Control of fire blight (Erwinia amylovora) on apple trees with trunk-injected plant resistance inducers and antibiotics and assessment of induction of pathogenesis-related protein genes.

Srdâ¯An G Aä‡Imoviä‡,Quan Zeng,Gayle C Mcghee,John C Wise,George W Sundin

doi:10.3389/fpls.2015.00016

Abstract

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 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.

Highlights

Erwinia amylovora (Burrill 1882) Winslow et al (1920) is a devastating bacterial pathogen of plant species in the Rosaceae family, causing the disease fire blight
We found that trunk-injected oxytetracycline in apples provides control of fire blight incidence of 60%, well surpassing kasugamycin and copper chelate effects (Acimovicet al., 2014a)
We uniquely investigated the effect of trunkinjected and not sprayed plant resistance inducer, potassium phosphites, and bactericides on the development of fire blight and for the first time we correlated disease suppression and pathogenesis related (PR) gene expression on mature apple trees

Summary

Introduction

Erwinia amylovora (Burrill 1882) Winslow et al (1920) is a devastating bacterial pathogen of plant species in the Rosaceae family, causing the disease fire blight. In the USA, fire blight losses and control costs per year are estimated to be more than $100 million (Norelli et al, 2003). In Michigan, a fire blight epidemic in 2000 resulted in economic losses of $42 million due to removal of approximately 400,000 apple trees (Longstroth, 2001). In Washington and northern Oregon, economic losses on pome fruits due to fire blight were over $68 million (Stockwell et al, 2002). An added difficulty in fire blight management is the occurrence and spread of strains of E. amylovora with resistance to the antibiotic streptomycin, limiting the efficacy of this compound as a plant disease control agent

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