Angular Leaf Spot of Cucurbits is Associated With Genetically Diverse Pseudomonas syringae Strains

A deletion in FLS2 and its expansion after domestication caused global dissemination of melon cultivars defective in flagellin recognition

FLAGELLIN SENSING 2 (FLS2) encodes a pattern recognition receptor that perceives bacterial flagellin. While putative FLS2 orthologs are broadly conserved in plants, their functional characterization remains limited. Here, we report the identification of orthologs in cucumber (Cucumis sativus) and melon (C. melo), named CsFLS2 and CmFLS2, respectively. Homology searching identified CsFLS2, and virus-induced gene silencing (VIGS) demonstrated that CsFLS2 is required for flg22-triggered ROS generation. Interestingly, genome re-sequencing of melon cv. Lennon and subsequent genomic PCR revealed that Lennon has two CmFLS2 haplotypes, haplotype I encoding full-length CmFLS2 and haplotype II encoding a truncated form. We show that VIGS-mediated knockdown of CmFLS2 haplotype I resulted in a significant reduction in both flg22-triggered ROS generation and immunity to a bacterial pathogen in melon cv. Lennon. Remarkably, genomic PCR of CmFLS2 revealed that 68% of tested commercial melon cultivars possess only CmFLS2 haplotype II: these cultivars thus lack functional CmFLS2. To explore evolutionary aspects of CmFLS2 haplotype II occurrence, we genotyped the CmFLS2 locus in 142 melon accessions by genomic PCR and analyzed 437 released sequences. The results suggest that CmFLS2 haplotype II is derived from C. melo subsp. melo. Furthermore, we suggest that the proportion of CmFLS2 haplotype II increased among the improved melo group compared with the primitive melo group. Collectively, these findings suggest that the deleted FLS2 locus generated in the primitive melo subspecies expanded after domestication, resulting in the spread of commercial melon cultivars defective in flagellin recognition, which is critical for bacterial immunity.

Assessment of the Dissipation Behaviors, Residues, and Dietary Risk of Oxine-Copper in Cucumber and Watermelon by UPLC-MS/MS

During production, agricultural products are often susceptible to potential harm caused by residual traces of pesticides. Oxine-copper is a broad spectrum and efficient protective fungicide widely used in the production of fruits and vegetables. The present study was carried out to profile the dissipation behaviors and residues of oxine-copper on cucumber and watermelon using QuEChERS pretreatment and UPLC-MS/MS. Its storage stability and dietary risk assessment were also estimated. The method validation displayed good linearity (R 2 ≥ 0.9980), sensitivity (limits of quantification ≤0.01 mg/kg), and recoveries (75.5-95.8%) with relative standard deviations of 2.27-8.26%. According to first-order kinetics, the half-lives of oxine-copper in cucumber and watermelon were 1.77-2.11 and 3.57-4.68 d, respectively. The terminal residues of oxine-copper in cucumber and watermelon samples were within <0.01-0.264 and <0.01-0.0641 mg/kg, respectively. Based on dietary risk assessment, the estimated long-term dietary risk probability value of oxine-copper in cucumber and watermelon is 64.11%, indicating that long-term consumption of cucumber and watermelon contaminated with oxine-copper would not pose dietary risks to the general population. The results provide scientific guidance for the rational utilization of oxine-copper in field ecosystems of cucumber and watermelon.

Unexpected Diversity of Pseudomonads Associated with Bacterial Leaf Spot of Cucurbits in the Southeastern United States

