Control Of Fire Blight Research Articles

Effect of Prohexadione-Calcium Dose Level on Shoot Growth and Fire Blight in Young Apple Trees

Prohexadione-calcium suppresses both shoot growth and fire blight in apple. In young apple orchards, there are conflicting requirements to control fire blight and allow sufficient tree growth for tree establishment. Application of prohexadione-calcium to various cultivars of orchard-grown apple trees ranging in age from newly planted to fifth-leaf trees indicated that fewer high-dose (125 or 250 mg ·liter-1) applications of prohexadione-calcium provided a better balance between fire blight control and growth in young orchards than multiple low-dose (30 or 63 mg·liter-1) applications. The response of early-season shoot growth to prohexadione-calcium treatment dose was linear. However, trees that received high doses of prohexadione-calcium tended to grow more in the latter part of the season, resulting in little or no difference in total seasonal growth between trees that received a few high or multiple low doses of prohexadione-calcium. Enhancement of fire blight resistance by prohexadione-calcium was correlated with shoot growth suppression at the time of inoculation, and the resistance response to prohexadione-calcium treatment dose was linear. Fire blight management strategies that use prohexadione-calcium in young apple orchards are discussed.

Biological control of fire blight in pear orchards with a formulation of Pantoea agglomerans strain Eh 24

Biological control by using epiphytic bacteria against Erwinia amylovora has been considered as an alternative method for controlling the disease. Talc-based formulation of Pantoea agglomerans strain Eh-24 was applied at 30% and 100% bloom on two pear orchards which were selected from different locations in the Aegean Region in Turkey. Pear orchard trials were replicated for two years (1999 and 2000) in each place. Talc-based formulation of P. agglomerans strain Eh-24 was sprayed on pear trees which were naturally infected with E. amylovora. In the orchard trials conducted in 1999 and 2000, talc-based formulation of P. agglomerans strain Eh-24 reduced the percentage of blighted blossoms on pear orchards by 63% to 76%, approximately. Copper oxychloride+maneb was less effective in reducing the incidence of blossom infection by E. amylovora in each pear orchard than the bioformulation treatment. P. agglomerans strain Eh-24 labelled with StrR+ was applied at 30% and 100% bloom to monitor the colonization and population dynamics of P. agglomerans on pear blossoms. The population size of P. agglomerans strain Eh-24 strR+ on pear blossoms increased from 2x104 to 1.3x106 cfu per blossom over 18 days.

PRESENT STATUS OF ORGANIC FRUIT GROWING IN EUROPE

Organic fruit growing in Europe has experienced remarkable growth rates since the mid 1990's. Southern states, especially Italy, Spain and France have the largest land area with organic fruit, are also growing olives, citrus and chestnuts. Mainly increasing interest of supermarket chains is responsible for this buoyancy, but also the availability of better plant protection products e.g. granulosis virus and mating disruption against codling moth, and Neem oil against Rosy Apple Aphid. State subsidies varying from 600 to more than 1600 Euro /ha/y in the EU-countries (15) are less decisive for the conversion of top fruit production. Market share of organic table fruit is only 1 to 2 %, reaching 4 to 5 % in Switzerland. For Switzerland, we estimate a market potential of around 12 to 15 %, which is already achieved with organic vegetables. In order to reach that percentage, better solutions for several key problems have to be found, e.g. control of scab, fire blight, sooty blotch, brown rot, weed management, fertilisation and crop load regulation. Also the assortment of organically produced “modern-standard” varieties is not satisfactory, in particular with stone fruit. The economics of organic fruit growing is comparatively healthy, however, it depends on receiving a one third higher farm gate price for the product. In Switzerland the benefit of organic orchards is 16 % higher compared to integrated fruit production; but labour hours exceed those of IFP by 7%, due to blossom thinning by hand, manual weed control and mice control. Supermarkets have a tendency to just “substitute” conventional with organic fruit if requiring organic fruit from disease susceptible varieties with no cosmetic blemishes. This can/does feed back to the growers resulting in “substitutional” production with disease and pest sensitive orchards managed with intensive “organic” spray and fertilisation programs. This certainly does not correspond with either the original concept of organic farming or with expectations of organic consumers. Thus, still a lot of development - also on the marketing side - has to be undertaken.

