Bacterial Leaf Spot Of Lettuce Research Articles

Genotypic and Phenotypic Characterization of Lettuce Bacterial Pathogen Xanthomonas hortorum pv. vitians Populations Collected in Quebec, Canada

Bacterial leaf spot of lettuce, caused by Xanthomonas hortorum pv. vitians, is an economically important disease worldwide. For instance, it caused around 4 million CAD in losses in only a few months during the winter of 1992 in Florida. Because only one pesticide is registered to control this disease in Canada, the development of lettuce cultivars tolerant to bacterial leaf spot remains the most promising approach to reduce the incidence and severity of the disease in lettuce fields. The lack of information about the genetic diversity of the pathogen, however, impairs breeding programs, especially when disease resistance is tested on newly developed lettuce germplasm lines. To evaluate the diversity of X. hortorum pv. vitians, a multilocus sequence analysis was performed on 694 isolates collected in Eastern Canada through the summers of 2014 to 2017 and two isolates in 1996 and 2007. All isolates tested were clustered into five phylogroups. Six pathotypes were identified following pathogenicity tests conducted in greenhouses, but when phylogroups were compared with pathotypes, no correlation could be drawn. However, in vitro production of xanthan and xanthomonadins was investigated, and isolates with higher production of xanthomonadins were generally causing less severe symptoms on the tolerant cultivar Little Gem. Whole-genome sequencing was undertaken for 95 isolates belonging to the pathotypes identified, and de novo assembly made with reads unmapped to the reference strain’s genome sequence resulted in 694 contigs ranging from 128 to 120,795 bp. Variant calling was performed prior to genome-wide association studies computed with single-nucleotide polymorphisms (SNPs), copy-number variants and gaps. Polymorphisms with significant p-values were only found on the cultivar Little Gem. Our results allowed molecular identification of isolates likely to cause bacterial leaf spot of lettuce, using two SNPs identified through genome-wide association study.

Bacterial Leaf Spot of Lettuce Disease: An Unpredictable Enemy to the Florida Lettuce Industry

Bacterial leaf spot (BLS) of lettuce is caused by the bacterium Xanthomonas hortorum. BLS is more damaging in Florida, where the climate is conducive for disease development. The most effective control method is host resistance currently being developed by the UF/IFAS lettuce breeding program in all lettuce types. This new 4-page publication of the UF/IFAS Horticultural Sciences Department was written by Germán Sandoya and D. Calvin Odero.

Determination of bacterial leaf spot of lettuce caused by Pseudomonas viridiflava (Burkholder 1930) Dowson 1939 and the reactions of some lettuce cultivars

Pseudomonas viridiflava (Burkholder 1930) Dowson 1939 is a multihost plant pathogenic bacterium all over the world and also in Turkey. The aim of this study was to identify the pathogenic bacteria causing the symptoms of water-soaked necrotic leaf spots, dark brown spots on leaves, tip burn of leaf in curly type lettuce (Lactuca sativa var. crispy) grown in the field conditions in Beypazarı region and to reveal the reactions of some lettuce cultivars against the bacteria. For this purpose, twelve isolates were obtained from 4 different locations. Pathogenicity tests were conducted on curly type lettuce and all isolates were found pathogenic on the host. Isolated bacteria were evaluated for Gram reaction, LOPAT tests, colony morphology on King’s Medium B and carbon sources utilization by VITEK-2 system and identified as Pseudomonas viridiflava. The 16S rDNA region of a selected isolate representing the similar isolates in all tests was sequenced for genetic confirmation and the obtained sequence was submitted to the GeneBank under the accession number MN_539659. The result of the obtained sequence was 99.75% similar to the respective reference sequence after BLAST analysis. Reaction of the three different widely produced lettuce types, iceberg, curly and romaine types, with ten varieties against the causative bacterial agent was evaluated and the difference between the varieties found significant statistically. In this study, P. viridiflava was detected in lettuce in Central Anatolia for the first time.

