Incidence Of Fire Blight Research Articles

Specific Weather Factors Affecting the Incidence of Fire Blight in Korea from 2020 to 2023

Since its initial outbreak in Korea in 2015, fire blight has consistently emerged annually. Fire blight outbreaks usually begin in May, peak in June, and decline in July in Korea. In this study, we analyzed cases that exhibit a distinct pattern of disease occurrence based on yearly weather conditions from 2020 to 2023. In 2020, fire bight disease occurrence began in late May. Although the disease incidence started late by the low temperatures in April, which caused flowering period delayed, the incidence increased significantly due to the high risk of blossom infection. In 2021, the first outbreak began in late April because the flower infection started in early April. In 2022, despite the high blossom infection risk during the flowering period in April and the high incidence of fire blight in May, the incidence decreased sharply from June due to the low rainfall in May. In 2023, due to torrential rains and hail in late June, the incidence of fire blight increased even in July. Considering the weather factors that affect the increase of fire blight disease, it is suggested that control measures to prevent the fire blight infection should be carried out before and after wind-driven rains.

Emergence Characteristics of Fire Blight from 2019 to 2023 in Korea

Erwinia amylovora is a gram-negative plant pathogen that causes fire blight in apple and pear trees, resulting in significant damage worldwide. In this study, we monitored the emergence of fire blight from 2019 to 2023 to determine the emergence patterns and the factors affecting the outbreak of the disease. As a result of the 5-year survey on the emergence of fire blight, a total of 2,029 cases have emerged, mostly in apple trees of 1,378 cases (67.9%) followed by 645 cases (31.8%) in pear trees, and from quince, hawthorn, and mountain ash trees. Fire blight appeared in specific areas of Gyeonggi, Chungnam, Gangwon, and Chungbuk provinces in 2019, but spread to Andong and Yesan in 2021, Muju and Bonghwa in 2023. In 2020 and 2021, there were 744 and 618 cases of fire blight outbreaks, respectively, compared to other years (188–245 cases/year). Notably, 914 of these cases were observed in apple trees from May to July, with 667 cases reported in Chungju and Jecheon. The incidence of fire blight was positively correlated with the daily maximum temperatures and rainy days in January and February, as well as the rainy days in May and June. The average age of the diseased pear trees was 25 years, higher than the 10-year average age of the apple trees. This study provides fundamental information to understand the status and factors affecting the fire blight emergence in Korea. Prevention measures should be established through continuous analysis of the status of fire blight.

Contribution of Native Plasmids of Pantoea vagans C9-1 to Epiphytic Fitness and Fire Blight Management on Apple and Pear Flowers and Fruits.

Pantoea vagans C9-1 (C9-1) is a biological control bacterium that is applied to apple and pear trees during bloom for suppression of fire blight, caused by Erwinia amylovora. Strain C9-1 has three megaplasmids: pPag1, pPag2, and pPag3. Prior bioinformatic studies predicted these megaplasmids have a role in environmental fitness and/or biocontrol efficacy. Plasmid pPag3 is part of the large Pantoea plasmid (LPP-1) group that is present in all Pantoea spp. and has been hypothesized to contribute to environmental colonization and persistence, while pPag2 is less common. We assessed fitness of C9-1 derivatives cured of pPag2 and/or pPag3 on pear and apple flowers and fruit in experimental orchards. We also assessed the ability of a C9-1 derivative lacking pPag3 to reduce populations of E. amylovora on flowers and disease incidence. Previously, we determined that tolerance to stresses imposed in vitro was compromised in derivatives of C9-1 lacking pPag2 and/or pPag3; however, in this study, the loss of pPag2 and/or pPag3 did not consistently reduce the fitness of C9-1 on flowers in orchards. Over the summer, pPag3 contributed to survival of C9-1 on developing apple and pear fruit in two of five trials, whereas loss of pPag2 did not significantly affect survival of C9-1. We also found that loss of pPag3 did not affect C9-1's ability to reduce E. amylovora populations or fire blight incidence on apple flowers. Our findings partially support prior hypotheses that LPP-1 in Pantoea species contributes to persistence on plant surfaces but questions whether LPP-1 facilitates host colonization.