Bacterial leaf spot of cucurbits (BLS) is an emerging disease in the southeastern United States that is capable of causing widespread outbreaks under conducive conditions. Historically attributed solely to the bacterium Pseudomonas syringae pv. lachrymans, recent studies have identified additional P. syringae pathovars as causal agents of the disease. To further investigate the identity and diversity of P. syringae strains associated with BLS in the southeastern United States, 47 bacterial isolates were recovered from symptomatic cucurbits from Florida, Alabama, and Georgia. Strains were characterized using the LOPAT testing scheme, fluorescence, and pathogenicity to watermelon and squash seedlings. Thirty-eight fluorescent isolates underwent whole-genome sequencing and were further characterized with 16S rRNA, four gene multilocus sequence analysis (MLSA) phylogeny, and average nucleotide identity analysis. Thirty-four isolates were identified as members of the P. syringae species complex, including P. syringae sensu stricto (12), P. alliivorans (12), P. capsici (nine), and P. viridiflava (one). An additional four isolates were found to belong to the Pseudomonas genus outside of the syringae species complex, though they did not share 95% or greater average nucleotide identity to any validly published species and are believed to belong to three novel Pseudomonas species. These results reveal an unpredicted level of diversity of Pseudomonas strains associated with BLS in the region and show the benefits of whole-genome sequencing for strain identification. Identification of P. capsici, which is capable of causing disease at higher temperatures than P. syringae, as a causal agent of BLS may also affect management strategies in the future.

Identification of the Causal Agent of Bacterial Leaf Spot on Watermelon in China

Watermelon diseases caused by pathogenic bacteria were endemic in Liaoning and Jilin Provinces from 2019 to 2020 in China, resulting in serious economic losses to the watermelon industry. This study characterized 56 strains isolated from symptomatic watermelon leaves collected from Liaoning and Jilin Provinces. Through morphological observation, 16S rRNA and gyrB sequence analysis, and BIOLOG profiles, the pathogen was identified as Pseudomonas syringae. In China, the watermelon disease caused by P. syringae was reported for the first time. The multilocus sequence analysis showed that the isolated strains belonged to three different clades within P. syringae phylogroup 2. Interestingly, most of them (79%) belonged to clade 2a, 14% were clade 2b, and 7% were clade 2d. This indicates that bacterial leaf spot outbreaks of watermelon in China were caused by multiple sources and mainly by P. syringae clade 2a.

Comparative genomics and genomic diversity of Pseudomonas syringae clade 2b-a in Australia

BACKGROUND

A zucchini disease outbreak with unusual symptoms associated with Pseudomonas syringae clade 2b was identified in Bundaberg, Australia during autumn 2016. To investigate the genetic diversity of the 11 Australian isolates obtained from the outbreak, the genomes were compared to the publicly available P. syringae strains in phylogroup 2.

RESULTS

Average nucleotide identity refined the P. syringae clade 2b-a into four clusters (Cluster A, B, C1 and C2), an expansion from the previously identified A, B and C. Australian isolates were in Cluster A, C1 and C2. Genomic analyses highlighted several key factors that may contribute to the virulence of these isolates. Six orthologous groups, including three virulence factors, were associated with P. syringae phylogroup 2 cucurbit-infecting strains. A region of genome plasticity analysis identified a type VI secretion system pathway in clade 2b-a strains which could also contribute to virulence. Pathogenicity assays on isolates KL004-k1, KFR003-1 and 77-4C, as representative isolates of Cluster A, C1 and C2, respectively, determined that all three isolates can infect pumpkin, squash, watermelon and zucchini var. Eva with different levels of disease severity. Subsequently, type III effectors were investigated and four type III effectors (avrRpt2, hopZ5, hopC1 and hopH1) were associated with host range. The hopZ effector family was also predicted to be associated with disease severity.

CONCLUSIONS

This study refined the taxonomy of the P. syringae clade 2b-a, supported the association between effector profile and pathogenicity in cucurbits established in a previous study and provides new insight into important genomic features of these strains. This study also provided a detailed and comprehensive resource for future genomic and functional studies of these strains.