Clarifying the taxonomy of the causal agent of bacterial leaf spot of lettuce through a polyphasic approach reveals that Xanthomonas cynarae Trébaol et al. 2000 emend. Timilsina et al. 2019 is a later heterotypic synonym of Xanthomonas hortorum Vauterin et al. 1995

Assessment of the taxonomy and diversity of Xanthomonas strains causing bacterial leaf spot of lettuce (BLSL), commonly referred to as Xanthomonas campestris pv. vitians, has been a long-lasting issue which held back the global efforts made to understand this pathogen. In order to provide a sound basis essential to its study, we conducted a polyphasic approach on strains obtained through sampling campaigns or acquired from collections. Results of a multilocus sequence analysis crossed with phenotypic assays revealed that the pathotype strain does not match the description of the nomenspecies provided by Brown in 1918. However, strain LMG 938=CFBP 8686 does fit this description. Therefore, we propose that it replaces LMG 937=CFBP 2538 as pathotype strain of X. campestris pv. vitians.Then, whole-genome based phylogenies and overall genome relatedness indices calculated on taxonomically relevant strains exhibited the intermediate position of X. campestris pv. vitians between closely related species Xanthomonas hortorum and Xanthomonas cynarae. Phenotypic profiles characterized using Biolog microplates did not reveal stable diagnostic traits legitimizing their distinction. Therefore, we propose that X. cynarae Trébaol et al. 2000 emend. Timilsina et al. 2019 is a later heterotypic synonym of X. hortorum, to reclassify X. campestris pv. vitians as X. hortorum pv. vitians comb. nov. and to transfer X. cynarae pathovars in X. hortorum as X. hortorum pv. cynarae comb. nov. and X. hortorum pv. gardneri comb. nov. An emended description of X. hortorum is provided, making this extended species a promising model for the study of Xanthomonas quick adaptation to different hosts.

Isolation and Characterization of Biosurfactant-Producing Bacteria From Oil Well Batteries With Antimicrobial Activities Against Food-Borne and Plant Pathogens.

Microbial biosurfactants, produced by fungi, yeast, and bacteria, are surface-active compounds with emulsifying properties that have a number of known activities, including the solubilization of microbial biofilms. In an on-going survey to uncover new or enhanced antimicrobial metabolite-producing microbes from harsh environments, such as oil-rich niches, 123 bacterial strains were isolated from three oil batteries in the region of Chauvin, Alberta, and characterized by 16S rRNA gene sequencing. Based on their nucleotide sequences, the strains are associated with 3 phyla (Actinobacteria, Proteobacteria and Firmicutes), as well as 17 other discrete genera that shared high homology with known sequences, with the majority of these strains identified to the species level. The most prevalent strains associated with the three oil wells belonged to the Bacillus genus. Thirty-four of the 123 strains were identified as biosurfactant-producers, among which Bacillus methylotrophicus strain OB9 exhibited the highest biosurfactant activity based on multiple screening methods and a comparative analysis with the commercially available biosurfactant, Tween 20. B. methylotrophicus OB9 was selected for further antimicrobial analysis and addition of live cultures of B. methylotrophicus OB9 (or partially purified biosurfactant fractions thereof) were highly effective on biofilm disruption in agar diffusion assays against several Gram-negative food-borne bacteria and plant pathogens. Upon co-culturing with B. methylotrophicus OB9, the number of either Salmonella enterica subsp. enterica Newport SL1 or Xanthomonas campestris B07.007 cells significantly decreased after 6 h and were not retrieved from co-cultures following 12 h exposure. These results also translated to studies on plants, where bacterized tomato seedlings with OB9 significantly protected the tomato leaves from Salmonella enterica Newport SL1 contamination, as evidenced by a 40% reduction of log10 CFU of Salmonella/mg leaf tissue compared to non-bacterized tomato leaves. When B. methylotrophicus 0B9 was used for bacterized lettuce, the growth of X. campestris B07.007, the causal agent of bacterial leaf spot of lettuce, was completely inhibited. While limited, these studies are noteworthy as they demonstrate the inhibition spectrum of B. methylotrophicus 0B9 against both human and plant pathogens; thereby making this bacterium attractive for agricultural and food safety applications in a climate where microbial-biofilm persistence is an increasing problem.

