Apoplexy of Peach Trees Caused by Pseudomonas viridiflava

A sudden apoplexy of young nectarine trees was observed during early autumn 1995 in Piedmont (northern Italy). At collar level, extensive wet necrosis of the tissues beneath the bark was observed. When the infection girdled the trunk, the tree died. LOPAT tests, fluorescence on single-carbon sources and comparison of whole-cell protein profiles with type-strains indicated that the bacterial isolates belong to Pseudomonas viridiflava and P. syringae pv. syringae. In the pathogenicity tests, the first bacterium incited symptoms similar to those observed in the field. This is the first record of P. viridiflava as a pathogen of peach.     

Nutritional similarity between leaf-associated nonpathogenic bacteria and the pathogen is not predictive of efficacy in biological control of bacterial spot of tomato

It has been demonstrated that for a nonpathogenic, leaf-associated bacterium, effectiveness in the control of bacterial speck of tomato is correlated with the similarity in the nutritional needs of the nonpathogenic bacterium and the pathogen Pseudomonas syringae pv. tomato. This relationship was investigated further in this study by using the pathogen Xanthomonas campestris pv. vesicatoria, the causal agent of bacterial spot of tomato, and a collection of nonpathogenic bacteria isolated from tomato foliage. The effects of inoculation of tomato plants with one of 34 nonpathogenic bacteria prior to inoculation with the pathogen X. campestris pv. vesicatoria were quantified by determining (i) the reduction in disease severity (number of lesions per square centimeter) in greenhouse assays and (ii) the reduction in leaf surface pathogen population size (log(10) of the number of CFU per leaflet) in growth chamber assays. Nutritional similarity between the nonpathogenic bacteria and X. campestris pv. vesicatoria was quantified by using either niche overlap indices (NOI) or relatedness in cluster analyses based upon in vitro utilization of carbon or nitrogen sources reported to be present in tomato tissues or in Biolog GN plates. In contrast to studies with P. syringae pv. tomato, nutritional similarity between the nonpathogenic bacteria and the pathogen X. campestris pv. vesicatoria was not correlated with reductions in disease severity. Nutritional similarity was also not correlated with reductions in pathogen population size. Further, the percentage of reduction in leaf surface pathogen population size was not correlated with the percentage of reduction in disease severity, suggesting that the epiphytic population size of X. campestris pv. vesicatoria is not related to disease severity and that X. campestris pv. vesicatoria exhibits behavior in the phyllosphere prior to lesion formation that is different from that of P. syringae pv. tomato.     

Nutritional Similarity between Leaf-Associated Nonpathogenic Bacteria and the Pathogen Is Not Predictive of Efficacy in Biological Control of Bacterial Spot of Tomato

It has been demonstrated that for a nonpathogenic, leaf-associated bacterium, effectiveness in the control of bacterial speck of tomato is correlated with the similarity in the nutritional needs of the nonpathogenic bacterium and the pathogen Pseudomonas syringae pv. tomato. This relationship was investigated further in this study by using the pathogen Xanthomonas campestris pv. vesicatoria, the causal agent of bacterial spot of tomato, and a collection of nonpathogenic bacteria isolated from tomato foliage. The effects of inoculation of tomato plants with one of 34 nonpathogenic bacteria prior to inoculation with the pathogen X. campestris pv. vesicatoria were quantified by determining (i) the reduction in disease severity (number of lesions per square centimeter) in greenhouse assays and (ii) the reduction in leaf surface pathogen population size (log₁₀ of the number of CFU per leaflet) in growth chamber assays. Nutritional similarity between the nonpathogenic bacteria and X. campestris pv. vesicatoria was quantified by using either niche overlap indices (NOI) or relatedness in cluster analyses based upon in vitro utilization of carbon or nitrogen sources reported to be present in tomato tissues or in Biolog GN plates. In contrast to studies with P. syringae pv. tomato, nutritional similarity between the nonpathogenic bacteria and the pathogen X. campestris pv. vesicatoria was not correlated with reductions in disease severity. Nutritional similarity was also not correlated with reductions in pathogen population size. Further, the percentage of reduction in leaf surface pathogen population size was not correlated with the percentage of reduction in disease severity, suggesting that the epiphytic population size of X. campestris pv. vesicatoria is not related to disease severity and that X. campestris pv. vesicatoria exhibits behavior in the phyllosphere prior to lesion formation that is different from that of P. syringae pv. tomato.     

