiosa was consistently absent from fruits, flowers, and roots, whereas it was detected in leaves as well as in shoots and branches. We demonstrate that the absence of X. fastidiosa in the roots is likely linked to the inability of this isolate to infect the peach-almond hybrid rootstock GF677. X. fastidiosa incidence in shoots and branches remained consistent throughout the year, whereas in leaf petioles, it varied across developmental stages, with lower detection during the early and late stages of the season. Similarly, viable X. fastidiosa cells were isolated from shoots and branches at all developmental stages, but no successful isolations were achieved from leaf petioles during the vegetative and nut growth stage. Studying the progression of almond leaf scorch symptoms in trees with initial infections showed that once symptoms emerged on one branch, symptomless branches were likely already infected by the bacterium. Therefore, selectively pruning symptomatic branches is unlikely to cure the tree. This study enhances our understanding of X. fastidiosa dynamics in almond trees and may have practical applications for its detection and control.

ABSTRACT This research focused on studying the dynamics of the bacterial pathogen Xylella fastidiosa in almond trees at different developmental stages and in various tree parts. The objective was to understand the annual distribution and concentration of X. fastidiosa within almond trees. Different tree parts, including leaf petioles, annual and perennial shoots, fruit parts, flowers, and roots, from ten X. fastidiosa -infected almond trees were sampled over two years. The distribution and concentration of X. fastidiosa were determined using qPCR and serial dilution plating. Throughout the study, X. fastidiosa was never found in the fruit, flowers, and roots of almond trees, but it was present in leaves and annual and perennial shoots. We show that the inability of X. fastidiosa to colonize roots is likely due to incompatibility with the GF677 rootstock. The presence of X. fastidiosa in shoots remained consistent throughout the year, while in leaf petioles it varied across developmental stages, with lower detection during early and late stages of the season. Similarly, viable X. fastidiosa cells could be isolated from shoots at all developmental stages, while in leaf petioles no successful isolations were achieved during the vegetative and nut growth stage. Examining the development of almond leaf scorch symptoms over time in trees with preliminary infections revealed that once symptoms have appeared on a single branch, other asymptomatic limbs were likely already colonized by the bacterium, hence, selective pruning of symptomatic branches is unlikely to cure the tree. Overall, this study enhances our understanding of X. fastidiosa dynamics in almonds and may have practical applications for its detection and control in almond orchards.

‘ Candidatus Liberibacter solanacearum’ is an insect-transmitted bacterium associated with several plant diseases. In the Mediterranean Basin, ‘ Ca. L. solanacearum’ haplotype D is vectored by Bactericera trigonica and can severely infect carrot plants leading to abnormal growth phenotypes and significant yield losses. Insecticide applications are insufficient to suppress disease spread and damage, and additional means for disease control are needed. In the current study, we evaluated the resistance of 97 carrot accessions to the bacterial pathogen ‘ Ca. L. solanacearum’ and its associated symptoms. Accessions (Western and Asian types) were first screened in two commercial carrot fields. We found that Western type accessions were less prone to develop disease symptoms in both fields and were less frequently visited by the insect vector in one field. Overall, 22 Asian and five Western accessions with significantly lower disease incidence compared with the commercial cultivar were found. These accessions were then inoculated with ‘ Ca. L. solanacearum’ under controlled conditions and were assessed for disease incidence, insect oviposition, and bacterial relative titer. Five accessions (three Asian and two Western) had significantly lower disease incidence compared with the reference cultivar. Interestingly, disease incidence was not necessarily in line with insect oviposition or in planta bacterial titer, which may indicate that other, perhaps physiological, differences among the accessions may govern the susceptibility of plants to the disease. The resistant accessions found in this study could be used in future resistance breeding programs and to better understand the underlying mechanisms of resistance to ‘ Ca. L. solanacearum’.

Symptoms resembling those associated with European stone fruit yellows reported from Europe, including leaf discoloration and early leaf defoliation, were observed in several nectarine and peach orchards in northern Israel. A wide survey which included 20 orchards revealed that the symptomatic trees were present in all the surveyed orchards, varying in incidences from sporadic and up to 26% of the trees in the orchard. Nested-PCR and amplicon sequencing analyses of multiple samples revealed that the observed symptoms were associated with ‘Candidatus Phytoplasma pyri’, 16SrX-C subgroup presence. Interestingly, this subgroup is known to be associated primarily with pear decline disease, however, results clearly showed that in Israel ‘Ca. P. pyri’ is widely spread in nectarine and presumably is leading to symptoms resembling those of European stone fruit yellows.

