Written by Tianna DuPont, WSU Extension. April 4, 2025
If we have warm temperatures and moisture during bloom be prepared to prevent fire blight infections. Consider the following reminders.
Risk
Fire blight risk depends on temperature and moisture during bloom and the amount of bacteria present from overwintering cankers or primary infections. When temperatures are low, below 50° F, bacterial cell division is minimal, and relatively slow at air temperatures between 50 to 70° F. At air temperatures above 70° F, the rate of cell division increases rapidly. Moisture from rain or dew moves bacteria into the floral cup where they can infect the flower through the nectaries.
Use products with high efficacy
Check product efficacy WSU Fire Blight webpage or the Crop Protection guide. A new publication covers efficacy of organic products.
Apply products BEFORE an infection event
Consider how each antibiotic works when deciding which to use. Antibiotics have the highest efficacy when applied shortly before a moisture event. While kasugamycin and streptomycin can also be applied up to 12 hours after a moisture event, efficacy is greatly reduced. Streptomycin has locally systemic activity and kasugamycin is effective on bacteria which have been washed into the floral cup but not yet invaded the flower (for a short time). Oxytetracycline is considered bacteriostatic (inhibits bacterial growth). Thus, to be effective it must be applied prior to rains where it can prevent growth on stigmas.
Consider mitigating antibiotic resistance
In a recent antibiotic resistance survey, researchers did not find isolates from collected samples to be resistant to streptomycin and tetracycline, but some isolates exhibited resistance and tolerance to kasugamycin (Zhao 2023-24). To minimize the risk of resistance development, it is recommended to apply streptomycin in combination with oxytetracycline and limit streptomycin application to once per season. Kasugamycin could be applied in a mixture with oxytetracycline or rotation with oxytetracycline.
Do not irrigate during bloom
Running sprinklers right now during bloom increases the humidity in your orchard and contributes to infections.
Optimize your spray by buffering
All WSU antibiotic trials are done with tank water buffered to 5.6. At higher pH, antibiotics are not as effective. This is because both the residual of the antibiotic is much longer at a lower pH and because the low pH itself reduces bacterial growth. Research by Ken Johnson at OSU showed that buffering to pH 4 can further improve antibiotic efficacy.
Additional information
Contact
Tianna DuPont, WSU Extension
(509) 713-5346