Relationships between the incidence of fire blight in apple orchards and plant nutritional imbalance indices

ABSTRACT Fire blight disease is a major threat to the apple orchards in various regions of the world. The multivariate compositional nutrient diagnosis (CND) was considered to evaluate the effect of plant nutrient imbalance in development of fire blight in ‘golden delicious’ apple orchards in Fars province, Iran. To establish CND norms and the fire blight severity index, 81 commercial orchards were used. The CND reference norms (V*x) for a low fire blight severity subpopulation were calculated as V*N = 2.813 ± 0.186, V*P = 0.297 ± 0.147, V*K = 2.660 ± 0.149, V*Ca = 2.442 ± 0.218, V*Mg = 1.188 ± 0.061, V*S = 0.595 ± 0.399, V*Fe = −0.952 ± 0.212, V*Mn = −3.132 ± 0.195, V*Zn = −4.040 ± 0.366, V*Cu = −5.032 ± 0.177, V*B = −3.361 ± 0.120, V*Rd = 6.521 ± 0.128. According to the calculated leaf nutrient indices based on the CND reference norms and principal component analysis (PCA), leaf Ca, S, B and Zn deficiency and leaf K and P excess significantly correlated with the enhancement of fire blight severity index (FBSI). The reduction of global nutrient imbalance index (CND-r2) to below 20.38 decreases the severity of fire blight in ‘golden delicious’ apple orchards.

Assessing and Minimizing the Development and Spread of Fire Blight Following Mechanical Thinning and Pruning in Apple Orchards.

The adoption of mechanical thinning and pruning in commercial apple orchards has been limited largely by the risk of development and spread of fire blight. This devastating disease, caused by the bacterial pathogen Erwinia amylovora, may be transmitted by mechanical injury such as pruning, especially under warm, moist conditions conducive to bacterial growth, infection, and disease development. However, risk may be mitigated by avoiding highest-risk times and applying a bactericide, such as streptomycin, after mechanical thinning or pruning. In 'Gala' and 'Idared' orchards, we evaluated the risk of fire blight development and spread after mechanical thinning early in bloom (20% bloom), when seasonal temperatures are cooler and there are few open flowers available for infection. In both orchards, we also evaluated the spread and development of fire blight by mechanical pruning in July and in August, before and after terminal bud set, when shoot growth is slowed and the tree is less susceptible to infection. We also assessed the potential efficacy of a streptomycin or Bacillus subtilis biopesticide application after mechanical thinning and pruning to mitigate the spread of fire blight. In the 'Gala' orchard, disease never developed beyond the inoculated tree after thinning or pruning, which was unexpected for this highly susceptible cultivar. In the 'Idared' orchard, incidence of blossom or shoot blight from the point source, represented as relative area under the disease progress curve, was rarely different for trees that underwent mechanical thinning or mechanical pruning compared with untreated trees, and it was often reduced or eliminated when the antibiotic streptomycin or the B. subtilis biopesticide was applied within 24 h of mechanical thinning or pruning. For both thinning and pruning, incidence of fire blight dropped off quickly beyond the inoculated tree in the 'Idared' orchard and generally was not observed in trees beyond 10 to 15 m from the inoculated point source or predicted beyond 10 m by exponential and power law models fit to the disease progress curves. The results of this work demonstrate the low risk for fire blight development and spread by mechanical thinning and pruning when practiced under low-risk conditions-early in bloom for mechanical thinning and after terminal bud set (in August) for mechanical pruning-especially when paired with a subsequent bactericide application. This study demonstrates the safe use of mechanical thinning and pruning in commercial apple production, corroborated by anecdotal evidence from apple growers in western New York State.