A decade ago, shoot proliferation symptoms (i.e., witches’ broom) in carrots were believed to be the cause of ‘Candidatus Phytoplasma’ and Spiroplasma infection, yet in recent years this association appeared to have weakened, and a closer association was found with the yet-unculturable, psyllid-transmitted Gram-negative bacterium ‘Candidatus Liberibacter solanacearum’. In Israel, carrots are grown throughout the year, yet shoot proliferation symptoms tend to appear only in mature plants and mostly in late spring to early summer. We hypothesized that factors such as plant age, temperature, and vector load, which vary during the year, have a critical effect on symptom development and examined these factors under controlled conditions. Here we show that young carrot seedlings are as prone as older plants to develop shoot proliferation symptoms after ‘Ca. L. solanacearum’ inoculation. Surprisingly, we found that the local ‘Ca. L. solanacearum’ haplotype was extremely sensitive to constant temperature of 30°C, which led to a significant reduction in bacterial growth and symptom development compared with 18°C, which was very conducive to symptom development. We have also found that inoculations with 10 or 20 psyllids per plant results in faster symptom development compared with inoculations with two psyllids per plant; however, the difference between vector loads in disease progress rate was not significant. These data provide important insights to the effects of plant age, growth temperature, and vector load on ‘Ca. L. solanacearum’ and its associated symptoms and further strengthen the notion that ‘Ca. L. solanacearum’ is the main responsible agent for carrot witches’ broom in Israel. [Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Diseases caused by the insect-transmitted bacterium Xylella fastidiosa have been reported in the Americas since the 19th century, causing diseases such as Pierce's disease of grapevine, almond leaf scorch (ALS), and citrus variegated chlorosis. In the last decade X. fastidiosa was reported from different parts of the world, most notably from southern Italy, infecting olives. In 2017, X. fastidiosa was reported to be associated with ALS symptoms in Israel. Here, we investigated the causal agent of ALS in Israel, its genetic diversity, and host range, and we characterized the temporal and spatial distribution of the disease. X. fastidiosa subsp. fastidiosa sequence type 1 was isolated from symptomatic almond trees and was used to infect almond and grapevine by mechanical inoculation. The pathogen, however, did not infect olive, peach, cherry, plum, nectarine, clementine, and grapefruit plants. Genomic analysis of local isolates revealed that the local population is derived from a single introduction and that they are closely related to X. fastidiosa strains from grapevines in California. Distribution analyses revealed that ALS did not expand from 2017 to 2019; however, since 2020, newly symptomatic trees appeared in the tested orchards. Symptomatic trees were located primarily in clusters, and symptoms tended to spread within rows. Our study confirms that X. fastidiosa is the causal agent of ALS in Israel and describes its genetic and host range characteristics. Although there is no clear evidence yet for the identity of the vectors in Israel, ALS spread continues to threat the almond and grapevine industries in Israel.

Potato common scab (CS) and peanut pod wart diseases cause substantial economic losses every year in Israel. In this comprehensive study on the Streptomyces spp. population, isolates were collected during 2004–2016 from potato tubers with symptoms grown in Israel, seed tubers imported from Europe and from peanut pods with symptoms grown in Israel. A total of 142 isolates were characterized by PCR using three primer sets ( txtA , tomA and nec1 genes) and by three pathogenicity tests. Seven species were identified among the isolates, S . bottropensis , S . europaeiscabiei , S . griseus , S . scabiei , S . sampsonii , S . turgidiscabies , and S . venezuelae . S . europaeiscabiei was the most dominant among isolates from imported seed tubers, while S . scabiei and S . turgidiscabies were isolated from tubers grown in Israel. Isolates originating from peanuts were significantly more virulent than those from imported seed tubers, as determined by a pathogenicity assay on radish plants. The presence of pathogenic Streptomyces was detected by quantitative PCR (qPCR) in 289 soil samples collected from 31 commercial potato fields with CS history. Eighty percent to 100% of samples collected from fields with a high disease incidence on tubers (50%) gave positive results in qPCR, whereas in samples collected from fields with low CS incidence (2%–30%) the qPCR results were very variable. Potato seed lots imported from Europe during 2008–2021 were found contaminated with CS symptoms at different levels on visual examination. The results of this study indicate that Streptomyces species, in particular S . europaeiscabiei , were introduced to Israel through seed lots imported from Europe.