Resistance and Pathogenicity of Salmonella Thompson Isolated from Incubation End of a Poultry Farm

Simple Summary

Non-typhoid Salmonella is the general term of Salmonella other than typhoid and paratyphoid, which often causes foodborne gastroenteritis in humans, but some serotypes have been proved to be pathogenic to poultry. Salmonella Enterica and Salmonella Typhimurium are the common serotypes pathogenic to poultry and have been systematically studied, but other serotypes have rarely been studied. During Salmonella surveillance in farms, we discovered by chance that Salmonella Thompson, a common non-typhoid Salmonella, is also pathogenic to avian embryos. Therefore, this study aimed to explore antimicrobial resistance and pathogenicity of clinical S. Thompson. Firstly, we found that the core-genome multilocus sequence typing of 14 clinical S. Thompson was consistent with two strains of S. Thompson from humans in China. Secondly, the antimicrobial resistance gene analysis demonstrated that all strains carried the polymyxin resistance gene mcr-9, which had not appeared resistance phenotype. Meanwhile, many essential virulence genes were also found in each S. Thompson isolate. Finally, the bacterial inoculation experiment revealed that clinical S. Thompson was highly pathogenic to newborn chicks after yolk sac inoculation. This study suggests that Salmonella Thompson can circulate between humans and poultry farms and transmit drug resistance genes and demonstrated that Salmonella Thompson is highly pathogenic to chicks and should be guarded against in the hatching stage of poultry farms.

Abstract

Salmonella Thompson, an important foodborne pathogen, is rarely found to be pathogenic to poultry. Accidentally, S. Thompson was found to be pathogenic to embryos of white feather broiler at a poultry farm in China. Therefore, this study aimed to explore antimicrobial resistance and pathogenicity of clinical S. Thompson isolated from dead poultry embryos. The phylogenetic tree based on 16S rRNA and seven housekeeping genes showed that the 14 clinical S. Thompson were closely related. The core-genome multilocus sequence typing of 14 clinical S. Thompson based on whole-genome sequencing was cgST-12774, consistent with the only two strains of S. Thompson from humans in China as reported in the NCBI database. The antimicrobial resistance gene analysis demonstrated that all strains carried aac(6′)-Iaa and the polymyxin resistance gene mcr-9. Antimicrobial sensitivity tests for 18 antibiotics showed that S. Thompson isolates displayed resistance against streptomycin (100%), ampicillin (35.7%), and doxycycline (14.3%), but sensitivity to polymyxin B, proving that the mcr-9 gene had not appeared resistance phenotype. Virulence genes Salmonella pathogenicity island (SPI) SPI1-5, type I fimbriae gene (fimA), flagellar assembly genes (bcfC, flhD, fliA, fliC, fljB, flgK, and lpfC), and other virulence genes (iroN, pagC, and cigR) were found in each S. Thompson isolate. Additionally, the bacterial inoculation experiment with 1-day-old chicks revealed that clinical S. Thompson was highly pathogenic to newborn chicks after yolk sac inoculation. This study highlighted that the S. Thompson isolated from poultry embryos and the S. Thompson causing human foodborne diarrhea in some parts of China belong to the same cgMLST typology (cgST-12774) and showed the pathogenicity of this clinical S. Thompson to chicks.

Characterization of Pseudomonas syringae Isolated from Systemic Infection of Zucchini in Australia

Zucchini plants with symptoms including twisted petioles, necrotic leaves, crown rot, and internal fruit rot were found in Bundaberg, Australia, at a commercial field for the first time during late autumn 2016, resulting in direct yield losses of 70 to 80%. Three Pseudomonas syringae strains isolated from symptomatic leaf (KL004-k1), crown (77-4C), and fruit (KFR003-1) were characterized and their pathogenicity evaluated on pumpkin, rockmelon, squash, and zucchini. Biochemical assays showed typical results for P. syringae. The three isolates differed, however, in that two produced fluorescent pigment (KFR003-1 and 77-4C) whereas the third, KL004-k1, was nonfluorescent. Multilocus sequence analysis classified the isolates to phylogroup 2b. The single-nucleotide polymorphism analysis of core genome from the Australian and closely related international isolates of P. syringae showed two separate clusters. The Australian isolates were clustered based on fluorescent phenotype. Pathogenicity tests demonstrated that all three isolates moved systemically within the inoculated plants and induced necrotic leaf symptoms in zucchini plants. Their identities were confirmed with specific PCR assays for P. syringae and phylogroup 2. Pathogenicity experiments also showed that the Eva variety of zucchini was more susceptible than the Rosa variety for all three isolates. Isolate KL004-k1 was more virulent than 77-4C on pumpkin, rockmelon, squash, and zucchini. This study expands the knowledge of P. syringae isolates that infect cucurbits and provides useful information for growers about the relative susceptibility of a range of cucurbit species.