Evaluation of a contact sterilant as a niche-clearing method to enhance the colonization of apple flowers and efficacy of Aureobasidium pullulans in the biological control of fire blight

Due to rapid expansion of organic apple production and issues with antibiotic resistance in conventional production, there is a need for novel strategies to protect orchards from outbreaks of fire blight caused by the bacterial pathogen Erwinia amylovora. The biological control material, Blossom Protect (consisting of two strains of the fungus Aureobasidium pullulans), has shown promising results in some apple growing regions but has shown reduced efficacy in humid apple-growing regions such as the Midwestern and Eastern United States. In an attempt to increase both flower colonization by A. pullulans and disease control efficacy, we applied a contact sterilant to clear the flower niche of native microbial populations 4 hr prior to application of Blossom Protect. Results from four years of field testing showed that application of the contact sterilant resulted in a significant reduction in populations of bacteria, fungi, and native yeast populations; however, all of these microbial populations recovered after 24 h. Examination of fire blight incidence revealed that use of the contact sterilant did not significantly impact disease control. Two applications of Blossom Protect, at 70–80% bloom and full bloom, exhibited the best efficacy in disease control over four years. In addition, we observed marginal differences in the marketability of the final fruit between treatments, each with less than 8% deemed unmarketable due to russet. Our results indicate that niche-clearing of apple flowers did not enhance flower colonization by A. pullulans, nor produce an increase in biological control efficacy. However, our current experimental results (2014–2018) do indicate consistency and an increase in efficacy of Blossom Protect compared to results from previous years.

English

The bacterial antagonist Pantoea agglomerans P10c was evaluated for efficacy to control fire blight in Morocco. It was evaluated in the laboratory on detached blossoms (pre and post-inoculation of pathogen) and under field for its ability to suppress growth of Erwinia amylovora on blossoms of pear trees (Pyrus communis); and for its aptitude to establish on and to colonize the blossoms of apple tree (Malus domestica) under field conditions. Results revealed that the installation of P. agglomerans P10c strain on detached blossom before the inoculation with the E. amylovora (preventive treatment) allows an efficient control of the disease (96%), with an incidence that never exceed the level 2.5%. but for curative treatment the incidence increased to 40% and the efficiency was only about 32%. Under field, P. agglomerans P10c applied a twice (10 and 60% bloom time) at 108 CFU/ml reduced significantly the incidence of fire blight by 64.7 to 76.9%. The experiment of the establishment and the survivability of P. agglomerans P10c in the stigma showed a positive evolution which increased from 4.5×104 to 7.6×105 CFU/flower. Results from this work illustrate that this antagonistic strain is able to control efficiently fire blight in apple and pear trees under the conditions of the arias of Meknes. Keys words: Erwinia amylovora, Pantoea agglomerans P10c, antagonist, blossom, preventive treatment, apple tree, pear tree, Meknes.

Fire Blight Disease Caused by <i>Erwinia amylovora</i> on <i>Rosaceae</i> Plants in Kyrgyzstan and Biological Agents to Control This Disease

Erwinia amylovora species were isolated from the blossoms, exudates, infected fruits, leaves and bent branches of diseased apple, pear and hawthorn trees, selected in the Chy, Osh and Jalal Abad regions. Biochemical and pathogenicity tests, alongside PCR analyses, were conducted to identify the local isolates of Erwinia amylovora. The alternative antagonistic microorganisms which combat bacterium E. amylovora were tested within in vitro and in vivo conditions. The results revealed the ability of Streptomyces antagonistic bacteria to decrease fire blight severity on pear and apple trees during the first stage of the fire blight disease in leaf tissues. Streptomyces strain C1-4 suppressed E. amylovora disease symptoms in the leaf tissues and excised apple and pear shoots. The incidence of fire blight on leaves was reduced by about 70% with two applications of bacterial antagonists. Further studies at different locations in Kyrgyzstan, using large scale application, would allow for stronger recommendations to be made, including studies and recommendations on their ability to prevent disease and to use them as main components in an integrated pest management program.

Real‐time PCR detection of Erwinia amylovora on blossoms correlates with subsequent fire blight incidence

Fire blight is the most devastating bacterial disease of rosaceous plants. Forecasting fire blight infections is important to allow for countermeasures that reduce economic damage in pome fruit production. Current computerized forecasting models are solely based on physical factors such as temperature and moisture, but not on the actual presence of the pathogen Erwinia amylovora. Although the inoculum concentration is considered to be crucial for infection and disease outbreak, most current approaches used for identification of fire blight inoculum including morphological, biochemical, serological, and DNA‐based methods are nonquantitative. Based on a real‐time PCR approach previously published, an improved protocol to be used directly on whole bacteria in the field is described. The method allows for early detection and quantification of the pathogen prior to the occurrence of first symptoms. There is a clear correlation between bacterial abundance and subsequent disease development. However, in some cases, no disease symptoms could be observed despite a pathogen load of up to 3·4 × 106 cells per blossom. Integration of the amount of pathogen detected into refined prediction algorithms may allow for the improvement of applied forecasting models, finally permitting a better abatement of fire blight.