Differential expression of tomato proteinase inhibitor I and II genes during bacterial pathogen invasion and wounding

Expression of proteinase inhibitor I and II genes was investigated during infection by Pseudomonas syringae pv. tomato, the causal agent of bacterial speck disease in tomato. Inoculation of leaves with P. s. pv. tomato of two inbred tomato lines that are resistant and susceptible to the pathogen resulted in the accumulation of proteinase inhibitor I and II mRNAs in this organ. Our data showed that in the lines used in this study, proteinase inhibitor II mRNAs accumulated in leaves to higher levels than proteinase inhibitor I mRNA in response to P. s. pv. tomato infection and wounding. Proteinase inhibitor II mRNAs accumulated more rapidly in disease-resistant than in disease-susceptible plants. Proteinase inhibitor I mRNAs were first detected in the disease-susceptible line during infection and wounding. In contrast to wounding, the systemic induction of these genes during pathogen ingression was limited. These data show that the plant proteinase inhibitors constitute one of the components of the plant defense system that are induced in response to bacterial pathogen invasion.     

Systemic Migration of Pseudomonas syringae pv. avellanae in Twigs and Young Trees of Hazelnut and Symptom Development

At the beginning of October 1992 and 1993, healthy adult and young trees of hazelnut, cultivars Tonda Gentile Romana and Nocchione, were inoculated with a Pseudomonas syringae pv. avellanae strain through leaf scars located midway from the tip of the twig. In adult trees, the systemic migration and the population level of the pathogen were monitored during autumn and winter by means of weekly to monthly isolation and countings. In young plants, the possible displacement of the bacterium was checked in May by sampling different sites of the plant. At each sampling, the presence of internal and external symptoms was carefully assessed. During the 2-year study, the results on migration and symptom development were consistent between the cultivars. Until the end of February. P.s. pv. avellanae colonized for approximately 45 mm, only the trait of the twig below the site of inoculation. At that time the population level ranged, in the farthest point reached by the pathogen, from 4.1 to 5.6 = 10⁴ cfu/3 mm of twigportion. During March the pathogen was recovered from the tip of the twig. The first sign of disease was noticed in November as brown discolouration of the epidermis near the site of inoculation as well as internal necrosis of the vascular tissue. In young trees. P.s. pv. avellanae from the twig reached the branches, collar and roots from where it also colonized the branch which did not carry an inoculated leaf scar. Extensive twig dieback was noticed during April, whereas in June, most of the adult trees inoculated in autumn with approximately 25000 cells of the pathogen, randomly distributed in 25 sites of the crown, wilted completely. The role of leaf scars as a favourite site of penetration exploited by the pathogen as well as phytosanitary measures are discussed.     

Type IV pilin is glycosylated in Pseudomonas syringae pv. tabaci 6605 and is required for surface motility and virulence