Yellows diseases in grapevine, associated with the presence of different phytoplasmas, are a major problem for growers, with no environmentally friendly means of control. Frateuria defendens (Frd), a bacterium with endophytic traits, has been shown to reduce yellows symptoms in grapevine plantlets under laboratory conditions. The objective of this study was to test whether similar effects could be achieved under field conditions. A trial was conducted in a heavily infected vineyard in northern Israel for two consecutive years. A suspension of Frd cells (108·mL-1) was applied bi-weekly by foliar spray on grapevines from bud burst to leaf senescence. Frd penetrated the leaves during the growing period but not during leaf senescence and could be detected in the leaves by PCR analysis up to 14 days post-spraying. The rate of yellows infection was lower in the treated grapevines compared to its increase in untreated grapevines and the yield of symptomatic plants was improved by 10 to 20 %. Taken together, the results suggest Frd acted as a biological control agent in vineyards under the experimental conditions tested.

Pectobacterium brasiliense (Pbr) infects a wide range of crops worldwide, causing potato blackleg and soft rot and vegetable soft rots. This study aimed to characterize the genetic diversity and virulence variability among 68 Pbr strains isolated from either symptomless potato progeny tubers, diseased potato plants, ware potatoes wash water, or vegetables grown in Israel, as well as strains isolated from symptomless seed tubers grown in Europe, or diseased potato plants grown in France. The collection was typed using PCR and TaqMan real-time PCR analyses, dnaX sequence analysis, pulsed-field gel electrophoresis (PFGE), and pectolytic activity. dnaX phylogeny grouped almost all strains in a common genetic clade related to Pbr, which was distinct from the other Pectobacterium species. PFGE analysis identified two main clusters, including one major group of 47 strains with 95%-100% similarity. Maceration assays on two potato cultivars showed significant differences between strains but with no correlations with the source of the strains nor the status of the host (with/without symptoms). Molecular (dnaX sequences and PFGE profiles) and phenotypic analyses (tuber maceration tests) showed that the tested Pbr strains are not a homogeneous group. Analysis of the tested Pbr strains isolated from potato and vegetables grown in fields with a history of potato cultivation suggests that seed tubers imported from Europe may be the main source for Pbr in Israel. To the best of our knowledge, this is the first study that describes biodiversity and population structure of P. brasiliense isolated from potato and vegetables under hot climate conditions.

Acidovorax citrulli is the causal agent of bacterial fruit blotch disease of cucurbits. Strains of this pathogen are distributed into two major groups: Group I strains have been mainly isolated from melon and other non-watermelon cucurbits, while Group II strains have been mainly recovered from watermelon. Here we report the characterization of strains T1 and EP isolated from diseased tomato and eggplant plants, respectively, and further confirmed to belong to A. citrulli species. Based on PCR, PFGE, and rep-PCR, these strains showed high similarity to the Group II strain 7a1. Sequencing and comparative analyses revealed that the genomes of T1 and EP aligned with that of the Group II model strain AAC00-1, over 97.88% and 99.22%, respectively. The virulence of T1, EP, and 7a1 determined on tomato, eggplant, and watermelon was similar and significantly higher than that of Group I strain M6. In contrast, M6 was more virulent on melon. Expression levels of seven virulence genes measured 24 hr after inoculation of tomato, eggplant, watermelon, and melon showed that the expression pattern was generally similar in strains 7a1, T1, and EP, whereas for M6 the expression was high only on melon. Overall, our results indicate that the solanaceous strains belong to Group II. To the best of our knowledge, this is the first study that reports characterization of A. citrulli strains isolated from solanaceous species. The fact that A. citrulli is able to naturally colonize and cause disease in non-cucurbit crops poses additional challenges for management of this important pathogen.