Bacterial leaf spot of lettuce caused by Xanthomonas hortorum pv. vitians in the Aegean Region of Turkey

Samples of diseased plants were collected from 12 lettuce fields in Aydin province of the Aegean Region of Turkey. A total of 8 bacterial isolates were identified by morphological characterization, sequence analysis of 16S rDNA and gyrB genes, and pathogenicity. The survey included twelve lettuce fields of three different cultivars [Yedikule (romaine type), Robinson and Chianti (iceberg type)]. The incidence of the disease ranged from 48 to 95%. Plants of cv. Chianti were the most severely infected cultivars. To our knowledge, this is the first report of the presence of Xanthomonas hortorum pv. vitians in lettuce plants in Turkey.

Survival of Xanthomonas campestris pv. vitians on lettuce in crop debris, irrigation water, and weeds in south Florida

Bacterial leaf spot of lettuce, caused by Xanthomonas campestris pv. vitians (Xcv), can be a disease of economic importance but it occurs sporadically in production fields in Florida. We investigated the role of weeds, irrigation water, and crop debris as potential inoculum sources for Xcv. Detection and identification of Xcv was facilitated using semi-selective media and PCR with two primers pairs. During field outbreaks of the disease, epiphytic populations of Xcv were detected on 13 weed species belonging in the Asteraceae, Amaranthaceae, Aizoaceae, Chenopodiaceae, Portulacaceae, Solanaceae, and Malvaceae families. In greenhouse assays, Xcv epiphytic populations on 12 weed hosts varied greatly. Bacterial leaf spot-like symptoms developed on Sonchus oleraceus and significantly higher populations of Xcv were recovered from this weed. Bacteria were also recovered from surface irrigation water monitored during a 4-month period covering two growing seasons at three commercial sites. In two seasons of survival studies on crop debris, Xcv was recovered up to 1 month after placement of diseased tissue on the soil surface but not detected in tissue buried 15 or 30 cm beneath the soil surface. Therefore, weeds and surface irrigation water may play a role in the survival of Xcv whereas crop debris may be limited as an inoculum source during the fallow period in Florida.

Multilocus Sequence Typing of Strains of Bacterial Spot of Lettuce Collected in the United States.

Studies on genetic diversity and recombination in bacterial pathogens are providing a better understanding of the mechanisms shaping bacterial diversity, which can affect disease control. Xanthomonas campestris pv. vitians, causal agent of bacterial leaf spot of lettuce, is a threat to the worldwide lettuce industry. We examined the genetic variation within a sample of 83 strains from California, Florida, and Ohio using multilocus sequence typing of six housekeeping genes, totaling 2.7 kb. Additionally, polymorphism in two virulence-related genes, hrpB2 and a putative glycosyl hydrolase, were examined. Based on housekeeping genes, we found three genetic groups of strains that were all able to induce the disease. These included strains collected from weeds and irrigation water that had haplotypes identical to strains from diseased lettuce. High linkage disequilibrium across the sequenced loci indicates that the pathogen is predominantly clonal but recombination has contributed to the observed sequence variation. Although there was significant genetic variation in X. campestris pv. vitians within and among sampled states, identical haplotypes were observed across all three states. This finding suggests that seedborne inoculum may contribute to the diversity of X. campestris pv. vitians in the United States. Knowledge of the genetic structure of the pathogen may be used for developing resistant lettuce varieties.

Organic compounds increase the efficacy of famoxadone + cymoxanil in the control of bacterial leaf spot of lettuce

Bacterial leaf spot of lettuce, induced by Xanthomonas campestris pv. vitians, constitutes a major disease on commercial lettuce (Lactuca sativa) production in south Florida. Under greenhouse conditions, resorcinol, two antioxidants, and various sugar alcohols such as mannitol, were evaluated alone, and in combination with Tanos® 50DF (25% a.i. of each component cymoxanil and famoxadone, E.I du Pont de Nemours and Company, Wilmington, DE) for control of bacterial leaf spot. Among the compounds evaluated, resorcinol (35–50% disease control) and mannitol (9–45%) were the most effective. In field trials, different mixtures containing propionic acid, mannitol, resorcinol, Tanos and or copper hydroxide (Kocide 3000®, 46.1%, 30% copper equivalent) were evaluated for the control of bacterial leaf spot of lettuce. Field data support a moderate impact of Tanos in disease reduction. Both greenhouse and field data support a synergistic interaction between Tanos and the compounds including copper, in disease reduction. The mixture Tanos + resorcinol, or copper/resorcinol, Tanos + copper provided better or similar disease control than copper alone. No phytotoxicity was observed in our trials. In an integrated management program for bacterial leaf spot of lettuce, Tanos may be useful in tank mixes with these compounds and copper.