Efficacy of alternatives to antibiotic chemicals for the control of fire blight of pears

SummaryThe efficacy of various chemicals as alternatives to antibiotics for the control of fireblight (Erwinia amylovora) on pear trees was tested. The chemicals were applied in two ways. In 1999 and 2000, preselected pear twigs (80–90% bloom stage) were sprayed once either preventively 1 day before inoculation or curatively one or three days after artificial inoculation with pathogen concentrations of 105 and 107cfu ml−1. In 2000 and 2001, whole trees were sprayed 2 and 4 days before artificial inoculation of the flowers. From the incidence of diseased flowers it appeared that Bion (50% benzothiadiazole) at 0.2 g litre−1 H2O and Aliette (80% fosetyl‐Al) at 2.5 g litre−1 H2O showed considerable preventive action by eliciting systemic acquired resistance mostly when they were applied in the whole trees. However the best control was achieved with the antibiotic Agrept (20% streptomycin) at 0.5 g litre−1 H2O. This showed both preventive and curative action. Kocide (77% copper hydroxide) at 0.9 g litre−1 H2O, Dentamet (citric acid in chelate) at 1.5 ml litre−1 H2O, Bactosan (an extract from the plant Pongamia pinnatd) at 3.0 g litre−1 H2O and Bion at 0.1 g litre−1 H2O, showed preventive action, but only when the inoculum concentration was low.

Fire Blight Management in the Twenty-first Century: Using New Technologies that Enhance Host Resistance in Apple.

Fire blight has been known as a destructive disease of apple and pear for over 200 years (3). The disease is caused by the bacterium Erwinia amylovora, which is capable of infecting blossoms, fruits, vegetative shoots, woody tissues, and rootstock crowns (Fig. 1). There are several distinct phases of the disease including blossom blight, shoot blight, and rootstock blight. The diversity of host tissues susceptible to infection, combined with the limited number of management tools available to control the disease, has made it difficult to stop or slow the progress of fire blight epidemics. Effective management of fire blight requires an integrated approach of several practices that are aimed at (i) reducing the amount of inoculum that is available to initiate new infections, (ii) imposing barriers to successful establishment of the pathogen on the host, and (iii) reducing host susceptibility to infection (1,55). Most fire blight management strategies developed during the twentieth century focused on the reduction of inoculum in the orchard and the use of antimicrobial treatments to prevent infection. Although increasing host resistance has been recognized as an important component of fire blight management, its application has been limited by a lack of resistant cultivars suited to commercial needs and by a lack of management practices that could effectively increase resistance. Recent advances have made it feasible to change this paradigm in the twenty-first century. First, apple rootstock breeding programs have developed size-controlling (often dwarfing) rootstocks that are resistant to fire blight and are currently becoming available for commercial use (43). Second, genetic engineering of commercial apple cultivars for increased fire blight resistance has been demonstrated, and transgenic apple plants are now undergoing field trials (2). Third, chemical treatments that enhance host resistance have been demonstrated to be useful in the control of fire blight (9,33,61). Although these technologies are at the early stages of development and are either not available or not proven in the marketplace, incorporating the use of host resistance into fire blight management strategies has become a realistic goal in the twenty-first century. This article describes recent progress in the development of new fire blight control technologies that enhance host resistance by chemical or genetic means.

Changes in the Sensitivity of Erwinia amylovora Populations to Streptomycin and Oxolinic Acid in Israel.

A survey of streptomycin resistance in the fire blight pathogen, Erwinia amylovora, conducted in pear, apple, and quince orchards in Israel during 1998 to 2001 revealed a decrease in the frequency of locations with streptomycin-resistant strains, from 57% in 1998 to 15% in 2001. In 2001, streptomycin-resistant strains were detected in only five locations in two restricted areas in western Galilee and the Golan Heights, compared with 16 locations found in 1998 throughout the northern part of the country. Since the use of streptomycin for fire blight control was terminated in 1997, this antibiotic has been replaced with oxolinic acid (Starner) in commercial orchards. Strains resistant to oxolinic acid were isolated from two pear orchards in the northern part of Israel in 1999. In a nationwide survey conducted during the spring and winter of 2000 and 2001, 51 and 47 pome fruit orchards, respectively, were sampled. Oxolinic acid-resistant strains were detected in several orchards located in two restricted areas in northern Galilee. Strains with resistance to both streptomycin and oxolinic acid were not found during 2000 to 2001. Results of this survey are used in managing fire blight with bactericides.