HomePlant DiseaseVol. 104, No. 8First Report of Pectobacterium parmentieri, One of the Causal Agents of Potato Blackleg and Tuber Soft Rot Diseases, in Israel PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of Pectobacterium parmentieri, One of the Causal Agents of Potato Blackleg and Tuber Soft Rot Diseases, in IsraelL. Tsror (Lahkim), O. Erlich, S. Lebiush, I. Galilov, M. Hazanovsky, L. Chalupowicz, M. Reuven, O. Dror, and S. Manulis-SassonL. Tsror (Lahkim)†Corresponding author: L. Tsror (Lahkim); E-mail Address: [email protected]http://orcid.org/0000-0001-9759-3575Agricultural Research Organization (ARO), Department of Plant Pathology and Weed Research, Gilat Research Center, MP Negev 85280, IsraelSearch for more papers by this author, O. ErlichAgricultural Research Organization (ARO), Department of Plant Pathology and Weed Research, Gilat Research Center, MP Negev 85280, IsraelSearch for more papers by this author, S. LebiushAgricultural Research Organization (ARO), Department of Plant Pathology and Weed Research, Gilat Research Center, MP Negev 85280, IsraelSearch for more papers by this author, I. GalilovAgricultural Research Organization (ARO), Department of Plant Pathology and Weed Research, Gilat Research Center, MP Negev 85280, IsraelSearch for more papers by this author, M. HazanovskyAgricultural Research Organization (ARO), Department of Plant Pathology and Weed Research, Gilat Research Center, MP Negev 85280, IsraelSearch for more papers by this author, L. ChalupowiczARO, Department of Plant Pathology and Weed Research, The Volcani Center, Rishon LeZion 7528809, IsraelSearch for more papers by this author, M. ReuvenARO, Department of Plant Pathology and Weed Research, The Volcani Center, Rishon LeZion 7528809, IsraelSearch for more papers by this author, O. DrorARO, Department of Plant Pathology and Weed Research, The Volcani Center, Rishon LeZion 7528809, IsraelSearch for more papers by this author, and S. Manulis-SassonARO, Department of Plant Pathology and Weed Research, The Volcani Center, Rishon LeZion 7528809, IsraelSearch for more papers by this authorAffiliationsAuthors and Affiliations L. Tsror (Lahkim)1 † O. Erlich1 S. Lebiush1 I. Galilov1 M. Hazanovsky1 L. Chalupowicz2 M. Reuven2 O. Dror2 S. Manulis-Sasson2 1Agricultural Research Organization (ARO), Department of Plant Pathology and Weed Research, Gilat Research Center, MP Negev 85280, Israel 2ARO, Department of Plant Pathology and Weed Research, The Volcani Center, Rishon LeZion 7528809, Israel Published Online:16 Jun 2020https://doi.org/10.1094/PDIS-02-20-0226-PDNAboutSections ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Potato (Solanum tuberosum) blackleg and tuber soft rot diseases in Israel, caused by Pectobacterium and Dickeya spp., originate from seed tubers imported from Europe. These diseases are of great concern due to the warm climatic conditions during the growing season that favor disease expression and may result in establishment of the pathogens in potato fields and their spread to weeds and other crops (Tsror (Lahkim) et al. 2009, 2012). P. parmentieri was recently reported in Europe as an emerging threat to potato production, among other Pectobacterium and Dickeya spp. (Suárez et al. 2017; van der Wolf et al. 2017; Zoledowska et al. 2018). Surveys for pectinolytic bacteria were carried out during spring 2019 in an experimental plot located at Gilat Research Center Israel (Agricultural Research Organization), where different imported seed lots were planted, and during 2017 in a commercial packhouse. P. parmentieri was detected in diseased potato plants (cv. Sifra) and in wash water sampled during the process of packing ware potato, respectively. Plant material was surface sterilized with 0.3% (v/v) hypochlorite for 3 min, and 10 segments from the stem base of each plant were homogenized with 10 ml of sterile distilled water. Sample homogenates were plated on crystal violet pectate medium (CVP). The wash water surveys were done during May, July, and August 2017, with processed tubers (cvs. Sifra, Winston, Allians, Evolution, and Rosanna) obtained from commercial spring plots, located in Western Negev, in which imported seeds