Progress 07/01/05 to 06/30/07
Outputs
Severe fire blight epidemics in 1998 and 2000 contributed significantly to this reduction in the size of the Michigan apple industry. New strategies are needed for controlling fire blight. The objectives of this proposal were to integrate new control strategies based on growth regulation, novel antibiotics, and biological control agents for management of fire blight in apple orchards, develop a mechanistic understanding of the shoot blight and trauma blight phases of fire blight disease in orchards, characterize the role of type II secretion as a virulence determinant in fire blight disease, optimize apple transgenic lines expressing the MpNPR-1 gene for disease resistance. In the 2005-2007 report period, we found that biological control agents such as BlightBan C9-1 and Bloomtime Biological E325 were only effective in controlling blossom blight if used in conjunction with at least one antibiotic spray. We also found that both antibiotics gentamicin and kasugamycin
have potential as substitutes for streptomycin in fire blight management programs. We have also begun evaluating the impact of gentamicin and kasugamycin use on the indigenous microflora residing in apple orchards. We created a mutant deficient in type II secretion in the laboratory; however, this mutant was not impacted in ability to cause fire blght disease. This work has subsequently been dropped. We initiated alternate research efforts in characterizing the role of biofilms in fire blight disease. Growth in a biofilm represents an important mechanism of pathogenesis utilized by many bacterial pathogens. We found that the fire blight pathogen Erwinia amylovora forms biofilms in vitro and during infection of apple and that the biofilm growth is essential for disease. The genetics of production of the exopolysaccharide amylovoran has been studied previously, however, the role of specific amylovoran genes in biofilm formation and virulence in host plants has not been studied. We are
actively studying this now. Transgenic approaches, including the overexpression of the resistance gene MpNPR-1 have proven successful in conferring increased fire blight resistance to apple. While the regulatory climate for transgenic apple crops currently is uncertain, this approach still holds promise in the battle against debilitating disease such as fire blight.
Impacts
Field research evaluating new materials and streptomycin alternatives for fire blight control is critical for the continued management of fire blight on existing apple trees. Biological control materials are important in possibly reducing the need for antibiotics for fire blight control. Research on genes required for virulence of the fire blight bacterium may offer the promise of finding novel control strategies, including the rational design of control compounds for fire blight.
Publications
- Malnoy, M., Q. Jin, E.E. Borejsza-Wysocka, S.Y. He, and H.S. Aldwinckle. 2007. Overexpression of the apple MpNPR1 gene confers increased disease resistance in Malus x domestica. Mol. Plant-Microbe Interact. (in press).
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Progress 07/01/05 to 06/30/06
Outputs
The apple acreage in Michigan has declined by about 18% in the last five years; severe fire blight epidemics in 1998 and 2000 contributed significantly to this reduction in the size of the Michigan apple industry. New strategies are needed for controlling fire blight. The objectives of this proposal are to integrate new control strategies based on growth regulation, boosting the resistance of apple trees, and antibiotics into disease management programs; characterize the genetic variation between Erwinia amylovora and closely related species; evaluate the transgenic expression of the MpNPR-1 gene in apple, and characterization of the Hrp virulence system in E. amylovora. In the 2005-06 report period, we found that streptomycin resistance is becoming more widepsread in Michigan, further limiting the ability of growers to control the disease with the best bactericide available. Streptomycin resistance was detected in the Hart apple-growing area of Michigan, the third
(of four) major apple-growing areas in the state to harbor streptomycin-resistant E. amylovora. We tested the efficacy of the experimental antibiotics gentamicin and kasumin in fire blight control and found that both antibiotics have potential as a subsititue for streptomycin in fire blight management programs. The biological control agents Pantoea agglomerans C9-1 and E325 also performed well in disease control trials performed in 2006. We tested two new compounds, the fungicide famoxate and the plant elicitor laminarin, for control of the blossom blight phase of fire blight. While both of these materials were effective in 2005 under low to moderate disease pressure, the materials did not effectively control blossom blight in 2006 under high disease pressure. We identified a new E. amylovora virulence gene, avrRpt2Ea, that is similar to a virulence gene of the plant pathogen Pseudomonas syringae. We demonstrated the role of this gene in pathogen virulence and also showed that it is
functionally related to the P. syringae gene. We are currently investigating new strategies for plant host resistance based on recognition of the AvrRpt2Ea protein. In addition, we continue to focus our efforts on determining the function of DspE, the critical effector for disease. We found that one of the functions of DspE is to suppress the innate immunity response of plants. We have been examining the secretion of DspE into plant cells and further studying other functions of DspE in causing plant disease. Knowledge of the genes expressed by E. amylovora during infection and studies of critical effectors such as DspE now reveals potential targets for novel disease management strategies.
Impacts
Field research evaluating new materials and streptomycin alternatives for fire blight control is critical for the continued management of fire blight on existing apple trees. Research on genes required for virulence of the fire blight bacterium may offer the promise of finding novel control strategies, including the rational design of control compounds for fire blight.
Publications
- Zhao, Y., S.Y. He, and G.W. Sundin. 2006. The Erwinia amylovora avrRpt2Ea gene contributes to virulence on pear and AvrRpt2Ea is recognized by Arabidopsis RPS2 when expressed in Pseudomonas syringae. Mol. Plant-Microbe Interact. 19:644-654.
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