Evaluation of Local and Imported Fire Blight Warning Systems in Israel

ABSTRACT The possibility of using local and imported warning systems for the management of fire blight (caused by the bacterium Erwinia amylovora) in pears was tested in Israel from 1997 to 2000. Three imported systems (MARYBLYT 4.3, BIS95, and Cougarblight 98C) and one local system (Fire Blight Control Advisory [FBCA]) were used. All systems were tested in simulation experiments; MARYBLYT 4.3 and FBCA were also tested in orchard experiments under natural infections. Simulation experiments included 193 orchard-plots in which the time of disease onset enabled us to determine the date of infection. Thirty-five experiments were conducted in commercial orchards; in 10 of these, fire blight developed naturally. The performance of the imported warning systems was too variable to be accurately used under Israeli conditions. In the simulation experiments, the success rate (i.e., the capacity of the systems to predict the exact date of the occurrence of infection episodes) of the imported systems was low (3 to 55%) with considerably large variability among years (CV = 30 to 67%). Similar results were obtained in the orchard experiments for MARYBLYT 4.3: in only two of five experiments where plots were managed according to that system was disease severity significantly lower than that recorded in untreated control plots. In comparison, the local system, FBCA, predicted most infection episodes in the simulation experiments with low variability (99%, CV = 1.0%). In the orchard experiments, adequate disease suppression was achieved in all eight experiments in which FBCA recommendations were followed. We concluded that it was not possible to import and successfully implement fire blight warning systems in Israel that have been developed in regions with dissimilar environmental conditions.

Antibiosis Contributes to Biological Control of Fire Blight by Pantoea agglomerans Strain Eh252 in Orchards

ABSTRACT Fire blight, caused by Erwinia amylovora, is the most serious bacterial disease of pear and apple trees. Biological control with strains of Pantoea agglomerans (syn. Erwinia herbicola) may provide an effective disease management strategy for fire blight. Most strains of P. agglomerans evaluated for suppression of fire blight produce compounds that inhibit the growth of E. amylovora in culture. The role of these inhibitory compounds in fire blight suppression in orchard environments has not been studied. In seven field trials in Oregon, we compared the population dynamics and disease suppression with P. agglomerans Eh252, a strain that produces a single antibiotic, with its near-isogenic antibiotic-deficient derivative, strain 10:12. Water or suspensions of Eh252 or 10:12 (1 x 10(8) CFU/ml) were applied at 30 and 70% bloom to pear or apple trees. Aqueous suspensions of freeze-dried cells of E. amylovora (3 x 10(5) CFU/ml) were applied at full bloom. Additional trees were treated with streptomycin or oxytetracycline at 30 and 70% bloom and in some experiments, 1 day after application of the pathogen. Population sizes of Eh252 or 10:12 on pear blossoms were estimated by spreading dilutions of blossom washes on culture media. Average population sizes of Eh252 and 10:12 on blossoms ranged from 10(5) to 10(7) CFU, and in five of six trials, the relative area under the population curve of Eh252 was not significantly different than that of its derivative 10:12. Both Eh252 and 10:12 reduced the growth of the pathogen on blossoms compared with inoculated water-treated controls. Eh252 significantly decreased the incidence of fire blight in six of seven field trials compared with the incidence on water-treated trees, and 10:12 similarly reduced the incidence of fire blight in four of seven trials. In three of seven field trials, trees treated with Eh252 had a significantly lower incidence of fire blight compared with trees treated 3 with 10:12. Overall,3 Eh252 reduced the incidence of fire blight by 55 +/- 8%, 10:12 by 30 +/- 6%, streptomycin by 75 +/- 4%, and oxytetracycline by 16 +/- 14%. The effectiveness of strain 10:12 compared with water treatment indicates that other mechanisms (e.g., competitive exclusion or habitat modification) also contribute to disease suppression by P. agglomerans. The increased suppression of fire blight by the parental strain Eh252 compared with the antibiotic-deficient mutant 10:12 indicates that antibiosis is an important mechanism of biological control of fire blight.