Type IV pilin (PilA) is a major constituent of pilus and is required for bacterial biofilm formation, surface motility and virulence. It is known that mature PilA is produced by cleavage of the short leader sequence of the pilin precursor, followed by methylation of N-terminal phenylalanine. The molecular mass of the PilA mature protein from the tobacco bacterial pathogen Pseudomonas syringae pv. tabaci 6605 (Pta 6605) has been predicted to be 12 329 Da from its deduced amino acid sequence. Previously, we have detected PilA as an approximately 13-kDa protein by immunoblot analysis with anti-PilA-specific antibody. In addition, we found the putative oligosaccharide-transferase gene tfpO downstream of pilA. These findings suggest that PilA in Pta 6605 is glycosylated. The defective mutant of tfpO (ΔtfpO) shows reductions in pilin molecular mass, surface motility and virulence towards host tobacco plants. Thus, pilin glycan plays important roles in bacterial motility and virulence. The genetic region around pilA was compared among P. syringae pathovars. The tfpO gene exists in some strains of pathovars tabaci, syringae, lachrymans, mori, actinidiae, maculicola and P. savastanoi pv. savastanoi. However, some strains of pathovars tabaci, syringae, glycinea, tomato, aesculi and oryzae do not possess tfpO, and the existence of tfpO is independent of the classification of pathovars/strains in P. syringae. Interestingly, the PilA amino acid sequences in tfpO-possessing strains show higher homology with each other than with tfpO-nonpossessing strains. These results suggest that tfpO and pilA might co-evolve in certain specific bacterial strains.     

Protection of tomato seedlings against infection by Pseudomonas syringae pv. tomato by using the plant growth-promoting bacterium Azospirillum brasilense

Pseudomonas syringae pv. tomato, the causal agent of bacterial speck of tomato, and the plant growth-promoting bacterium Azospirillum brasilense were inoculated onto tomato plants, either alone, as a mixed culture, or consecutively. The population dynamics in the rhizosphere and foliage, the development of bacterial speck disease, and their effects on plant growth were monitored. When inoculated onto separate plants, the A. brasilense population in the rhizosphere of tomato plants was 2 orders of magnitude greater than the population of P. syringae pv. tomato (10(7) versus 10(5) CFU/g [dry weight] of root). Under mist chamber conditions, the leaf population of P. syringae pv. tomato was 1 order of magnitude greater than that of A. brasilense (10(7) versus 10(6) CFU/g [dry weight] of leaf). Inoculation of seeds with a mixed culture of the two bacterial strains resulted in a reduction of the pathogen population in the rhizosphere, an increase in the A. brasilense population, the prevention of bacterial speck disease development, and improved plant growth. Inoculation of leaves with the mixed bacterial culture under mist conditions significantly reduced the P. syringae pv. tomato population and significantly decreased disease severity. Challenge with P. syringae pv. tomato after A. brasilense was established in the leaves further reduced both the population of P. syringae pv. tomato and disease severity and significantly enhanced plant development. Both bacteria maintained a large population in the rhizosphere for 45 days when each was inoculated separately onto tomato seeds (10(5) to 10(6) CFU/g [dry weight] of root). However, P. syringae pv. tomato did not survive in the rhizosphere in the presence of A. brasilense. Foliar inoculation of A. brasilense after P. syringae pv. tomato was established on the leaves did not alleviate bacterial speck disease, and A. brasilense did not survive well in the phyllosphere under these conditions, even in a mist chamber. Several applications of a low concentration of buffered malic acid significantly enhanced the leaf population of A. brasilense (>10(8) CFU/g [dry weight] of leaf), decreased the population of P. syringae pv. tomato to almost undetectable levels, almost eliminated disease development, and improved plant growth to the level of uninoculated healthy control plants. Based on our results, we propose that A. brasilense be used in prevention programs to combat the foliar bacterial speck disease caused by P. syringae pv. tomato.     

Characterization of Pseudomonas pathovars isolated from rosaceous fruit trees in East Algeria

A survey of bacterial diseases due to Pseudomonas on rosaceous fruit trees was conducted. In forty two orchards located in the Constantine region ( East Algeria). Pseudomonas isolates were identified on the bases of their cultural and biochemical characteristics . A total of fifty nine phytopathogenic bacteria were isolated from diseased pome and stone fruit trees. Thirty one strains comparable to Pseudomonas syringae pv. syringae were isolated from cherry (Prunus avium L.), plum (P. domestica L.), apricot (P. armeniaca L.), almond (P. dulcis L.) and pear trees (Pirus communis L.); sixteen strains comparable to Pseudomonas syringae pv. morsprunorum were obtained from samples of cherry and plum. Twelve strains of Pseudomonas viridiflava were isolated from cherry, apricot and peach (Prunus persica L.).     

