Search

With the goal to develop management tools to mitigate the severity of shoot blight on pear caused by Erwinia amylovora and prevent fire blight canker development from infected shoots on pear tree wood, we evaluated five preventive treatments in inoculated experiments over 2-years. We focused on foliar spray and trunk injection applications of extract of giant knotweed, Reynoutria sachalinensis (RSE), as a plant resistance activator candidate and on comparisons to antibiotics. Five foliar applications of RSE (2.24 L/ha) on 6-year-old trees of cultivar ‘Bartlett’ resulted in 100% control of shoot blight severity and canker incidence in both years of evaluation. The RSE trunk injection in fall of the first year did not control the disease allowing 75% of shoot severity and 38.5% of canker incidence. The opposite happened in the second year, when we injected RSE in spring, providing 85.7% control of shoot blight severity and 89.8% control of canker incidence. Antibiotics oxytetracycline (OTC) and streptomycin (STM) did not provide consistent control in the second year in comparison to the first year. In the first year, both antibiotics, OTC (trunk injection and foliar spray) and STM were effective, providing 94.8%, 100% and 86.3% control of shoot blight, respectively. They also provided 100, 91 and 84% control of canker incidence, respectively. However, in the second they were not effective allowing 69 – 96% of shoot blight severity and 70 – 92% of canker incidence. Regardless of the inconsistency in the effectiveness of antibiotics, which are not recommended for shoot blight control due to risks of promoting antibiotic resistance, five preventive spray applications of RSE could be used on pear trees to prevent shoot blight and canker development. Thus, the RSE spray program we developed could serve as a valuable control tool that could help prevent sudden outbreaks of shoot blight during summer and pear tree death from fire blight cankers with no risk for pathogen resistance.

Erwinia amylovora causes fire blight, a disease responsible for enormous economic losses in the pome fruit-producing areas where it is present. Despite the abundant research on fire blight, information about E. amylovora population dynamics and survival in fire blight cankers and the plant defense responses to this pathogen in the infected bark are limited. In our study, we obtained fire blight cankers in apple, pear, and Asian pear cultivars showing differing resistance to the disease by shoot inoculation with E. amylovora. We collected cankers from irrigated and non-irrigated trees every 3 months in two independent field experiments and analyzed samples by viability digital PCR. We also assessed the expression of pathogenicity-related (PR) genes in the bark of selected apple and Asian pear cultivars. A logistic regression analysis revealed the impact of environmental and host factors on E. amylovora detection rates in cankers. The chances of detecting live E. amylovora cells in cankers increased significantly in those collected from irrigated trees, in July, and/or during an experiment performed in a year with an expected average rainfall when compared to samples from non-irrigated trees, collected in January, and/or during an experiment performed under environmental conditions dominated by drought. We found a positive correlation between the pathogen detection rates in cankers and the host resistance to fire blight that might be explained by lower E. amylovora survival rates in more damaged tissues of susceptible hosts. The genes PR-1, PR-2, PR-5, and PR-8 were induced in the bark surrounding apple and Asian pear fire blight cankers. Our study, involving the analysis of more than 800 canker samples, provides new knowledge about the fire blight disease cycle and lays the foundation for improved fire blight management and eradication strategies in pome fruit orchards. National Institute of Food and Agriculture through New York State Specialty Crop Block Grant Program; National Institute of Food and Agriculture, U.S. Department of Agriculture, Hatch/Multi-State Project [SCG 82535/A001-SCG 17 005]; USDA National Institute of Food and Agriculture [1014444, NYG-625857]; [1020561] Published version This material is based upon work supported by the National Institute of Food and Agriculture through New York State Specialty Crop Block Grant Program, project number SCG 82535/A001-SCG 17 005 to SGA in 2017, and by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Hatch/Multi-State Project W4185, under Accession Number 1014444 Project Number NYG-625857, to SGA in 2017. This work was supported by the USDA National Institute of Food and Agriculture, Hatch Project Accession Number 1020561 to SGA, in 2019.

Management of fire blight is complicated by limitations on use of antibiotics in agriculture, antibiotic resistance development, and limited efficacy of alternative control agents. Even though successful in control, preventive antibiotic sprays also affect non-target bacteria, aiding the selection for resistance which could ultimately be transferred to the pathogen Erwinia amylovora. Trunk injection is a target-precise pesticide delivery method that utilizes tree xylem to distribute injected compounds. Trunk injection could decrease antibiotic usage in the open environment and increase the effectiveness of compounds in fire blight control. In field experiments, after 1-2 apple tree injections of either streptomycin, potassium phosphites (PH) or acibenzolar-S-methyl (ASM), significant reduction of blossom and shoot blight symptoms was observed compared to water- or non-injected control trees. Overall disease suppression with streptomycin was lower than typically observed following spray applications to flowers. Trunk injection of oxytetracycline resulted in excellent control of shoot blight severity, suggesting that injection is a superior delivery method for this antibiotic. Injection of both ASM and PH resulted in the significant induction of PR-1, PR-2 and PR-8 protein genes in apple leaves indicating induction of systemic acquired resistance (SAR) under field conditions. The time separating SAR induction and fire blight symptom suppression indicated that various defensive compounds within the SAR response were synthesized and accumulated in the canopy. ASM and PH suppressed fire blight even after cessation of induced gene expression. With the development of injectable formulations and optimization of doses and injection schedules, the injection of protective compounds could serve as an effective option for fire blight control.