The role of insects as fire blight vectors: Implications for sustainable disease management in Northeast apple orchards

View the project final report

• 2016 annual report

Commodities

• Fruits: apples, general tree fruits
• Animals: bees, Flies

Practices

• Education and Training: extension, on-farm/ranch research
• Natural Resources/Environment: biodiversity
• Pest Management: disease vectors, field monitoring/scouting, integrated pest management
• Sustainable Communities: community services, partnerships, public participation, sustainability measures

Fire blight (Erwinia amylovora) is a devastating bacterial disease of apple, causing severe damage and economic loss in the Northeast, where over 20% of the nation’s apples are produced. To date, growers depend on streptomycin applications to control the disease, but this method is not sustainable due to occurrences of, and continued risk for, antibiotic resistance. Though insects have long been implicated as fire blight vectors, the potential benefits of controlling vectors as part of a fire blight management program are unknown. Thus, the purpose of this project is to investigate molecular, behavioral, and ecological interactions between fire blight and its insect vectors. We propose to: identify key vectors and their significance in the ecology of the disease; evaluate fire blight performance within the insect gut; and characterize vector preferences for volatiles of diseased apple tissue. We hypothesize that: key vectors carry the pathogen internally; a novel genetic construct in the bacteria genome mediates interactions between pathogen and vector; and infection causes changes in the volatile profile of apple tissue, resulting in vector preference for diseased trees. We will investigate these hypotheses at three distinct levels, using field surveys, behavioral bioassays and molecular characterizations to complete the project objectives. If successful, this project has significant implications for future sustainable management of fire blight, reducing selection pressures favoring antibiotic resistance and subsequent financial burdens on growers. We will disseminate new knowledge in various ways, including agricultural expos, stakeholder publications and presentations, and updated fire blight and pest insect fact sheets.

1. Identify candidate insect vectors of E. amylovora.

Existing data on insect transmission of fire blight is outdated and characterizes the vectoring process as passive. We hypothesize that certain insects actively vector fire blight by transporting the pathogen internally. We will conduct field surveys throughout the growing season to monitor changes in community structure, using molecular methods to confirm E. amylovora presence in insect guts. We will focus on pollinators visiting blossoms; hemipterans who provide entry wounds for shoot colonization by E. amylovora; and dipterans visiting oozing cankers on diseased trees. Finally, we will use live captured candidate vectors to conduct laboratory acquisition and transmissions assays to fully confirm vectoring capability. These data will be instrumental for monitoring and control of insect vectors in commercial and small holding orchards.

2. Investigate persistence of E. amylovora within insect guts.

Two novel type III secretion systems (T3SSs) in the fire blight genome were recently characterized as non-pathogenic, showing greater genetic homology to human enteropathogens and insect endosymbionts rather than phytopathogens. The role of these secretion systems in the fire blight disease cycle is unknown. We hypothesize that these T3SSs mediate interactions between vectors and E. amylovora, allowing the pathogen to persist in the insect gut. We will challenge insects with ΔT3SS mutant or wild type (WT) E. amylovora and use qPCR at multiple time points to evaluate pathogen abundance. We will compare insect acquisition and transmission of ΔT3SS and WT E. amylovora in laboratory assays. These data will provide insight into the cellular relationship between fire blight and its vectors, with implications for disease control in other agricultural pathosystems.

3. Evaluate insect preference for healthy or diseased apple shoots/blossoms.

Many plant pathogens alter volatile profiles of their hosts, preferentially attracting vectors. In apple, changes in shoot and blossom volatile profiles due to fire blight infection are unknown, as is the resultant impact on insects. We hypothesize that infection induces changes in volatile emissions from blossoms and shoots, resulting in preferential insect aggregation and visitation on diseased tissue. We will conduct choice and non-choice bioassays to evaluate insect preferences for diseased tissue. We will also characterize the volatile profiles of diseased and healthy shoots and blossoms to identify key chemical differences between the two. These data lend insight into multi-partite interactions between plant pathogens, their hosts, and insect vectors, justifying future assays aimed at identifying and utilizing behavior altering compounds for vector management.