The cloned avirulence gene avrPto induces disease resistance in tomato cultivars containing the Pto resistance gene.

Resistance of tomato plants to the bacterial pathogen Pseudomonas syringae pv. tomato race 0 is controlled by the locus Pto. A bacterial avirulence gene was cloned by constructing a cosmid library from an avirulent P. syringae pv. tomato race, conjugating the recombinants into a strain of P. syringae pv. maculicola virulent on a tomato cultivar containing Pto, and screening for those clones that converted the normally virulent phenotype to avirulence. The cloned gene, designated avrPto, reduced multiplication of P. syringae pv. tomato transconjugants specifically on Pto tomato lines, as demonstrated by bacterial growth curve analyses. Analysis of F2 populations revealed cosegregation of resistance to P. syringae pv. tomato transconjugants carrying avrPto with resistance to P. syringae pv. tomato race 0. Surprisingly, mutation of avrPto in P. syringae pv. tomato race 0 does not eliminate the avirulent phenotype of race 0, suggesting that additional, as yet uncharacterized, avirulence genes and/or resistance genes may contribute to specificity in the avrPto-Pto interaction. Genetic analysis indicates that this resistance gene(s) would be tightly linked to Pto. Interestingly, P. syringae pv. glycinea transconjugants carrying avrPto elicit a typical hypersensitive resistant response in the soybean cultivar Centennial, suggesting conservation of Pto function between two crop plants, tomato and soybean.     

Arabidopsis NHO1 is required for general resistance against Pseudomonas bacteria

Nonhost interactions are prevalent between plants and specialized phytopathogens. Although it has great potential for providing crop plants with durable resistance, nonhost resistance is poorly understood. Here, we show that nonhost resistance is controlled, at least in part, by general resistance. Arabidopsis plants are resistant to the nonhost pathogen Pseudomonas syringae pv phaseolicola NPS3121 and completely arrest bacterial multiplication in the plant. Ten Arabidopsis mutants were isolated that were compromised in nonhost (nho) resistance to P. s. phaseolicola. Among these, nho1 is caused by a single recessive mutation that defines a novel gene. nho1 is defective in nonspecific resistance to Pseudomonas bacteria, because it also supported the growth of P. s. tabaci and P. fluorescens bacteria, both of which are nonpathogenic on Arabidopsis. In addition, the nho1 mutation also compromised resistance mediated by RPS2, RPS4, RPS5, and RPM1. Interestingly, the nho1 mutation had no effect on the growth of the virulent bacteria P. s. maculicola ES4326 and P. s. tomato DC3000, but it partially restored the in planta growth of the DC3000 hrpS(-) mutant bacteria. Thus, the virulent bacteria appear to evade or suppress NHO1-mediated resistance by means of an Hrp-dependent virulence mechanism.     

Polysaccharides of <Emphasis Type="Italic">Pseudomonas</Emphasis> Pathovar Strains That Infect Pea,Tomato, and Soya Bean

In order to understand the mode of action of taxonomically related Pseudomonas syringae pathovar strains that infect pea, tomato, and soya bean, we examined their extracellular polysaccharides (EPS). Maximum production of polysaccharide in shake culture of these pathogens was observed between 24 and 60 h. P. syringae pv. pisi 519, the bacterial blight pathogen of pea, produced a higher amount of polysaccharide (34.87 g/mL) at 60 h compared with 32.67 g/mL produced by P. syringae pv. glycinea NCPPB 1783, the bacterial blight pathogen of soya bean, and 30.03 g/mL produced by P. syringae pv. tomato NCPPB 269, the bacterial speck pathogen of tomato. EPS produced by P. syringae pv. pisi 519, P. syringae pv. tomato NCPPB 269, and P. syringae pv. glycinea NCPPB 1783 was characterized with infrared (FTIR), nuclear magnetic resonance (NMR), high performance thin layer chromatography, (HPTLC), and matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. HPTLC profiles revealed the presence of glucose and glucuronic acid in all bacteria and mannose only in P. syringae pv. tomato. Molecular mass of EPS of P. syringae pv. pisi (m/z 933.8), P. syringae pv. tomato (m/z 950.4), and P. syringae pv. glycinea (m/z 933.5) was confirmed by MALDI-TOF mass spectrometry.     