Evaluation of Strategies for Fire Blight Control in Organic Pome Fruit Without Antibiotics.

Apple and pear produced organically under the U.S. National Organic Program (NOP) standard can be treated with antibiotics for suppression of fire blight caused by Erwinia amylovora. Recent regulatory actions by the NOP, however, have lessened the likelihood of antibiotic use after the 2014 season. In response, western U.S. organic apple and pear stakeholders identified two immediate-need research objectives related to fire blight control: development of effective non-antibiotic control programs based on combinations of registered biological products; and, in apple, integration of these products with lime sulfur, which is sprayed at early bloom to reduce fruit load. In orchard trials in Oregon, increasing the frequency of treatment with biological products improved suppression of floral infection. In apple, fruit load thinning with 2% lime sulfur plus 2% fish oil (LS+FO) at 30 and 70% bloom significantly (P ≤ 0.05) reduced the proportion of blighted flower clusters in four of five orchard trials. Moreover, lime sulfur significantly (P ≤ 0.05) suppressed epiphytic populations of E. amylovora after their establishment on apple flowers. Over four trials, treatment with Aureobasidium pullulans (Blossom Protect) after LS+FO reduced the incidence of fire blight by an average of 92% compared with water only; this level of control was similar to treatment with streptomycin. In three seasons, a spray of a Pantoea agglomerans product after the 70% bloom treatment of LS+FO established the antagonist on a significantly (P ≤ 0.05) higher proportion of flowers compared with a spray of this bacterium before the thinning treatment. Consequently, in apple, biological treatments for fire blight control are not advised until after lime sulfur treatments for fruit load thinning are completed.

Evaluation of the Risk of Spreading Fire Blight in Apple Orchards with a Mechanical String Blossom Thinner

The risk of spreading fire blight in apples after mechanical thinning with a rotating string blossom thinner was evaluated in field and potted-tree experiments. In the field experiment, using the mechanical thinner on noninoculated trees immediately after operating the equipment on inoculated trees significantly (P < 0.01) increased fire blight incidence resulting in 90 ± 20.01 (mean ± SE) infected shoots compared with 23.5 ± 8.97 diseased shoots in similar trees that were not thinned mechanically. A similar result was obtained in greenhouse experiments whereby healthy apple plants positioned adjacent to diseased plants before the group was subjected to the mechanical thinner developed more than twice the number of infected shoots as that on similar plants that were not thinned. These results indicate that under conditions conducive to infection, the mechanical blossom thinner significantly increases the risk of spreading Erwinia amylovora. The use of the thinner should therefore be limited to orchards with no history of disease in the last 3 years and on days when predicted weather is not suitable for tree infection by E. amylovora; otherwise, a severe fire blight epidemic could develop in the orchard.

Implications of Pathogenesis by Erwinia amylovora on Rosaceous Stigmas to Biological Control of Fire Blight

As a prerequisite to infection of flowers, Erwinia amylovora grows epiphytically on stigmas, which provide a conducive habitat for bacterial growth. Stigmas also support growth of several other bacterial genera, which allows for biological control of fire blight; although, in practice, it is very difficult to exclude E. amylovora completely from this habitat. We investigated the dynamics of growth suppression of E. amylovora by comparing the ability of virulent and avirulent strains of E. amylovora to compete with each other on stigmas of pear, apple, and blackberry, and to compete with a co-inoculated mixture of effective bacterial antagonists. When strains were inoculated individually, virulent E. amylovora strain Ea153N attained the highest population size on stigmas, with population sizes that were approximately double those of an avirulent hrpL mutant of Ea153 or the bacterial antagonists. In competition experiments, growth of the avirulent derivative was suppressed by the antagonist mixture to a greater extent than the virulent strain. Unexpectedly, the virulent strain enhanced the population size of the antagonist mixture. Similarly, a small dose of virulent Ea153N added to inoculum of an avirulent hrpL mutant of Ea153 significantly increased the population size of the avirulent strain. A pathogenesis-gene reporter strain, Ea153 dspE::gfp, was applied to flowers and a subset of the population expressed the green fluorescent protein while growing epiphytically on stigmas of apple. These results are consistent with the hypothesis that virulent E. amylovora modifies the epiphytic habitat presented by the stigma through a pathogenesis-related process, which increases host resources available to itself and, coincidentally, to nonpathogenic competitors. Over nine orchard trials, avirulent Ea153 hrpL significantly suppressed the incidence of fire blight four times compared with six for the antagonist mixture. The degree of biological control achievable with an avirulent strain of E. amylovora likely is limited by its inability to utilize the stigmatic habitat to the same degree as a virulent strain.