from Europe were planted in January. Wash water samples (200 ml) were filtered through rough filter paper followed by fine filtering using Millipore filter paper (45 µm). The Millipore filter paper was resuspended in 2 ml of sterile distilled water, and suspensions were plated on CVP. Single colonies obtained on CVP medium were purified by repeated subculturing on nutrient agar (Difco), and selected strains were further characterized. Most of the isolated colonies were identified as Pectobacterium carotovorum subsp. carotovorum, P. carotovorum subsp. brasiliense, or Dickeya solani; however, one isolate from the diseased plants and three isolates from the wash water were characterized as Pectobacterium parmentieri. DNA extracted from these colonies reacted positively in a qPCR assay based on the YD repeat protein gene PW7011F/R specific primers (Kim et al. 2012) and also in a TaqMan assay based on the mdh sequence (van der Wolf et al. 2017). Sequencing the gapA gene (Cigna et al. 2017) identified the isolate as P. parmentieri (GenBank accession no. MN887102), with 100% similarity to P. parmentieri strain SCC3193 (CP003415.1). All four isolates caused maceration of potato tubers at 30°C (Tsror (Lahkim) et al. 2013). In a pathogenicity assay with the isolate from the diseased plant and one of the isolates from the wash water, conducted at 30°C on potato plants (cv. Allians), wilting symptoms appeared 48 h after stem inoculation (10 μl of bacterial suspension at 108 cells/ml). Reisolated bacteria from the inoculated plants were confirmed as P. parmentieri by PCR. This is the first report on the presence of P. parmentieri in Israel. These findings are of phytosanitary significance due to the recent reports from Europe indicating that P. parmentieri has an important role in the pectinolytic bacterial complex causing potato blackleg and thus has the potential to cause disease under warm climate conditions as well.The author(s) declare no conflict of interest.References:Cigna, J., et al. 2017. Plant Dis. 101:1278. https://doi.org/10.1094/PDIS-12-16-1810-RE Link, ISI, Google ScholarKim, M. H., et al. 2012. Plant Dis. 96:253. https://doi.org/10.1094/PDIS-06-11-0511 Link, ISI, Google ScholarSuárez, M. B., et al. 2017. Plant Dis. 101:1029. https://doi.org/10.1094/PDIS-01-17-0013-PDN Link, ISI, Google ScholarTsror (Lahkim), L., et al. 2009. Eur. J. Plant Pathol. 123:311. https://doi.org/10.1007/s10658-008-9368-0 Crossref, ISI, Google ScholarTsror (Lahkim), L., et al. 2012. Plant Pathol. 61:161. Crossref, ISI, Google ScholarTsror (Lahkim), L., et al. 2013. Plant Pathol. 62:1097. https://doi.org/10.1111/ppa.12030 Crossref, ISI, Google Scholarvan der Wolf, J. M., et al. 2017. Plant Pathol. 66:571. https://doi.org/10.1111/ppa.12600 Crossref, ISI, Google ScholarZoledowska, S., et al. 2018. Plant Dis. 102:154. https://doi.org/10.1094/PDIS-05-17-0761-RE Link, ISI, Google ScholarThe author(s) declare no conflict of interest.Funding: This study was supported by the Israeli Ministry of Agricultural and Rural Development grant no. 20‐02‐0051.DetailsFiguresLiterature CitedRelated Vol. 104, No. 8 August 2020SubscribeISSN:0191-2917e-ISSN:1943-7692 DownloadCaptionApple scab lesions on h(11) Malus baccata ‘Jackii’ found in 2011 at Skierniewice, Poland (A. Patocchi et al.). Photo credit: A. Patocchi. Healthy `ōhi`a seedling growing from a crack in a lava field (M. A. Hughes et al.). Photo credit: K. Hughes. Metrics Article History Issue Date: 28 Jul 2020Published: 16 Jun 2020First Look: 25 Mar 2020Accepted: 24 Mar 2020 Pages: 2288-2288 Information© 2020 The American Phytopathological SocietyFundingIsraeli Ministry of Agricultural and Rural DevelopmentGrant/Award Number: 20‐02‐0051KeywordsvegetablesSolanum tuberosumThe author(s) declare no conflict of interest.Cited bySurvey of Soft Rot Pectobacteriaceae Infecting Potatoes in South Africa5 November 2022 | Potato Research, Vol. 215Characterization of Pectobacterium brasiliense strains from potato and vegetables in Israel30 August 2021 | Plant Pathology, Vol. 70, No. 9Wild Potatoes: A Genetic Reservoir for Potato BreedingDiseases Caused by Pectobacterium and Dickeya Species Around the World5 January 2021