Identification of Bacteriophages for Biocontrol of the Kiwifruit Canker Phytopathogen Pseudomonas syringae pv. actinidiae

Pseudomonas syringae pv. actinidiae is a reemerging pathogen which causes bacterial canker of kiwifruit (Actinidia sp.). Since 2008, a global outbreak of P. syringae pv. actinidiae has occurred, and in 2010 this pathogen was detected in New Zealand. The economic impact and the development of resistance in P. syringae pv. actinidiae and other pathovars against antibiotics and copper sprays have led to a search for alternative management strategies. We isolated 275 phages, 258 of which were active against P. syringae pv. actinidiae. Extensive host range testing on P. syringae pv. actinidiae, other pseudomonads, and bacteria isolated from kiwifruit orchards showed that most phages have a narrow host range. Twenty-four were analyzed by electron microscopy, pulse-field gel electrophoresis, and restriction digestion. Their suitability for biocontrol was tested by assessing stability and the absence of lysogeny and transduction. A detailed host range was performed, phage-resistant bacteria were isolated, and resistance to other phages was examined. The phages belonged to the Caudovirales and were analyzed based on morphology and genome size, which showed them to be representatives of Myoviridae, Podoviridae, and Siphoviridae. Twenty-one Myoviridae members have similar morphologies and genome sizes yet differ in restriction patterns, host range, and resistance, indicating a closely related group. Nine of these Myoviridae members were sequenced, and each was unique. The most closely related sequenced phages were a group infecting Pseudomonas aeruginosa and characterized by phages JG004 and PAK_P1. In summary, this study reports the isolation and characterization of P. syringae pv. actinidiae phages and provides a framework for the intelligent formulation of phage biocontrol agents against kiwifruit bacterial canker.     

Detection of <Emphasis Type="Italic">Xanthomonas oryzae</Emphasis> pv. <Emphasis Type="Italic">oryzae</Emphasis> in seeds using a specific TaqMan probe

Xanthomonas oryzae pv. oryzae is the pathogen that causes bacterial leaf blight in rice. Bacterial leaf blight is the main cause for severe rice underproduction in many countries. However, with conventional methods it is difficult to quickly and reliably distinguish this pathogen from other closely related pathogenic bacteria, especially X. oryzae pv. oryzicola, the causal organism of bacterial leaf streak in rice. We have developed a novel and highly sensitive real-time method for the identification of this specific bacteria based on a TaqMan probe. This probe is designed to recognize the sequence of a putative siderophore receptor gene cds specific to X. oryzae pv. oryzae, and can be identified from either a bacterial culture or naturally infected rice seeds and leaves in only 2 h. The sensitivity of the method is 100 times higher than that of the current polymerase chain reaction (PCR) gel electrophoresis method for diagnosis.     

Formulation of Bacteria for Biological Weed Control Using the Stabileze Method

Two plant pathogenic Pseudomonas spp. were formulated using the 'Stabileze' method which involves the incorporation of bacteria in a water-absorbent starch matrix with oil and sucrose, then granulating the matrix with hydrated silica. In one experiment, P. syringae pv. tabaci formulated with the standard Stabileze formula was evaluated for storage viability at -15, 2 and 22°C. Bacteria stored for 1 year at -15 and 2°C lost only 0.2 and 0.5 log 10 colony forming units (CFU) g -1 respectively compared to a loss of log 10 3.5 CFU at 22°C. In a second experiment, the same pathogen was evaluated using variations of the formula with and without oil, and with and without sucrose. P.s. pv. tabaci formulated with sucrose and oil in combination, and sucrose and oil alone survived better than the formulation without oil or sucrose. A third experiment tested the effect of four levels of oil and four levels of sucrose (4 ×4 factorial) on survival of P.s. pv. tagetis over a 28 month period. Sucrose alone enhanced survival more than oil alone, and the beneficial effect of the sucrose was reduced when it was combined with oil. These experiments suggest that the Stabileze protocol is effective for stabilizing bacteria, but there are differences in response to different formulation components between species of bacteria.     