Effect of Treating Apple Trees with Acibenzolar-S-Methyl on Fire Blight and Expression of Pathogenesis-Related Protein Genes.

Acibenzolar-S-methyl (ASM, Actigard 50 WG), a synthetic inducer of systemic acquired resistance (SAR) and pathogenesis-related (PR) proteins, was evaluated for the control of fire blight on apple trees in the field and for PR protein gene expression in apple seedlings in the greenhouse. Expression of putative genes related to SAR induction was elevated in ASM-treated apple seedlings 2 to 7 days after treatment; levels of PR-1 and PR-8 RNA were increased 10-fold and PR-2 RNA was increased 100-fold in ASM-treated seedlings over levels in untreated seedlings. Spurs and shoots on Jonathan trees sprayed with ASM at 75 mg a.i./liter at pink and at weekly intervals thereafter had significantly less fire blight than untreated trees, both from natural infection and following artificial inoculation with the pathogen. Less fire blight was observed on trees sprayed weekly with ASM than on trees sprayed biweekly. The severity of fire blight on inoculated shoots of Fuji apple decreased with increasing rates of ASM (0 to 300 mg a.i./liter), and ASM combined with streptomycin showed enhanced activity over ASM at 75 mg a.i./liter alone. In general, ASM was not superior to streptomycin for fire blight control, but integrating a weekly schedule of ASM, preferably at 150 mg a.i./liter, with a schedule of streptomycin designed for blossom blight control appears promising for overall improvement in fire blight control.

Biological Control Agents for Fire Blight of Apple Compared Under Conditions Limiting Natural Dispersal.

The efficacy of Pantoea agglomerans strain E325 for control of fire blight of apple was determined in comparative field trials involving other bacterial antagonists. Concurrently, the importance of the natural dispersal of bacteria as a complicating factor was assessed. Tests were performed under two sets of conditions, those that allowed for the dispersal of bacteria via honeybee activity and those that minimized it through the use of translucent polyethylene enclosures around single trees. The enclosures also raised daytime temperatures and allowed for controlled wetting, two factors important to the development of blossom blight. Singleantagonist treatments with strain E325, Pseudomonas fluorescens strain A506, and P. agglomerans strain C9-1 were applied to open blossoms on each of 10 enclosed trees and 10 nonenclosed trees. During bloom, suspensions of antagonists (108 CFU/ml) were applied twice with a brush, and a suspension of Erwinia amylovora (107 CFU/ml) was subsequently applied once using the same method. Two days after inoculation with the pathogen, trees were misted to simulate precipitation. Flower-to-flower spread of antagonistic bacteria was less frequent on trees surrounded by plastic enclosures than on nonenclosed trees. The range and statistical separation of means for population size of E. amylovora and disease incidence among treatments were greater for enclosed trees than for nonenclosed trees. Based on these results, the natural spread of antagonists being compared may mask differences in their efficacy as biocontrol agents. Such distortions and resulting misinterpretations could be lessened by separating treatments widely in large orchard blocks and by monitoring microbial populations. Strain E325 from fresh or lyophilized cultures was consistently more effective than standard antagonists in suppressing E. amylovora and reducing disease incidence. To fully assess its potential use for fire blight, larger-scale trials under various conditions will be necessary.