The inheritance of resistance to bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians in three lettuce cultivars.

Lettuce yields can be reduced by the disease bacterial leaf spot (BLS) caused by the pathogen Xanthomonas campestris pv. vitians (Xcv) and host resistance is the most feasible method to reduce disease losses. The cultivars La Brillante, Pavane and Little Gem express an incompatible host–pathogen interaction as a hypersensitive response (HR) to California strains of Xcv resulting in resistance. Little was known about the inheritance of resistance; however, resistance to other lettuce pathogens is often determined by resistance gene candidates (RGCs) encoding nucleotide-binding leucine-rich repeat (NB-LRR) proteins. Therefore, we determined the inheritance of BLS resistance in the cultivars La Brillante, Little Gem and Pavane and mapped it relative to RGCs. The reaction to Xcv was analyzed in nine F1, F2 and recombinant inbred line populations of lettuce from HR×compatible or HR×HR crosses. The HR in La Brillante, Pavane and Little Gem is conditioned by single dominant genes, which are either allelic or closely linked genes. The resistance gene in La Brillante was designated Xanthomonas resistance 1 (Xar1) and mapped to lettuce linkage group 2. Xar1 is present in a genomic region that contains numerous NB-LRR encoding RGCs and functional pathogen resistance loci in the RGC2 family. The Xar1 gene confers a high level of BLS resistance in the greenhouse and field that can be introgressed into commercial lettuce cultivars to reduce BLS losses using molecular markers.

Association analysis of bacterial leaf spot resistance and SNP markers derived from expressed sequence tags (ESTs) in lettuce (Lactuca sativa L.)

Bacterial leaf spot of lettuce, caused by Xanthomonas campestris pv. vitians, is a devastating disease of lettuce worldwide. Since there are no chemicals available for effective control of the disease, host-plant resistance is highly desirable to protect lettuce production. A total of 179 lettuce genotypes consisting of 29 leaf, 15 crisphead, one stem, 21 romaine, and 113 butterhead types were evaluated for response to X. c. vitians. One source of high resistance and five sources of moderate resistance were identified with four being butterhead lettuce and two leaf lettuce. The population genetic structure based on 350 expressed-sequence-tag-derived single nucleotide polymorphism generated two clades: Clade I and Clade II. The butterhead type was genetically distinct from romaine and crisphead types, while the leaf type was found to frequently exchange genes with other types through the history of breeding. Association mapping identified one single nucleotide polymorphism (QGB19C20.yg-1-OP5) associated with disease severity in Q general linear model and Q + K mixed linear model. Two SNP markers (Contig15389-1-OP1 and Contig6039-19-OP1) were associated with the resistance in the leaf lettuce PI 358000-1 which had no disease symptoms. The marker QGB19C20.yg-1-OP5 is in linkage group 2, while both of Contig15389-1-OP1 and Contig6039-19-OP1 are in linkage group 4. The resistant lines and the associated SNPs should be useful to develop resistant cultivars to battle against the devastating disease in lettuce.

A Novel Screening Method for Evaluation of Lettuce Germplasm for Bacterial Leaf Spot Resistance

Bacterial leaf spot of lettuce, caused by Xanthomonas campestris pv. vitians, is a devastating disease of lettuce worldwide. Because there are no chemicals available for effective control of the disease, host-plant resistance is highly desirable to protect lettuce production. A new method for fast screening and accurate identification of bacterial leaf spot (BLS)-resistant lettuce has been developed in our laboratory. A total of 79 lettuce genotypes (69 germplasm lines and 10 adapted cultivars) were evaluated with this technique for response to X. c. vitians. Disease incidences ranged from 92% to 100% and disease severities were between 1.6 and 3.6 on the 0 to 4 scale. No highly resistant genotypes were identified. However, 12 genotypes did not significantly differ for disease severities from the moderately resistant ‘Little Gem’ lettuce that was used as a resistant control. Comparison of disease severities of 10 commercial cultivars and three moderately resistant germplasm lines tested at the seedling stage and adult stage showed a high positive correlation (r = 0.87, P < 0.0001) between tests. The new screening method should be useful in breeding programs, in which great numbers of plants need to be tested during germplasm evaluation, and for single plant selection as well as other studies. The identification of new sources of moderate resistance in this study could facilitate development of cultivars with a higher level of resistance through the gene pyramiding approach.