Transcriptome Profiling of Cucumber (Cucumis sativus L.) Early Response to Pseudomonas syringae pv. lachrymans

Bacterial angular leaf spot disease (ALS) caused by Pseudomonas syringae pv. lachrymans (Psl) is one of the biological factors limiting cucumber open-field production. The goal of this study was to characterize cytological and transcriptomic response of cucumber to this pathogen. Plants of two inbred lines, B10 (susceptible) and Gy14 (resistant), were grown, and leaves were inoculated with highly virulent Psl strain 814/98 under growth chamber conditions. Microscopic and transcriptional evaluations were performed at three time points: before, 1 and 3 days post inoculation (dpi). Investigated lines showed distinct response to Psl. At 1 dpi bacterial colonies were surrounded by necrotized mesophyll cells. At 3 dpi, in the susceptible B10 line bacteria were in contact with degraded cells, whereas cells next to bacteria in the resistant Gy14 line were plasmolyzed, but apparently still alive and functional. Additionally, the level of H₂O₂ production was higher in resistant Gy14 plants than in B10 at both examined time points. In RNA sequencing more than 18,800 transcripts were detected in each sample. As many as 1648 and 2755 differentially expressed genes (DEGs) at 1 dpi as well as 2992 and 3141 DEGs at 3 dpi were identified in B10 and Gy14, respectively. DEGs were characterized in terms of functional categories. Resistant line Gy14 showed massive transcriptomic response to Psl at 1 dpi compared to susceptible line B10, while a similar number of DEGs was detected for both lines at 3 dpi. This suggests that dynamic transcriptomic response to the invading pathogen may be related with host resistance. This manuscript provides the first transcriptomic data on cucumber infected with the pathovar lachrymans and helps to elucidate resistance mechanism against ALS disease.

Aerosol transmission of Pseudomonas amygdali pv. lachrymans in greenhouses

Most previous studies on aerosols have focused on the role of aerosols in the transmission of human and animal pathogens; however, little is known about the role of aerosols in the transmission of plant bacterial disease. In this study, experimental evidence for the emission and transmission of Pseunomonas. amygdali pv. lachrymans (Pal) aerosol was provided, and the results supported that diseased cucumber plants served as the main source of Pal aerosol. Bacterial aerosols released by infected plants played a significant role in the epidemiology of cucumber angular leaf spot (ALS) disease. Aerosol chambers were constructed to study the characteristics of Pal aerosols released by artificially infested cucumber plants. The particle size of Pal aerosol was predominately distributed from 1.1 to 4.7 μm, accounting for 72.16% of the total amount of Pal aerosol. The infection threshold of aerosolized Pal to cause ALS disease was 84-179 CFU/m³. In addition, the transmission dynamics of Pal aerosols from donor cucumber plants to recipient cucumber plants were also confirmed in exposure chambers and greenhouses. The results from the present study verified the hypothesis that aerosol dissemination is a potential route for the epidemiology of plant bacterial disease, and these data will contribute to the development of new strategies for the effective alleviation and control of plant bacterial diseases.

Zucchini Vein Clearing Disease Is Caused by Several Lineages Within Pseudomonas syringae Species Complex