Pseudomonas syringae pv. syringae B728a hydrolyses indole-3-acetonitrile to the plant hormone indole-3-acetic acid

Nitrilase enzymes catalyse the hydrolysis of nitrile compounds to the corresponding carboxylic acid and ammonia, and have been identified in plants, bacteria and fungi. There is mounting evidence to support a role for nitrilases in plant–microbe interactions, but the activity of these enzymes in plant pathogenic bacteria remains unexplored. The genomes of the plant pathogenic bacteria Pseudomonas syringae pv. syringae B728a and Pseudomonas syringae pv. tomato DC3000 contain nitrilase genes with high similarity to characterized bacterial arylacetonitrilases. In this study, we show that the nitrilase of P. syringae pv. syringae B728a is an arylacetonitrilase, which is capable of hydrolysing indole-3-acetonitrile to the plant hormone indole-3-acetic acid, and allows P. syringae pv. syringae B728a to use indole-3-acetonitrile as a nitrogen source. This enzyme may represent an additional mechanism for indole-3-acetic acid biosynthesis by P. syringae pv. syringae B728a, or may be used to degrade and assimilate aldoximes and nitriles produced during plant secondary metabolism. Nitrilase activity was not detected in P. syringae pv. tomato DC3000, despite the presence of a homologous nitrilase gene. This raises the interesting question of why nitrilase activity has been retained in P. syringae pv. syringae B728a and not in P. syringae pv. tomato DC3000.     

Construction and Use of a Nonradioactive DNA Hybridization Probe for Detection of Pseudomonas syringae pv. Tomato on Tomato Plants

Pseudomonas syringae pv. tomato, the causal agent for bacterial speck of tomato, produces the phytotoxin coronatine. A 5.3-kilobase XhoI fragment from the chromosomal region controlling toxin production was cloned into the plasmid pGB2, and the resulting recombinant plasmid, pTPR1, was tested for its ability to serve as a diagnostic probe for P. syringae pv. tomato. In a survey of 75 plant-associated bacteria, pTPR1 hybridized exclusively to those strains that produced coronatine. The detection limit for this probe, which was labeled with the Chemiprobe nonradioactive reporter system, was approximately 4 × 10³ CFU of lesion bacteria. During the 1989 growing season, a total of 258 leaf and fruit lesions from nine tomato fields were screened for P. syringae pv. tomato by using pTPR1 and the culture method of detection. The best agreement between the two methods, 90%, occurred early in the season with samples taken from relatively young (5-week-old) plants. Young plants also had a higher percentage of P. syringae pv. tomato-positive lesions. P. syringae pv. tomato was the only coronatine producer recovered from the nine tomato fields. All 244 P. syringae pv. tomato strains isolated during this study reacted strongly with the probe. The P. syringae pv. tomato population of healthy field tomato leaves was determined by a pTPR1 colony hybridization procedure. Every probe-positive colony that was isolated and characterized was identified as P. syringae pv. tomato. The pTPR1 probe should expedite disease diagnosis and facilitate epidemiological studies of this pathogen. It also should aid in screening transplant seedlings for bacterial speck infestation.     

Histopathology of Apple Proliferation in Malus Taxa and Hybrids of Different Susceptibility