Evaluation of Selected Bactericides for the Control of Fire Blight on Crabapple

Abstract Agrimycin 17 21.2W (streptomycin sulfate), Aliette WDG (fosetyl-Al), Kocide 101 77W (copper hydroxide), Phyton 27 (copper sulfate pentahydrate), and the experimental fungicide fluazinam 500F were evaluated for the control of fire blight (Erwinia amylovora) on field-grown crabapple. A block of bare-root flowering crabapple (Malus sp.) ‘Snowdrift’ was established in 1995, and the study was conducted in 1996, 1997, and 1998 at the Brewton Experiment Field in Brewton, AL (USDA Plant Hardiness Zone 8a). In all three years, streptomycin sulfate was the only product that significantly reduced the severity of fire blight. Typically, damage on the Agrimycin 17-treated trees was unobtrusive and restricted to the death of a few scattered bloom clusters (strikes). In two of three years, fire blight damage levels on the Kocide 101-treated trees were intermediate between those sprayed with streptomycin sulfate and the unsprayed control. Although intermediate disease control was also obtained in 1996 with Phyton 27, this fungicide/bactericide failed to significantly reduce disease severity in 1997 and 1998 as compared to the unsprayed control. In all three years, no significant reduction in the severity of fire blight was obtained with Aliette WDG or fluazinam 500F.

Induction of resistance to fire blight in apple (Erwinia amylovora)

Apple genotypes show different levels of resistance to fire blight caused by the bacterium E. amylovora. This resistance is considered to be polygenic, thus relying on a number of defense mechanisms. On the assumption that in susceptible genotypes these defenses may be present but not activated during the infection process, we attempted their induction by various abiotic and biotic elicitors. Two kinds of experiments were performed in the greenhouse and in the orchard: i) evaluation after controlled inoculation of the level of protection conferred to the plant by each elicitor and ii) investigation of various defense responses potentially involved in the protection. Results showed that some elicitors provided a level of overall protection of about 50% in the orchard while higher levels may be obtained on seedlings in the greenhouse. The comparison of some defense responses in protected and unprotected treated plants showed that i) elicitation of defense is not necessarily associated with control of fire blight and that ii) some responses at least may be associated with the observed protection.

Field Performance of Antagonistic Bacteria Identified in a Novel Laboratory Assay for Biological Control of Fire Blight of Pear

Abstract A laboratory assay to identify antagonists of Erwinia amylovora on flowers was tested for its accuracy in predicting the performance of antagonists in controlling fire blight disease of pear trees. Bacterial strains isolated from pear flowers were evaluated in the laboratory for their ability to suppress growth of E. amylovora on flowers on cut branches. The growth of E. amylovora was assessed with a flower-freezing assay using a recombinant strain of the pathogen exhibiting an ice-nucleating phenotype. A number of antagonistic and nonantagonistic bacterial strains selected by this method were tested for their control of flower infection by E. amylovora on trees in the greenhouse and the field. There was a significant ( P ≤ 0.01) relationship between the ability of antagonists to suppress growth of E. amylovora in the flower-freezing assay and their ability to control fire blight on trees. Some of the strains identified conferred similar or better disease control than the biocontrol agent Pseudomonas fluorescens A506. None of the strains produced antibiotics inhibitory to E. amylovora on the culture media tested. All antagonistic and nonantagonistic strains survived well on pear flowers on trees and reached population sizes of approximately 10 5 to 10 6 cells/flower 3 days after their application. The use of this flower-freezing assay has promise in the rapid identification of biocontrol agents of fire blight.

Prohexadione-Ca (Apogee®): Growth Regulation and Reduced Fire Blight Incidence in Pear

Prohexadione-Ca (Apogee®) was tested as a growth retardant and fire-blight control agent in the pear (Pyrus communis L. cv. Abbé Fétel) on both bearing trees in the orchard and on 1-year-old scions under greenhouse conditions. Four sprays of 50 and 100 mg·L-1 of the chemical were applied to trees in the orchard at 2-week intervals starting at petal fall, when terminal growth was 4 cm (mid-April). Scions received a single application (250 mg·L-1) and were transferred 2 weeks later to a greenhouse where the shoots were inoculated with a local, virulent strain of Erwinia amylovora (Burrill) Winslow et al. In the orchard, the higher prohexadione-Ca concentration was more effective in reducing shoot growth, enhancing fruit weight and controlling fire blight incidence and severity. Similar effects on growth parameters and disease progression were observed under greenhouse conditions. Chemical name used: calcium 3-oxido-4-propionyl-5-oxo-3-cyclohexene carboxylate (prohexadione-Ca)

Characterization of a Viral EPS-Depolymerase, a Potential Tool for Control of Fire Blight