Analysis of Fire Blight Shoot Infection Epidemics on Apple.

Fire blight incidence and spread of the shoot blight phase of the disease was studied in four apple cultivars in replicated blocks over 4 years (1994 to 1997). Cv. York was highly susceptible, followed by 'Fuji' and 'Golden Delicious,' which were moderately susceptible, and 'Liberty,' which was least susceptible. On York, the first appearance of shoot blight was within 48 h of its predicted appearance according to the Maryblyt model in 3 of the 4 years studied. Shoot blight epidemics in York in 1995 and 1996, and Fuji in 1995, were best described with a logistic model that showed apparent infection rates ranging from 0.05 to 0.20, indicating a low to moderately high rate of disease increase. The spatial positions (row and column) of all infected plants in each subplot were recorded on plot maps on each sampling date. The binomial and β-binomial distributions were fit to the data to test for spatial aggregation of disease incidence for each cultivar plot. Maximum likelihood estimation was possible for 92 (43.6%) of the 211 data sets subjected to this analysis. Of these, 35 data sets were better fit by the β-binomial distribution than the binomial distribution. The binary power law was used to characterize the relationship between the variance among quadrats within each plot to the variance expected for that plot given the observed level of disease incidence. The binary power law provided an excellent fit to the full data set and to nearly all of the subsets and, with b > 1, indicated that heterogeneity changed systematically with disease incidence. A covariance analysis was conducted to determine the effect of the factors 'year,' 'cultivar,' 'orchard plot,' and 'observation date' on the intercept and slope parameters of the binary power law. In general, plot followed by year had the greatest impact on parameter estimates and is an indication that location and seasonal factors impact heterogeneity of disease, although the specifics could not be ascertained from this study. Ordinary runs analysis was used to analyze the pattern of diseased trees within rows and detected significant nonrandom patterns of disease incidence in 63.5% of the orchard plots over the 4-year study. From these data sets, 68.7% had significantly fewer runs, particularly at disease incidences greater than 0.1. The fewer-than-expected runs at incidences greater than 0.10 provides strong evidence of localized spread.

FIRE BLIGHT INCIDENCE ON MALUS SIEVERSII GROWN IN NEW YORK AND MINNESOTA

Malus sieversii (Lebed.), a wild apple species native to Central Asia, has been recognized as the major progenitor of the domestic apple. This investigation summarizes the natural infection by Erwinia amylovora (fire blight) on 2590 M. sieversu seedlings grown as own-rooted trees. At a USDA orchard in Geneva, New York (NY), 1180 seedlings (106 populations) representing germplasm from eight sites in Kazakhstan were planted in 1997 and 1998. At a University of Minnesota (MN) orchard in Excelsior, MN, 1410 seedlings representing 124 populations from two climatically diverse sites in Kazakhstan were planted in 1998. Individuals in each population are half-sibs of each other from seeds collected from the same mother tree of wild M. sieversii. One of the Kazakh sites (site 9) selected for the MN trial was a mesic site at 47° N latitude chosen for its potential for extreme cold hardiness. The other group was collected from site 6, a relatively warm, xeric site. To compare results, 32 of the populations grown in MN were also grown in NY including 10 from site 6 and 22 from site 9. Natural infection by fire blight was recorded annually using a 5-point scale (1= no infection to 5= very severe). This rating was cumulative over years. In both MN and NY, we observed a 20% greater incidence of fire blight in the trees from Kazakh site 9 than from site 6, with many infected trees dying outright. Populations grown only in NY from the other six Kazakh sites had results similar to those from site 6. Where infection was medium to severe (3-5 rating), we defined seedlings as susceptible. We determined that 535 (45%) of the seedlings grown in NY were rated 1 or 2 and potentially resistant. To determine if these genotypes were truly resistant, scions from the field trees were grafted to seedling rootstock, and shoot tips of the resulting grafted plants were inoculated in a greenhouse. Of 289 of the genotypes checked to date, nearly 60% of this group that escaped infection in the orchard were resistant.