Malus taxa and hybrids (“taxa”) grafted with M. pumila cv.‘Golden Delicious’differ significantly in their susceptibility to apple proliferation which is caused by a mycoplasma-like organism (MLO). These differences are correlated with the severity of anatomical aberrations and the numbers of MLOs in the phloem. The roots of declining trees of highly susceptible taxa with a mortality of more than 50 % are characterized by extensive phloem necrosis and the depletion of starch. MLOs are either not detectable or are present in low numbers, or the population appears degenerate when viewed by fluorescence microscopy. In comparable trees of a hybrid of M. sieboldii×M. pumila which shows a high recovery rate, both phloem necrosis and starch depletion are less pronounced, and the MLO numbers are low or the organisms are not detectable. Decline-tolerant taxa such as M. silvestris or M. pumila×M. baccata are little affected. Their phloem conditions and starch content do not differ significantly from that of healthy trees. However, the MLO titer is high. The histopathology of the scion cultivar of all groups examined is rather similar to that of the roots of the decline-tolerant taxa. Only in a late stage of decline, phloem necrosis increases while starch content and MLO numbers decrease in the scions grafted onto highly susceptible stockls.     

Suppression of bacterial blight by a bacterial community isolated from the guttation fluids of anthuriums

Growth and survival of Xanthomonas campestris pv. dieffenbachiae in guttation fluids (xylem sap exuded from leaf margins) of anthuriums were suppressed by several bacterial strains indigenous to leaves of various anthurium cultivars. Inhibition of growth was not observed in filter-sterilized guttation fluids and was restored to original levels only by reintroducing specific mixtures of bacteria into filter-sterilized guttation fluids. The inhibitory effect was related to the species in the bacterial community rather than to the total numbers of bacteria in the guttation fluids. One very effective bacterial community consisted of five species isolated from inhibitory guttation fluids of two susceptible anthurium cultivars. The individual strains in this community had no effect on the pathogen, but the mixture was inhibitory to X. campestris pv. dieffenbachiae in guttation fluids. The populations of the individual strains remained near the initial inoculum levels for at least 14 days. The effect of the five inhibitory strains on reducing disease in susceptible anthurium plants was tested by using a bioluminescent strain of X. campestris pv. dieffenbachiae to monitor the progression of disease in leaves nondestructively. Invasion of the pathogen through hydathodes at leaf margins was reduced by applying the strain mixture to the leaves. When the strain mixture was applied directly to wounds created on the leaf margins, the pathogen failed to invade through the wounds. This bacterial community has potential for biological control of anthurium blight.     

Differences between Pseudomonas syringae pv. syringae B728a and Pantoea agglomerans BRT98 in epiphytic and endophytic colonization of leaves

The leaf colonization strategies of two bacterial strains were investigated. The foliar pathogen Pseudomonas syringae pv. syringae strain B728a and the nonpathogen Pantoea agglomerans strain BRT98 were marked with a green fluorescent protein, and surface (epiphytic) and subsurface (endophytic) sites of bean and maize leaves in the laboratory and the field were monitored to see if populations of these strains developed. The populations were monitored using both fluorescence microscopy and counts of culturable cells recovered from nonsterilized and surface-sterilized leaves. The P. agglomerans strain exclusively colonized epiphytic sites on the two plant species. Under favorable conditions, the P. agglomerans strain formed aggregates that often extended over multiple epidermal cells. The P. syringae pv. syringae strain established epiphytic and endophytic populations on asymptomatic leaves of the two plant species in the field, with most of the P. syringae pv. syringae B728a cells remaining in epiphytic sites of the maize leaves and an increasing number occupying endophytic sites of the bean leaves in the 15-day monitoring period. The epiphytic P. syringae pv. syringae B728a populations appeared to originate primarily from multiplication in surface sites rather than from the movement of cells from subsurface to surface sites. The endophytic P. syringae pv. syringae B728a populations appeared to originate primarily from inward movement through the stomata, with higher levels of multiplication occurring in bean than in maize. A rainstorm involving a high raindrop momentum was associated with rapid growth of the P. agglomerans strain on both plant species and with rapid growth of both the epiphytic and endophytic populations of the P. syringae pv. syringae strain on bean but not with growth of the P. syringae pv. syringae strain on maize. These results demonstrate that the two bacterial strains employed distinct colonization strategies and that the epiphytic and endophytic population dynamics of the pathogenic P. syringae pv. syringae strain were dependent on the plant species, whereas those of the nonpathogenic P. agglomerans strain were not.