Potato blackleg and tuber soft rots in Israel, caused by Pectobacterium and Dickeya spp., originating from seed tubers imported from Europe, are of a great concern due to the warm climatic conditions during the growing season that favour…

Potato blackleg and tuber soft rots in Israel, caused by Pectobacterium and Dickeya spp., originating from seed tubers imported from Europe, are of a great concern due to the warm climatic conditions during the growing season that favour disease expression and may result in the establishment of the pathogens in potato fields and their spread to weeds and other crops (Tsror et al., 3). In a previous survey in Israel, Dickeya solani was isolated only from Cyperus rotundus, out of symptomless plants of 12 weed species (Tsror et al., 4). Recently, Pectobacterium carotovorum subsp. brasiliense (Pcb) was reported as an emerging threat in Western Europe and the primary blackleg-causing pathogen for some countries in the region (van der Wolf et al., 6). The pathogen is also responsible for considerable disease in Israel (personal information). To study the dissemination of Pcb to weeds, a survey was conducted in potato fields where Pcb-infected plants were detected during the spring of 2018. Symptomless weed plants from 13 genera and 10 families, namely Solanum nigrum (Solanaceae), Polygonum equisetiforme (Polygonaceae), Centaurea procurrens, Sonchus oleraceus (Asteraceae), Lolium rigidum, Phalaris brachystachys, Avena sterilis (Poaceae), Malva nicaeensis (Malvaceae), Amaranthus blitoides (Amaranthaceae), Chenopodium murale (Chenopodiaceae), Chrozophora tinctoria (Euphorbiaceae), Orobanche aegyptiaca (Orobanchaceae) and Erucaria rostrate (Brassicaceae), were randomly collected from areas where potato plants infected by Pcb or D. solani had been identified. Roots or stems (in the case of O. aegyptiaca) of 6-15 plants of each weed were washed, surface sterilised, macerated in sterile distilled water and the suspensions were plated on crystal violet pectate medium. Cavity forming bacteria were transferred to nutrient agar for further characterisation. Pcb was isolated only from latently infected Malva nicaeensis plants and with an incidence of 16.7%. DNA extracted from these colonies reacted positively in a PCR assay using BR1f/L1r specific primers (Duarte et al., 2) and also in a TaqMan assay based on araC sequence (van der Wolf et al., 6). Sequencing the gapA gene (Cigna et al., 1) identified the isolate as Pcb (GenBank Accession No. MK086015). A maceration assay on potato tubers at 30°C (Tsror et al., 5) was positive. This is the first report of latent infection of Pectobacterium carotovorum subsp. brasiliense in one of the most prevalent weeds in potato fields in Israel. Malva nicaeensis may serve as an alternative host for Pcb allowing the pathogen to survive in the absence of the host crop. This study was supported by the Israeli Ministry of Agricultural and Rural Development (grant no. 20-02-0051).

Reliable detection and identification of plant pathogens are essential for disease control strategies. Diagnostic methods commonly used to detect plant pathogens have limitations such as requirement of prior knowledge of the genome sequence, low sensitivity and a restricted ability to detect several pathogens simultaneously. The development of advanced DNA sequencing technologies has enabled determination of total nucleic acid content in biological samples. The possibility of using the single‐molecule sequencing platform of Oxford Nanopore as a general method for diagnosis of plant diseases was examined. It was tested by sequencing DNA or RNA isolated from tissues with symptoms from plants of several families inoculated with known pathogens (e.g. bacteria, viruses, fungi, phytoplasma). Additionally, samples of groups of 200 seeds containing one infected seed of each of two or three pathogens, as well as samples with symptoms but unidentified pathogens were tested. Sequencing results were analysed with Nanopore data analysis tools. In all the inoculated plants, pathogens were identified in real time within 1–2 h of running the Nanopore sequencer and were classified to the species or genus level. DNA sequencing or direct RNA sequencing of samples with unidentified disease agents were validated by conventional diagnostic procedures (e.g. PCR , ELISA , Koch test), which supported the results obtained by Nanopore sequencing. The advantages of this technology include: long read lengths, fast run times, portability, low cost and the possibility of use in every laboratory. This study indicates that adoption of the Nanopore platform will be greatly advantageous for routine laboratory diagnosis.