ABSTRACT A 3.3-kb fragment of genomic DNA from bacteriophage Phi-Ea1h encoding an amylovoran-directed depolymerase lyase was sequenced, and three open reading frames (ORFs) were detected. The first ORF could encode a lysozyme and the second a holin that may form a pore supporting cell lysis by the lysozyme. The third ORF encodes a protein of 657 amino acids and deletion mutation in this DNA fragment abolished extracellular polysaccharide (EPS)-degrading activity. A putative promoter and a ribosome binding sequence were located in front of the gene. A polymerase chain reaction product spanning the gene was inserted into multi copy plasmids including fusions with a Histidine-tagged sequence to facilitate its purification on a nickel nitrilotriacetic acid column. Maximal activity of the purified protein was observed between pH 4 and 5 at 52 degrees C. Visualized by staining with fluorescein isothiocyanate-labeled lectin from Abrus precatorious, the enzyme degraded the EPS-capsules of Erwina amylovora. In virulence assays, no symptoms were detected for a low inoculum of an E. amylovora strain when pear slices were soaked in a solution of depolymerase in contrast to control slices without addition of the enzyme. Furthermore, gfp- or lux-labeled E. amylovora cells were not propagated, when their amylovoran capsules were removed by the depolymerase. The enzyme could be a tool for biological control of fire blight.

Acibenzolar-S-methyl induces the accumulation of defense-related enzymes in apple and protects from fire blight

Acibenzolar-S-methyl (Novartis) is a chemical inducer of systemic acquired resistance in several annual plants. The ability of this novel chemical to induce resistance was studied in a perennial plant (apple) affected by fire blight caused by Erwinia amylovora. Acibenzolar-S-methyl (100 and 200 mg/l active ingredient) protected Golden Delicious seedlings, scions and trees from artificial infection when applied before inoculation. The protection of apple seedlings was similar to the protection obtained with the standard for fire blight control, plantomycin (100 mg/l streptomycin sulfate), applied immediately before inoculation. The mean levels of control in scions in the greenhouse and in trees in orchards were approximately 69% and 50%, respectively. The protection of apple seedlings was constantly associated with the activation of two families of defense-related enzymes, peroxidases and β-1,3-glucanases. Accumulation of both enzymes was induced locally in treated leaves and systemically, especially for β-1,3-glucanases, in upper untreated leaves, and was sustained for at least 17 days. These results suggest that acibenzolar-S-methyl promotes induced systemic resistance in apple by increasing defense-related compounds. This chemical could provide a new approach of control of fire blight but its practical use needs further investigation.

Interference of Copper Sulphate in Growth of Erwinia amylovora

AbstractTo understand the toxicity of copper salts on Erwinia amylovora, which are used in the control of fire blight, bacterial growth and cell metabolism was assayed with copper sulphate in the presence or absence of complex‐forming compounds such as various amino acids or citrate. In minimal medium without amino acids copper sulphate strongly interfered with the growth of E. amylovora. A concentration of 15 μm CuSO4 resulted in about 50% growth inhibition. In contrast to a strong effect of streptomycin, copper ions barely killed the cells when incubated in minimal medium for 1 h. The addition of 4 g asparagine per litre relieved a‘bacteriostatic’effect of copper ions and allowed growth of the bacteria at 2 mm CuSO4. Other amino acids had a similar effect in the protection of E. amylovora against copper ions. This was in contrast to glycine betain, which was unable to suppress growth inhibition by CuSO4. Presumably, the free ammonium groups of amino acids participated in the protective effect. The addition of citrate, exceeding the amount of copper‐ions, was also protective. Bioluminescence of E. amylovora cells was expressed via a constitutive promoter from the lux‐operon of Vibrio fischeri. The light emission is dependent on active cell metabolism. In a novel approach to determine the immediate response of E. amylovora after the addition of copper sulphate, the change of bioluminescence was determined. Addition of copper ions to MM3 medium strongly affected the bioluminescence, but no change in light production was noticed, when citrate or asparagine were present in addition to copper sulphate. A decrease of bioluminescence to 50% was observed for 50 μm CuSO4 in the absence of amino acids.