Induction of polyphenol gene expression in apple (Malus x domestica) after the application of a dioxygenase inhibitor

A comprehensive study of the complex polyphenol biosynthesis in developing leaves of apple (Malus domestica) was performed comprising gene expression, enzyme activities and polyphenol composition. During leaf development, an early increase in gene expression was observed for phenylalanine ammonia lyase (PAL, EC 4.3.1.5), chalcone synthase (CHS, EC 2.3.1.74), flavanone 3‐hydroxylase (FHT, EC 1.14.11.9) and dihydroflavonol 4‐reductase/flavanone 4‐reductase (DFR/FNR, EC 1.1.1.219). Their enzyme activities showed a corresponding trend during the time course. A parallel set of experiments was carried out with leaves treated with prohexadione‐Ca (ProCa), which is an enzyme inhibitor of 2‐oxoglutarate dependent dioxygenases (2‐ODDs). ProCa is known to induce changes in polyphenol biosynthesis, which are accompanied by a reduced incidence of fire blight and scab, the two major pome fruit diseases. The application of ProCa led to an increase in activities of PAL, CHS, FHT and DFR/FNR, which was based on an enhanced gene expression. In contrast, an inhibition of gene expression was detected for anthocyanidin synthase (EC 1.14.11.19). These effects are interpreted as a feedback regulation by changed polyphenol levels. Because of the inhibition of the 2‐ODDs FHT and flavonol synthase (EC 1.14.11.23), some pronounced changes in polyphenol composition were observed. Eriodictyol, the substrate of FHT, accumulated as eriodictyol‐7‐O‐glucoside and 6″‐O‐trans‐p‐coumaroyleriodictyol 3′‐O‐glucoside. In addition, the 3‐deoxycatechin luteoliflavan was formed which is not present in untreated apple leaves. Hence, beyond the redirection of polyphenol biosynthesis by the enzyme inhibitor, changed polyphenol levels obviously cause a distinct induction of gene expression by feedback regulation.

Luteoforol, a flavan 4-ol, is induced in pome fruits by prohexadione-calciumand shows phytoalexin-like properties against Erwinia amylovoraand other plant pathogens

Treatments with prohexadione-calcium led to lowered incidence of fire blight, scab and other diseases in pome fruit trees and other crop plants. In addition to acting as a growth regulator, prohexadione-calcium interferes with flavonoid metabolism and induces the accumulation of the 3-deoxycatechin luteoliflavan in shoots of pome fruit trees. Luteoliflavan does not possess any remarkable antimicrobial activity. Therefore luteoforol, its unstable and highly reactive precursor, has been tested in vitro for its bactericidal and fungicidal activities. Luteoforol was found to be highly active against different strains of Erwinia amylovora, the causal agent of fire blight, and all other bacterial and fungal organisms tested. Phytotoxic effects were also observed in pear plantlets. The results obtained indicate that prohexadione-calcium induces luteoforol as an active principle with non-specific biocidal properties. It is proposed that luteoforol is released upon pathogen attack from its cellular compartment and inhibits further disease development by destroying pathogen cells as well as by inducing a hypersensitive-like reaction in the host plant tissue. This mechanism would be closely analogous to the one known for structurally related phytoalexins in sorghum.