Zucchini (Cucurbita pepo) is worldwide affected by Pseudomonas syringae, inducing vein clearing, stunting, and necroses during plantlet development. A collection of 58 P. syringae strains isolated from diseased zucchini plantlets was characterized by multilocus sequence analysis (MLSA). A subset of 23 strains responsible for vein clearing of zucchini (VCZ) was evaluated for pathogenicity on zucchini, and their genomes were sequenced. The host range of six VCZ strains was evaluated on 11 cucurbit species. Most VCZ strains belong to clades 2a and 2b-a within phylogroup 2 of P. syringae species complex and are closely related to other strains previously isolated from cucurbits. Genome analyses revealed diversity among VCZ strains within each clade. One main cluster, once referred to by the invalid pathovar name (peponis), gathers VCZ strains presenting a narrow host range including zucchini and squashes. Other VCZ strains present a large host range including zucchini, squashes, cucumber, melons, and in some cases watermelon. The VCZ strain pathogenic features are strongly associated with type III effector repertoires. The presence of avrRpt2 and absence of hopZ5 are associated with a narrow host range, whereas the presence of hopZ5 and absence of avrRpt2 are most generally associated with a large host range. To better detect the different clusters identified with whole genome sequence and pathogenicity analyses, we used a specific-k-mers approach to refine the MLSA scheme. Using this novel MLSA scheme to type P. syringae isolates from diseased cucurbits would give insight into distribution of worldwide strains and origin of epidemics.

Identification of Pseudomonas Isolates Associated With Bacterial Canker of Stone Fruit Trees in the Western Cape, South Africa

Bacterial canker is a common bacterial disease of stone fruit trees. The causal agents responsible for the disease include several pathovars in Pseudomonas syringae sensu lato and newly described Pseudomonas species. Pseudomonad strains were isolated from symptomatic stone fruit trees, namely apricot, peach, and plum trees cultivated in spatially separated orchards in the Western Cape. A polyphasic approach was used to identify and characterize these strains. Using a multilocus sequence typing approach of four housekeeping loci, namely cts, gapA, gyrB, and rpoD, the pseudomonad strains were delineated into two phylogenetic groups within P. syringae sensu lato: P. syringae sensu stricto and Pseudomonas viridiflava. These results were further supported by LOPAT diagnostic assays and analysis of clades in the rep-PCR dendrogram. The pseudomonad strains were pathogenic on both apricot and plum seedlings, indicative of a lack of host specificity between Pseudomonas strains infecting Prunus spp. This is a first report of P. viridiflava isolated from plum trees showing symptoms of bacterial canker. P. viridiflava is considered to be an opportunistic pathogen that causes foliar diseases of vegetable crops, fruit trees, and aromatic herbs, and thus the isolation of pathogenic P. viridiflava from twigs of plum trees showing symptoms of bacterial canker suggests that this bacterial species is a potentially emerging stem canker pathogen of stone fruit trees in South Africa.

Phenotypically and Genotypically Heterogeneous Strains of Pseudomonas syringae Associated With Alfalfa Leaf Spot Disease in Iran

In this study, we provide a polyphasic characterization of 18 Pseudomonas spp. strains associated with alfalfa leaf spot symptoms in Iran. All of the strains were pathogenic on alfalfa, although the aggressiveness and symptomology varied among the strains. All strains but one were pathogenic on broad bean, cucumber, honeydew, and zucchini, whereas only a fraction of the strains were pathogenic on sugar beet, tomato, and wheat. Syringomycin biosynthesis genes (syrB1 and syrP) were detected using the corresponding PCR primers in all of the strains isolated from alfalfa. Phylogenetic analyses using the sequences of four housekeeping genes (gapA, gltA, gyrB, and rpoD) revealed that all of the strains except one (Als34) belong to phylogroup 2b of P. syringae sensu lato, whereas strain Als34 placed within phylogroup 1 close to the type strain of P. syringae pv. apii. Among the phylogroup 2b strains, nine strains were phylogenetically close to the P. syringae pv. aptata clade, whereas the remainder were scattered among P. syringae pv. atrofaciens and P. syringae pv. syringae strains. Pathogenicity and host range assays of the bacterial strains evaluated in this study on a set of taxonomically diverse plant species did not allow us to assign a "pathovar" status to the alfalfa strains. However, these results provide novel insight into the host range and phylogenetic position of the alfalfa-pathogenic members of P. syringae sensu lato, and they reveal that phenotypically and genotypically heterogeneous strains of the pathogen cause bacterial leaf spot of alfalfa.