Carrot yellows disease has been associated for many years with the Gram-positive, insect-vectored bacteria, ‘Candidatus Phytoplasma’ and Spiroplasma citri. However, reports in the last decade also link carrot yellows symptoms with a different, Gram-negative, insect-vectored bacterium, ‘Ca. Liberibacter solanacearum’. Our study shows that to date ‘Ca. L. solanacearum’ is tightly associated with carrot yellows symptoms across Israel. The genetic variant found in Israel is most similar to haplotype D, found around the Mediterranean Basin. We further show that the psyllid vector of ‘Ca. L. solanacearum’, Bactericera trigonica, is highly abundant in Israel and is an efficient vector for this pathogen. A survey conducted comparing conventional and organic carrot fields showed a marked reduction in psyllid numbers and disease incidence in the field practicing chemical control. Fluorescent in situ hybridization and scanning electron microscopy analyses further support the association of ‘Ca. L. solanacearum’ with disease symptoms and show that the pathogen is located in phloem sieve elements. Seed transmission experiments revealed that while approximately 30% of the tested carrot seed lots are positive for ‘Ca. L. solanacearum’, disease transmission was not observed. Possible scenarios that may have led to the change in association of the disease etiological agent with carrot yellows are discussed. [Formula: see text] Copyright © 2018 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

Clavibacter michiganensis subsp. michiganensis (Cmm), the causal agent of bacterial wilt and canker of tomato, is one of the most important bacterial diseases of tomato in all major tomato-growing areas worldwide. Cmm strains may be divided into various distinct genetic groups using macrorestriction pulsed-field gel electrophoresis (PFGE) analysis. The objectives of this study were: (i) to document changes in Cmm population structure in the main tomato production areas of Israel, and to reveal (ii) differences in aggressiveness of the various strains and (iii) differences in survival capability among strains associated with different genetic groups. Of the 38 strains sampled between 1998 and 2008, 37 (97.3%) were associated with group B and only one strain (2.7%) was associated with group A. However, in the four years that followed, the prevalence of strains associated with group B declined gradually and that of other genetic groups, A, E and Z, increased; the latter was the most common in 2012, making up similar to 42% of the population. The aggressiveness of strains associated with different genetic groups was examined in two artificially inoculated experiments: the first included strains of groups A, B, E and Z and the second strains of groups A, B, K, L and Z. Two to four strains from each genetic group were included in each experiment. Differences in aggressiveness among the genetic groups were insignificant. There were, however, some differences in aggressiveness between individual strains within each genetic group. In another set of experiments, the survival of four bacterial strains associated with each of the A, B, E and Z genetic groups was determined in roots of infected plants or in infested soil (three soil types). Differences in survival capabilities were recorded among strains associated with the same genetic groups, but not among those associated with different genetic groups.

BACKGROUND Phytoplasma, the causative agent of Bois Noir disease of grapevines, are vectored by the planthopper Hyalesthes obsoletus (Hemiptera: Cixiidae). A Dyella-like bacterium (DLB) isolated from H. obsoletus inhibits the growth of Spiroplasma melliferum, a cultivable relative of phytoplasma. Additional evidence suggests that DLB can reduce the symptoms of yellows disease in grapevine plantlets. The aim of this study was to test whether DLB could colonize a range of phytoplasma- and liberibacter-sensitive crop plants, and thus assess its potential agricultural use. RESULTS Vitex agnus-castus, the preferred host plant of H. obsoletus was found to be a natural host of DLB, which was successfully introduced into a range of crop plants belonging to seven families. The most effective DLB application method was foliar spraying. Microscopy observation revealed that DLB aggregated on the leaf surface and around the stomata, suggesting that this is its route of entry. DLB was also present in the vascular tissues of plants, indicating that it moved systemically through the plant. CONCLUSIONS DLB is a potential biocontrol agent and its broad spectrum of host plants indicates the possibility of its future use against a range of diseases caused by phloem-limited bacteria. © 2017 Society of Chemical Industry.