Washington State University Tree Fruit Extension is warning growers of extreme fire blight risk continuing through the weekend with rain and warmer temperatures in the forecast. Continued management is critical, and the tree fruit extension team offers a few reminders:

—Use products with high efficacy.

—When deciding which antibiotic to use, consider how each works. Antibiotics have the highest efficacy when applied shortly before rain, however, kasugamycin and streptomycin can also be applied up to 12 hours after, but their effectiveness is reduced. Streptomycin is locally systemic and kasugamycin is effective for a short time on bacteria that have been washed into the floral cup but have not yet invaded the flower. 

—Oxytetracycline is considered bacteriostatic and inhibits bacterial growth, but to be effective it must be applied prior to rain so that it can prevent growth on stigmas.

—Open blooms are at risk.

—At 80 degrees Fahrenheit, bacterial populations in the flower can multiply fast enough to cause infection in just one day.

—Frequent applications are necessary — with rotating FRAC groups — to cover new blooms as they open and as antibiotics degrade in ultraviolet light during high-risk conditions.

—Lime sulfur applied for thinning reduces the number of flowers and the opportunity for infection, and it is also bactericidal but has no residual protection.

For more information, visit the WSU fire blight webpage at: https://treefruit.wsu.edu/crop-protection/disease-management/fire-blight/ or contact Tianna DuPont by email at: Tianna.dupont@wsu.eduor by phone at: 509-713-5346

by Jonelle Mejica