Effect of Nectar on Microbial Antagonists Evaluated for Use in Control of Fire Blight of Pome Fruits

ABSTRACT Under warm, dry conditions, Erwinia amylovora can become established in relatively high populations on apple (Malus domestica) or pear (Pyrus communis) flower stigmas, and subsequent wet conditions facilitate its movement to the flower hypanthium where infection generally is initiated through the nectarthodes. Research on biological control of fire blight has focused mainly on the flower stigma, and knowledge is lacking regarding the effect of nectar on microbial antagonists in the flower hypanthium. The biocontrol agents Pseudomonas fluorescens strain A506 and Pantoea agglomerans strain C9-1 were cultured in a basal liquid medium with various concentrations (0 to 50% total sugar) of sucrose or synthetic nectar (sucrose/glucose/fructose, 2:1:1). Strain A506 showed less growth and lower survival than strain C9-1 at high sugar levels, and A506 was less effective than C9-1 as a preemptive antagonist of E. amylovora in high-sugar media. Both antagonist strains were less tolerant to high sugar levels than E. amylovora (strain Ea153). The same bacteria were cultured in a medium with 25% total sugar consisting of various proportions of sucrose, glucose, and fructose, and growth response correlated strongly with solute potential. When 28 microbial strains were cultured in synthetic nectar (25% total sugar) and ranked based on growth, strains clustered according to taxonomic group. Yeasts were most osmotolerant, followed by strains of E. amylovora, Pantoea agglomerans, Bacillus spp., and Pseudomonas spp. Further studies done in planta are necessary to determine whether osmotolerance of antagonists is advantageous in the biological control of fire blight.

Inhibition of Monilinia oxycocci by bacteria isolated from a cranberry marsh

Bacteria inhibitory to Moniliniaoxycocci, the cranberry cotton ball pathogen, were identified. Eighty-three bacteria isolated from a cranberry marsh and Erwinia herbicola C9-1,which is being developed elsewhere for the biological control of fire blight of pome fruits, were tested fortheir ability to inhibit radial growth and conidial germination of M. oxycocci in vitro. IsolateBA35 from cranberry (tentatively identified as Pseudomonas syringae pv. morsprunorum) and E. herbicola C9-1 completely inhibited radial growth of M. oxycocci. BA35 and C9-1 were among the most effective of 21 bacteria tested for inhibition of conidial germination. Growth of bacteria in sclerotial extracts of M. oxycocci and Sclerotinia sclerotiorum was determined in order to select isolates that might be successful in degradingsclerotia, thereby reducing the viability of M.oxycocci. Populations of isolates 62 and S-18(identities unknown), and S-19 (tentatively identified as Micrococcus luteus), increased approximately3.0--5.0 log10 units in all extracts within 24 h. Populations of isolate S-10 increased by about 3.5log10 units in all sclerotial extracts within 48 h. Populations of isolate S-49 (identity unknown)were lower in extracts of M. oxycocci than S. sclerotiorum after 48-h. In buffer controls, bacterial populations remained stable or decreased over the 48-h period.

Distribution of streptomycin-resistant strainsof Erwinia amylovora in Israel and occurrence of blossom blight in the autumn

Following failure in control of fire blight with streptomycin, the distribution of streptomycin-resistant strains ofErwinia amylovora in Israel was surveyed. During 1994–1997 109 pear, apple, loquat and quince orchards were monitored. Streptomycin-resistant strains ofE. amylovora were recovered from flowers and from infected branches collected at 18 locations in the Sharon, Galilee and Golan Heights regions. In the Sharon region all the isolated strains ofE. amylovora were streptomycin-resistant, whereas in the Galilee and Golan Heights, resistant as well as sensitiveE. amylovora strains were recovered at different locations. In the southern coastal plain no resistance could be detected. Streptomycin-resistant strains ofE. amylovora did not hybridize with the DNA probe SMP3, and resistance could not be transferred by mating to a sensitive strain, suggesting that streptomycin resistance in Israel is not plasmid-mediated. Fire blight symptoms were observed, for the first time, on pear blossoms during the autumn of 1994. A high population of 2x 106-6x 107 CFU/flower in the autumn of 1995 and of 1996 was correlated with the appearance of blossom blight symptoms.