Potential of weed species to serve as a reservoir for Xanthomonas campestris pv. vitians, the causal agent of bacterial leaf spot of lettuce

The ability of Xanthomonas campestris pv. vitians (Brown) Dye, the causal agent of bacterial leaf spot of lettuce, to colonize plant surfaces as well as its ability to cause symptoms was investigated for several weed species belonging to the Asteraceae, Chenopodiaceae, Malvaceae, Polygonaceae and Portulacaceae families. A bacterial suspension of X. campestris pv. vitians was sprayed onto aerial parts of the weed plants. Bacterial population size was determined after 4 weeks of incubation in a greenhouse at 24 °C and 95% relative humidity. Bacterial population sizes were determined by dilution plating of the leaf washings and expressed in CFU (colony forming units) per gram of plant tissue. X. campestris pv. vitians was recovered from all weed species. The bacterial population size was found to vary significantly among the different weed plant species (P < 0.0001, F = 7.358). Significantly larger populations were recovered on Lactuca species (>105 CFU/g of plant) than on Chenopodiaceae, Polygonaceae and Portulacaceae and Astereae (a tribe of the Asteraceae) (<102 CFU/g of plant). Symptom development was investigated using two inoculation methods: spraying and injection. Symptoms were evaluated 2, 7 and 14 days after inoculation. In each trial, independently, symptoms developed on all Lactuca species, regardless of the method used. Symptoms were observed less consistently for other species of the family Asteraceae with the exception of members of the tribe Astereae, in which no symptoms developed. For species belonging to other families, no symptoms were observed, except on Nicotiana tabacum, which belongs to the family Solanaceae.

Leaf microbiota in an agroecosystem: spatiotemporal variation in bacterial community composition on field-grown lettuce.

The presence, size and importance of bacterial communities on plant leaf surfaces are widely appreciated. However, information is scarce regarding their composition and how it changes along geographical and seasonal scales. We collected 106 samples of field-grown Romaine lettuce from commercial production regions in California and Arizona during the 2009-2010 crop cycle. Total bacterial populations averaged between 10(5) and 10(6) per gram of tissue, whereas counts of culturable bacteria were on average one (summer season) or two (winter season) orders of magnitude lower. Pyrosequencing of 16S rRNA gene amplicons from 88 samples revealed that Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria were the most abundantly represented phyla. At the genus level, Pseudomonas, Bacillus, Massilia, Arthrobacter and Pantoea were the most consistently found across samples, suggesting that they form the bacterial 'core' phyllosphere microbiota on lettuce. The foliar presence of Xanthomonas campestris pv. vitians, which is the causal agent of bacterial leaf spot of lettuce, correlated positively with the relative representation of bacteria from the genus Alkanindiges, but negatively with Bacillus, Erwinia and Pantoea. Summer samples showed an overrepresentation of Enterobacteriaceae sequences and culturable coliforms compared with winter samples. The distance between fields or the timing of a dust storm, but not Romaine cultivar, explained differences in bacterial community composition between several of the fields sampled. As one of the largest surveys of leaf surface microbiology, this study offers new insights into the extent and underlying causes of variability in bacterial community composition on plant leaves as a function of time, space and environment.

Weeds and Epidemiology of Bacterial Leaf Spot of Lettuce in the Everglades Agricultural Area

This 2-page fact sheet briefly covers the history, symptoms, and epidemiology of this disease of lettuce. Written by D.C. Odero, and published by the UF Department of Agronomy, May 2011. SS-AGR-347/AG357: Weeds and Epidemiology of Bacterial Leaf Spot of Lettuce in the Everglades Agricultural Area (ufl.edu)

Population dynamics of Xanthomonas campestris pv. vitians on different plant species and management of bacterial leaf spot of lettuce under greenhouse conditions