Ten years of experience with fireblight in Austria1

In 1993, fireblight (Erwinia amylovora) was detected for the first time in Austria, in the most westerly part of the country (Vorarlberg). During the following years, especially since 1998, a stepwise migration from west to east was observed. For control, mechanical measures are mostly used. As these measures require early recognition of symptoms, broad information campaigns for the public and extensive surveys of host plants have been set up. Suspicious plant samples are sent to the central laboratory for diagnosis. Regional plant protection services are assisted by trained volunteers. Eradication and pruning measures are performed by trained working teams. Infected plant material is burned. In all pome‐fruit production areas, the disease forecasting system Maryblyt has been established. Regulations concerning the movement of beehives have been adopted, together with prohibition of the planting of certain ornamental host plants (Crataegus, Cotoneaster). Chemical treatments are limited to one copper oxychloride product. In 2003, temporary application of Prohexadione‐Ca for the prevention of shoot infections was authorized. In some areas, the rigorous application of control measures has reduced the incidence of fireblight and possibly slowed down its progress. However these measures are not sufficient to protect against new, unexpected outbreaks.

Effect of inoculum dose on infection of apple (Malus domestica) flowers by Erwinia amylovora

The effects of inoculum dose, flower age, and climate (temperature, relative humidity, and rainfall) on the incidence of fire blight following inoculation of apple (Malus domestica) flowers with Erwinia amylovora were studied in an orchard. Disease symptoms were only observed when populations of E. amylovora exceeded 106 colony‐forming units (cfu) on flowers 4 days after budburst. The rate at which these populations were reached depended on the climatic conditions over flowering. Ten cfu of E. amylovora inoculated on flower stigmas were capable of inciting fire blight in an orchard in which temperatures above 15°C and high rainfall provided ideal conditions for bacterial multiplication, infection, and subsequent expression of symptoms in flowers. The results from this study may lead to insights into improved management strategies and provide key information for phytosanitary risk assessments for exports of commercial apple fruit.

Temporal Dynamics of the Biocontrol Agent Pseudomonas fluorescens Strain A506 in Flowers in Inoculated Pear Trees

ABSTRACT The colonization of individual flowers in mature pear orchards by Pseudomonas fluorescens strain A506 applied at different times during bloom was measured to determine the receptivity of flowers to colonization and the extent of intra-tree movement over time. Strain A506 populations in flowers open at inoculation were initially about 10(4) cells per flower and increased to approximately 10(6) cells per flower in flowers that were inoculated within about 5 days of opening. However, eventual populations decreased with further increases in flower age at inoculation to as few as about 10(3) cells per flower when inoculated flowers were more than 10 days old. Populations of strain A506 on flowers that opened after inoculation was initially very low at the time of petal expansion (<100 cells per flower) but increased rapidly with time after flower opening. The maximum population of strain A506 that developed on such flowers decreased with increasing time between inoculation and petal expansion; 10(4) to 10(5) cells of strain A506 eventually colonized flowers that opened within 7 days of inoculation, whereas fewer than 100 cells colonized flowers that opened 24 days or more after inoculation. Large total bacterial populations on A506-treated trees were associated with significant reductions in populations of Erwinia amylovora and reduced incidence of fire blight and severity of fruit russet.

Temperature Declines During Storms and Irrigation May Contribute to Fire Blight Infection of Pear Fruit

During summer of 2002, fire blight caused by Erwinia amylovora appeared in immature pears (Pyrus communis) in five Ulster County pear orchards in southeastern New York State. Foliar symptoms of fire blight were absent or infrequent in many trees that had infected fruit, suggesting that fruit infections occurred after shoots had stopped growing and were no longer susceptible to infection. In one orchard where overhead irrigation was initiated on 23 July when air temperatures were 96°F, incidence of fruit infection in September ranged from 10 to 76% with a mean of 36% of all fruit showing fire blight symptoms. Laboratory experiments showed that non-wounded fruit preheated to 95°F and then submerged in inoculum suspensions at 70°F or 40°F developed fire blight infections whereas similar fruit preheated at 70°F and immersed in 70°F-inoculum did not develop infections. Overhead irrigation initiated during the heat of the day can disperse inoculum at the same time that rapid cooling from irrigation water may cause E. amylovora to be drawn into immature pear fruit. The same combination of water-disseminated inoculum and rapid cooling can occur during summer thunderstorms, and storm events may explain the unusual incidence of fire blight on immature pears that was observed in 2002. Accepted for publication 29 January 2003. Published 10 March 2003.