Inference of Convergent Gene Acquisition Among Pseudomonas syringae Strains Isolated From Watermelon, Cantaloupe, and Squash

Pseudomonas syringae sensu stricto (phylogroup 2; referred to as P. syringae) consists of an environmentally ubiquitous bacterial population associated with diseases of numerous plant species. Recent studies using multilocus sequence analysis have indicated the clonal expansion of several P. syringae lineages, located in phylogroups 2a and 2b, in association with outbreaks of bacterial spot disease of watermelon, cantaloupe, and squash in the United States. To investigate the evolutionary processes that led to the emergence of these epidemic lineages, we sequenced the genomes of six P. syringae strains that were isolated from cucurbits grown in the United States, Europe, and China over a period of more than a decade, as well as eight strains that were isolated from watermelon and squash grown in six different Florida counties during the 2013 and 2014 seasons. These data were subjected to comparative analyses along with 42 previously sequenced genomes of P. syringae stains collected from diverse plant species and environments available from GenBank. Maximum likelihood reconstruction of the P. syringae core genome revealed the presence of a hybrid phylogenetic group, comprised of cucurbit strains collected in Florida, Italy, Serbia, and France, which emerged through genome-wide homologous recombination between phylogroups 2a and 2b. Functional analysis of the recombinant core genome showed that pathways involved in the ATP-dependent transport and metabolism of amino acids, bacterial motility, and secretion systems were enriched for recombination. A survey of described virulence factors indicated the convergent acquisition of several accessory type 3 secreted effectors (T3SEs) among phylogenetically distinct lineages through integrative and conjugative element and plasmid loci. Finally, pathogenicity assays on watermelon and squash showed qualitative differences in virulence between strains of the same clonal lineage, which correlated with T3SEs acquired through various mechanisms of horizontal gene transfer (HGT). This study provides novel insights into the interplay of homologous recombination and HGT toward pathogen emergence and highlights the dynamic nature of P. syringae sensu lato genomes.

Comparative genomic analysis of Pseudomonas amygdali pv. lachrymans NM002: Insights into its potential virulence genes and putative invasion determinants

Pseudomonas amygdali pv. lachrymans is currently of important plant pathogenic bacteria that causes cucumber angular leaf spot worldwide. The pathogen has been studied for its roles in pathogenicity and plant inheritance resistance. To further delineate traits critical to virulence, invasion and survival in the phyllosphere, we reported the first complete genome of P. amygdali pv. lachrymans NM002. Analysis of the whole genome in comparison with three closely-related representative pathovars of P. syringae identified the conservation of virulence genes, including flagella and chemotaxis, quorum-sensing systems, two-component systems, and lipopolysaccharide and antiphagocytosis. It also revealed differences of invasion determinants, such as type III effectors, phytotoxin (coronatine, syringomycin and phaseolotoxin) and cell wall-degrading enzyme, which may contribute to infectivity. The aim of this study was to derive genomic information that would reveal the probable molecular mechanisms underlying the virulence, infectivity and provide a better understanding of the pathogenesis of the P. syringae pathovars.

Molecular Epidemiology of Pseudomonas syringae pv. syringae Causing Bacterial Leaf Spot of Watermelon and Squash in Florida

From 2013 to 2014, bacterial leaf spot epidemics incited by Pseudomonas syringae pv. syringae affected an estimated 3,000 ha of watermelon and squash in Florida, and caused foliar blighting and transplant losses in severely affected fields. To investigate the diversity of the causal agent, we isolated 28 P. syringae strains from diseased plants grown in 10 Florida and Georgia counties over the course of 2 years. Strains were confirmed as P. syringae through sequence analysis of the 16S ribosomal RNA, phenotypic, and biochemical profiling; however, 20 displayed an atypical phenotype by exhibiting nonfluorescent activity on King's medium B agar and being negative for ice-nucleating activity. Multilocus sequence analysis and BOX polymerase chain reaction revealed the presence of two haplotypes among the collected strains that grouped into two distinct clades within P. syringae phylogroup 2. Pathogenicity testing showed that watermelon, cantaloupe, and squash seedlings were susceptible to a majority of these strains. Although both haplotypes were equally virulent on cantaloupe, they differed in virulence on watermelon and squash. The distribution of one haplotype in 9 of 10 Florida and Georgia counties sampled indicated that these epidemics were associated with the recent introduction of a novel clonal P. syringae lineage throughout major watermelon production areas in Florida.