The population dynamics of Xanthomonas campestris pv. vitians ( Xcv) was studied both externally and internally in lettuce, tomato and pepper plants. In addition, the application of bactericides during transplant production period was carried out for the management of bacterial leaf spot of lettuce under greenhouse conditions. Epiphytic populations of Xcv were recovered on leaves of lettuce plants (10 5 CFU/g) 5 weeks after sprayed than the other plant species when inoculated with 10 8 CFU/ml of Xcv. When plants of each crop species infiltrated with the bacterium at 10 5 CFU/ml, the highest populations were developed in lettuce (10 8 CFU/cm 2) followed by pepper with 10 6 CFU/cm 2 and tomato with 10 5 CFU/cm 2 10-days after infiltration. Application of a mixture of Maneb and Kocide or Kocide alone as a foliar spray on lettuce significantly reduced the incidence and disease severity of bacterial leaf spot by 29 and 27% respectively. Spread of the bacterium and development of the disease may partly be managed by avoiding intercropping of these plants commonly grown in close proximity to lettuce. For the management of leaf-associated populations of Xcv in lettuce, use of a mixture of Maneb and Kocide is advocated to minimize the effect of attacks.

Occurrence of bacterial leaf spot disease on lettuce caused by Pseudomonas viridiflava in the Kingdom of Saudi Arabia

During the spring of 2007, Romaine type lettuce plants (Lactuca sativa cv. Darkland) grown in four commercial fields in the province of EL-Kharj region of Saudi Arabia were observed with numerous lesions typical of bacterial leaf spot. Lesions were irregular, small, pale green to black. Disease incidence approached 30%. Bacteria were isolated by cutting lesions into small (1 mm) pieces and soaking them in 2 mL sterile distilled water. The resulting suspensions were streaked onto sucrose peptone agar and King’s medium B. Three isolates from different fields, selected for further study were aerobic, Gram-negative, non-spore-forming rods. All isolates induced a hypersensitive response in tobacco plants, caused soft rot of potato tubers, and were negative for levan, oxidase, and arginine dihydrolase (Lelliott & Stead, 1987). Acid was produced aerobically from D-glucose, mannitol, and sorbitol, but not from D-arabinose, L-rhamnose, or sucrose; starch was not hydrolysed, glucose metabolism was oxidative, nitrates were not reduced to nitrites, gelatin was hydrolysed. Isolates were catalase positive and unable to grow in the presence of 5% NaCl. On this basis isolates were identified as Pseudomonas viridiflava. The identity of bacterial strains was confirmed by BIOLOG™ analysis (carbon source utilization at 37°C), with a similarity index of 0·75. Pathogenicity tests were performed by spraying leaves of five-week-old lettuce plants (cv. Darkland) with bacterial suspensions (ca. 108 cfu/mL) prepared from 48 h cultures on yeast dextrose chalk agar. Control plants were sprayed with sterile water. Following inoculation plants were covered with polyethylene bags for 48 h at 25°C, after which bags were removed and plants were transferred to a greenhouse at 25–28°C (Aysan et al., 2003). All isolates were pathogenic, causing leaf-spot symptoms, similar to those observed on the samples collected, within two weeks of inoculation. No symptoms developed on control plants. The bacterium was re-isolated from inoculated plants and identified as P. viridiflava. This is the first report of bacterial leaf spot of lettuce in Saudi Arabia caused by Pseudomonas viridiflava.

First Report of Bacterial Leaf Spot of Lettuce (Lactuca sativa) Caused by Xanthomonas campestris pv. vitians in Saudi Arabia.

