Wendy McFadden-Smith, OMAFRA Horticulture IPM Specialist; Kristy Grigg-McGuffin, OMAFRA Horticulture IPM Specialist; Amanda Green, OMAFRA Tree Fruit Specialist
A joint Growing Forward 2 project with the Ontario Apple Growers and the Ontario Tender Fruit Growers was launched this past year to investigate fire blight (Erwinia amlyovora) management in Ontario apple and pear orchards. The objectives of this project are to:
- Provide growers with tools to respond quickly and effectively to fire blight outbreaks.
- Gain information on the prevalence and distribution of fire blight resistance to streptomycin in apple and pear orchards across Ontario.
- Develop strategies for integrating biologicals, antibiotics and copper for the management of fire blight.
- Evaluate the impact of cultural management practices.
Streptomycin Resistance Survey
In 2016, samples of fire blight infected shoots were collected across Ontario from commercial apple and pear orchards. Environmental conditions were suitable for fire blight development and project consultants were able to collect from a total of 64 orchards (42 apple and 22 pear). These samples were tested for the presence of streptomycin resistant E. amlyovora. From each orchard, up to 250 isolates of E. amylovora were screened for growth on agar medium amended with 100 and 1000 ppm streptomycin to identify isolates with moderate or high resistance, respectively.
The following results were found:
- No E. amylovora was isolated from 7 of the orchard sites surveyed, leaving 57 sites for resistance analysis.
- Overall, no growth occurred on either amended medium in 27 sites, which means no resistant isolates were detected.
- In the remaining 30 sites across the province, the highest percentage of moderate and high resistant isolates were 3 and 2%, respectively, which suggests that very little selection for resistant isolates has occurred in any of the orchards sampled.
It is typical to have a low proportion of less susceptible isolates in an average population due to natural mutation. Grower cooperators in this project will receive their individual orchard results before spring 2017.
So, what does this mean for Ontario growers? The results indicate streptomycin is currently still a very effective product for management of fire blight. However, in order to keep this product as a viable tool, it is very important to incorporate other control products and management strategies to reduce selection pressure.
Fire blight management should:
- Consist of a season-long program, including pruning out infected material as soon as possible (see previous Hort Matters article) and maintaining good insect control to prevent spread of bacteria.
- Include a dormant copper spray at silver tip to ¼ inch green.
- Delaying copper after ½ inch green can result in phytotoxicity and severe fruit russetting, depending on the copper product used, especially on lighter skinned apples and some pear varieties such as Anjou.
- Do not apply under slow drying conditions or just prior to a predicted frost to also help minimize the risk of phytotoxicity.
- Use a sound rotational program of registered products during bloom, including antibiotics (Streptomycin, Kasumin), copper (Cueva, Copper 53W) and/or biologicals (Blossom Protect, Bloomtime, Double Nickel, Serenade OPTI).
- Ensure adequate water volumes are used and that sprays are deposited into blossoms. Do not use excessive air speed on sprayers as this will propel sprays past the flower targets. For more information on calibrating an airblast sprayer or troubleshooting coverage, refer to Airblast 101 Handbook.
- Products are most effective when applied just prior to an infection period. Since biologicals have a preventative action, these products should be applied when forecasting model (see below) says risk is coming in the next 3-4 days. Antibiotics will provide activity 24 hours before and after a wetting event.
- Products should be re-applied every 2-3 days during times of infection risk.
- Copper and some fungicides are not compatible with biologicals. Check the label.
- Apply control products when infection conditions are predicted using a forecasting model such as:
- Consider managing growth of succulent shoots by limiting excess nitrogen and incorporating Apogee (apples only) beginning at king bloom petal fall.
Integrating Biologicals, Antibiotics and Copper for Fire Blight Management at Bloom
Also in 2016, an experimental block of high density apples cv. Gala/M9 rootstock and pears cv. Bosc/Swiss Bartlett rootstock was established at AAFC Vineland to investigate the integration of antibiotics, biologicals and copper into a fire blight management program at bloom (Figure 1). The pears did not have a lot of bloom so only apples were included in the treatments this year (Table 1).
The targeted bloom stages for spray applications were trace bloom, 25-50% bloom and 50-80% bloom. However, bloom was inconsistent and high temperatures caused bloom to occur rapidly over a 3-day period. Treatments were applied with a calibrated CO2 backpack sprayer to first drip the mornings of May 30, 31 and June 1. Trees were inoculated with E. amylovora the evening of June 1. Symptom development was monitored in the orchard to compare efficacy of different treatments. Fire blight infected tissue was removed and destroyed.
Weather conditions were not conducive to the development of blossom blight symptoms and many of the blossoms were at petal fall stage by June 1 when inoculation took place; however, shoot blight symptoms continued to develop through the end of June. Some of this could be due to insect-vectored infections from a heavily infected research orchard adjacent to the trial block, although there was no apparent gradient in infection from one side of the block to the other. It is more likely that, despite conditions not being ideal for infection during bloom, some of the bacteria managed to enter the blossoms and the very warm weather following inoculation allowed rapid multiplication of bacteria resulting in shoot blight.
The number of trees with blossoms ranged from 0-5 per plot. Due to variability and small sample size, no statistically significant differences were found among treatments (Table 1, p=0.10). However, there were trends of lower infection with some treatments. In 2017, bloom should be more uniform and the % of flower clusters per tree will be evaluated to provide more statistically useful information.
Table 1. Bloom treatments for management of fire blight on apples cv. Gala/M9 rootstock, 2016.
|% trees infected|
|Double Nickel + Cueva||Streptomycin||Streptomycin||38.8|
|Double Nickel + Cueva||Double Nickel + Cueva||Double Nickel + Cueva||41.3|
|Blossom Protect||Blossom Protect||Blossom Protect||38.4|
|Double Nickel||Double Nickel||Double Nickel||74.2|
*The number of trees with blossoms ranged from 0-5 per plot. Due to variability and small sample size, no statistically significant differences were found among treatments (p=0.10)
Impact of Cultural Practices on Fire Blight Management
A second experimental orchard of pear cv. Bosc was also established at AAFC Vineland to explore cultural practices to manage fire blight in a young high density orchard including high and low nitrogen fertilization, rootstock susceptibility and use of Apogee in replicated five-tree plots. Pears were planted on two rootstocks, OHxF 97 and Swiss Bartlett.
The young orchard will be inoculated with E. amylovora at full bloom in 2017. Control trees will not receive the inoculation. Tree growth and fire blight development will be compared in each treatment.
We would like to sincerely thank all of the growers who participated in providing samples for testing and who offered their knowledge and assistance in establishing the experimental orchards. As well, our sincere gratitude to Antonet Svircev, Darlene Nesbitt and Kinga Bodnar at Agriculture & Agri-Food Canada for laboratory testing; Leslie Huffman, Lindsay Pink, Kathy Hoshkiw, Margaret Appleby and Kevin Schooley who conducted the sampling, and summer students, Peter Smith, Caitlin Smith, Chris Burnie, Curtis Burton, Aleyna Bingham, Anthony Miller, Justin Freeman-Phillips, Made Mekoya, Erica Sayles and Megan Bender for their assistance in carrying out the project.
Generous support was provided by NuFarm Agriculture Inc., Bayer CropScience, Bio-Ferm GmbH, Certis USA, Engage Agro Corp., Arysta LifeScience, Niagara Peninsula Fruit & Vegetable Growers Association, Ontario Orchard Supply, and Vanden Bussche Irrigation. This project is funded in part by Growing Forward 2 (GF2), a federal-provincial-territorial initiative. The Agricultural Adaptation Council assists in the delivery of GF2 in Ontario.