The current status on the taxonomy of Pseudomonas revisited: An update

The genus Pseudomonas described in 1894 is one of the most diverse and ubiquitous bacterial genera which encompass species isolated worldwide. In the last years more than 70 new species have been described, which were isolated from different environments, including soil, water, sediments, air, animals, plants, fungi, algae, compost, human and animal related sources. Some of these species have been isolated in extreme environments, such as Antarctica or Atacama desert, and from contaminated water or soil. Also, some species recently described are plant or animal pathogens. In this review, we revised the current status of the taxonomy of genus Pseudomonas and the methodologies currently used for the description of novel species which includes, in addition to the classic ones, new methodologies such as MALDI-TOF MS, MLSA and genome analyses. The novel Pseudomonas species described in the last years are listed, together with the available genome sequences of the type strains of Pseudomonas species present in different databases.

Evidence for a Novel Phylotype of Pseudomonas syringae Causing Bacterial Leaf Blight of Cantaloupe in China

Bacterial leaf blight (BLB) has caused severe yield losses in cantaloupe (Cucumis melo L.) in the major melon-growing regions of China since the beginning of the twentieth century. Historically, Pseudomonas syringae pv. lachrymans was considered to be the causal agent of BLB of cantaloupe and angular leaf spot of cucumber. In the process of characterizing bacteria isolated from cantaloupe, we observed that putative P. syringae pv. lachrymans yielded negative results in P. syringae pv. lachrymans-specific PCR assays. This suggested that the P. syringae pv. lachrymans-like strains from cantaloupe were distinct from those recovered from cucumber. To investigate the differences between P. syringae pv. lachrymans-like strains isolated from cantaloupe and cucumber, 13 P. syringae strains isolated from cantaloupe [12 from China and 1 from Zimbabwe (NCPPB2916)] and 7 additional P. syringae reference strains were analyzed by catabolic profiling, phylogenetic analysis by multilocus sequence analysis (MLSA) and pathogenicity tests on cantaloupe leaflets. Catabolic profiling and MLSA based on 10 housekeeping genes and 2 hypersensitive response and pathogenicity (hrp) genes allowed us to differentiate strains isolated from cantaloupe and cucumber. Pseudomonas syringae pv. lachrymans strains isolated from cucumber clustered with genomospecies 2, and 13 P. syringae strains isolated from cantaloupe belonged to genomospecies 1. While all cantaloupe strains were closely related to P. syringae pv. aptata, they could be differentiated from this pathovar based on metabolic tests and MLSA. Pathogenicity tests showed that all strains isolated from cantaloupe and cucumber were only pathogenic on their original hosts. Based on these observations we conclude that P. syringae pv. lachrymans strains recovered from cantaloupe in China represent a novel phylotype.

Epidemiology and management of bacterial leaf spot on watermelon caused by Pseudomonas syringae

Bacterial leaf spot of watermelon caused by Pseudomonas syringae has been an emerging disease in the southeastern United States in recent years. Disease outbreaks in Florida were widespread from 2013 to 2014 and resulted in foliar blighting at the early stages of the crop and transplant losses. We conducted a series of field trials at two locations over the course of two years to examine the chemical control options that may be effective in management of this disease, and to investigate the environmental conditions conducive for bacterial leaf spot development. Weekly applications of acibenzolar-S-methyl (ASM) foliar, ASM drip, or copper hydroxide mixed with ethylene bis-dithiocarbamate were effective in reducing the standardized area under the disease progress curve (P < 0.05). Pearson's correlation test demonstrated a negative relationship between the average weekly temperature and disease severity (-0.77, P = 0.0002). When incorporated into a multiple regression model with the square root transformed average weekly rainfall, these two variables accounted for 71% of the variability observed in the weekly disease severity (P < 0.0001). This information should be considered when choosing the planting date for watermelon seedlings as the cool conditions often encountered early in the spring season are conducive for bacterial leaf spot development.