Fighting Fire with a Twist

This article was reviewed by the editor of The Tender Fruit Grape Vine for technical accuracy and appropriateness.

Researchers at Agriculture and Agri-Food Canada (AAFC, A.M. Svircev and P.L. Sholberg) and Brock University (A.J. Castle) have taken a novel approach towards the control of fire blight in the orchard. The biological control project is funded by AAFC under the Improving Farming Systems and Practices Initiative, funded by AAFC's Pest Management Centre. The novel system relies on the 'double punch' approach of controlling Erwinia amylovora, the fire blight pathogen with Pantoea agglomerans and bacteriophages.

Biological control strategies have attracted more and more interest as regulatory bodies question the agricultural use of antibiotics. Two commercially available biocontrol products, the bacterial antagonists BlightBan® C9-1 and A506 provide control of the fire blight pathogen, Erwinia amylovora, through competition for nutrients and/or the production of microbial antibiotics. During the blossom time under ideal weather conditions, the pathogen can destroy (Fig. 1) an entire orchard within one growing season.

Fig. 1 Severe shoot blight in apples due to blossom infections in the spring. Photo by Ed Barszcz.

Our research has focused on developing a biological control strategy for fire blight that improves upon the efficacy and reliability of bacterial antagonists alone. We use P. agglomerans together with bacteriophages of E. amylovora to prevent growth of the pathogen in the pear and apple floral cup. Bacteriophages (or simply "phages") are bacterial viruses that infect specific host bacteria, replicate inside it, and then kill the host cell to release the new phages. P. agglomerans has a dual role in this system, acting as a biological control agent and as a carrier for the phages. The carrier permits the continuous production of fresh, infective phages on the flower surface, while competing with the pathogen for the ecological niche provided by the blossom.

Phages in our project were collected from the orchard ecosystem in which disease symptoms were visible in the form of cankers, blossoms and/or shoot blight. Phages belong to the order Caudovirales, specifically the tailed families Myoviridae (Fig. 2) and Podoviridae (Fig. 3). They form 6 distinct molecular groups based on the RFLP patterns and vary from 40-250 nm in size. Phage host ranges have been established on13 E. amylovora isolates. Some phage isolates may have very promiscuous host range while others are much more restricted, particularly when considering their relative behaviour on pathogen isolates from Ontario vs. British Columbia (Applied and Environmental Microbiology, Vol. 69: 2133-2138).

Fig. 2 Short tailed Erwinia sp. phages. Photo by Ron Smith

Fig. 3 Long tailed Erwinia sp. phages. Photo by Ron Smith

Quantitative PCR technology was applied by Dr. Won-Sik Kim (NSERC Visiting Fellow) to investigate the microbial populations present in the blossom. This technology, which we refer to as direct real-time PCR (DRT-PCR), allows rapid and sensitive quantification of organisms directly from plant tissue. Using blossoms from field based experiments, we can simultaneously detect and quantify populations of P. agglomerans, E. amylovora, and some phages. Field trials carried out by the group and Brock University graduate student Susan Lehman, indicates that a correlation exists between populations of the biocontrol agents and the pathogen and the ability of each treatment to control disease. In treatments in which disease control was successful, phages multiplied on the P. agglomerans carrier and once the pathogen was applied the phages grew preferentially on the pathogen in the floral cup. Early field trials in pear and apple orchards have demonstrated that the phage-carrier system can reduce the incidence of diseased blossom clusters by 50%. Research conducted with a laboratory-based pear blossom assay in Ontario and British Columbia continues to screen new isolates and to assess potential modes of application for incorporation into future field trials. Phage mixtures will be eventually used to prevent the pathogen from sequentially accumulating resistance to individual phages. How easily does the host develop resistance to the phages? Brock University, graduate student Dwayne Roach is studying this problem. He will determine the incidence and nature of phage-resistance among pathogen populations.

The major goals of this project are to establish an extensive collection of phages from Ontario and British Columbia that individually have a wide host range, handle well during scale up production, show potential for fire blight control under field conditions and develop formulations and application procedures that result in high levels of control in the field. Research is continuing to identify isolates with high field efficacy, determine the mechanisms of development of phage resistance in host bacterium, develop large scale processing of phage/carrier and to follow the environmental fate of the phages in the orchard ecosystem. The ultimate goal is to develop a biocontrol system that will have efficacy for disease control in the orchard comparable to streptomycin, the industry standard.

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