In April of 2008, lettuce (Lactuca sativa L. cv. Darkland) plants grown in the Al-Ouunia Region of Saudi Arabia were observed with numerous lesions typical of bacterial leaf spot. Leaf lesions were irregular, small, pale green to black, and 2 to 5 mm in diameter. Bacteria were isolated from diseased leaf tissues by cutting leaves into small pieces (0.5 mm) and soaking them in 2 ml of sterile distilled water. The resulting suspension was streaked onto yeast dextrose calcium carbonate agar (YDC) (1) and plates were incubated at 28°C. Large, round, butyrus, bright yellow colonies typical of Xanthomonas spp. formed after 48 h and five strains were selected for further tests. A yellow, mucoid bacterium was consistently isolated from lettuce samples with typical bacterial leaf spot symptoms. All five strains tested in this study were gram negative, oxidase negative, nitrate reduction negative, catalase and esculin hydrolysis positive, motile, and strictly aerobic. All were slightly pectolytic but not amylolytic. All were identified as Xanthomonas campestris pv. vitians. The bacterium was identified with specific oligonucleotide primers (2). This primer pair directed the amplification of an approximately 700-bp DNA fragment from total genomic DNA of all X. campestris pv. vitians strains tested. Pathogenicity tests were performed by using bacterial cultures grown on YDC for 48 h at 28°C. Each strain was suspended in sterile distilled water and the bacterial concentration was adjusted to 106 CFU/ml. Leaves of 5-week-old lettuce plants (cv. Darkland) were sprayed with the bacterial suspension. The inoculated and sterile-water-sprayed control plants were covered with polyethylene bags for 48 h at 25°C, after which the bags were removed and plants were transferred to a greenhouse at 25 to 28°C (1). All strains were pathogenic on the lettuce cv. Darkland, causing typical bacterial leaf spot symptoms by 2 weeks after inoculation. All inoculated plants showed typical symptoms of bacterial leaf spot and symptoms similar to those observed on the samples collected. No symptoms developed on the control plants. The bacterium was reisolated from inoculated plants and identified as X. campestris pv. vitians by morphological, physiological, and biochemical tests as described above. To our knowledge, this is the first report of bacterial leaf spot of lettuce by X. campestris pv. vitians in Saudi Arabia.

Development of Lettuce Breeding Lines Resistant to Bacterial Leaf Spot

Bacterial leaf spot of lettuce caused by Xanthomonas campestris pv. vitians (Xcv) is an important lettuce disease in California. No adequate control measures have been found, although resistance exists in several heirloom cultivars. Deployment of cultivars resistant to bacterial leaf spot will reduce these periodic and costly disease events. The objectives of this research were to 1) identify new sources of resistance within modern crisphead cultivars and 2) select for resistance in `Salad Crisp' × `Iceberg' progeny. Field plots were established and grown with overhead irrigation, and a three-strain mixture of Xcv was applied until runoff 1 week after thinning at 1 × 109 CFU/mL. Twenty-six crisphead cultivars were tested in unreplicated field trials and rated on a 1 (susceptible) to 4 (resistant) scale. Selection was carried out between and within families from the F2 to F4 generation. Sixteen F3 families were evaluated in unreplicated plots, and 12 F5 families were tested in replicated plots for disease incidence and severity. No usable levels of resistance were identified in the modern crisphead cultivars tested to date. All F3 families had resistance greater than `Iceberg', and 19 plants from eight families were selected for further breeding. Subsequently, 12 plants from two F4 families were selected. Replicated trials of 12 F5 families indicated that all lines have disease severity comparable to both parents. Breeding lines from crosses to `Salinas 88' are currently being developed.

Toxicity of mixed-oxidant electrolyzed oxidizing water to in vitro and leaf surface populations of vegetable bacterial pathogens and control of bacterial diseases in the greenhouse

Mixed-oxidant (MO) electrolyzed oxidizing (EO) water was generated by electrolysis of a 1.7% KCl (aq. w/v) brine solution. The MO EO water was a powerful bactericide in vitro at a dosage of mixed oxidants equivalent to 50 mg L −1 free available chlorine. Populations of Xanthomonas campestris pv. vitians, Pseudomonas syringae pv. coriandricola, and Erwinia carotovora subsp. carotovora were reduced from log 9 to log 10 CFU mL −1 to undetectable levels after 1 min exposure. Only E. carotovora subsp. carotovora was sensitive to a 5 mg L −1 dose of MO EO water. In greenhouse disease control experiments, 50 or 100 mg L −1 MO EO water failed to control bacterial leaf spot of lettuce, bacterial spot of tomato and pepper, or bacterial leaf spot of radish. A spray application of a copper hydroxide/mancozeb suspension was effective for control of bacterial leaf spot of lettuce and bacterial spot of tomato and pepper, reducing foliar disease levels up to 45%. Some phytotoxicity was observed at the 100 mg L −1 MO EO water dose. The lack of disease control on greenhouse plants with MO EO water may in large part be due to low mortality of the pathogen on leaf surfaces. In five of six experiments, no significant reductions in leaf surface populations were found for 50 mg L −1 MO EO water. In contrast, copper/mancozeb treatments reduced pathogen leaf surface populations by up to 5 log units when copper-sensitive strains were involved.