May 4, 2015
Penn State: Fire blight alert this week

All predictive models indicate a high risk for fire blight infection this week due to the high temperature, humidity, and chance of rain showers in the forecast. Growers should begin protecting blossoms with streptomycin on Monday. More strep applications will be needed as infection conditions persist.

I was worried this would happen: the blossoms open and Mother Nature decides to finally warm up and feel like spring for more than 24 hours. It’s shaping up to be another doozy of a spring for fire blight and you must heed the warning. Based on the prediction models, it might be a repeat of 2014. However, thanks to all of the information you received this winter and early spring, you are prepared to pull out your best Clint Eastwood impression to stare the fire blight bacteria in the face and say, “Go ahead…make my day.”

At the moment, May 5-9 indicate a significant risk for infection. To be clear: that is every day during that period, and perhaps into next week. Growers need to apply streptomycin to any apple or pear block with open blossoms. Strep (or any antibiotic) is expected to give 1-2 days of protection. If conditions persist over 4-5 days, you will need to apply two sprays during this period, especially if rainfall occurs and your apple trees are at different bloom stages.

For organic growers:

Due to the new rules in place, organic growers are no longer allowed to use antibiotics for fire blight control. Your best option right now:

Cueva 2 qts/A plus Double Nickel 1 qt/A

For best control, tank mix these two products instead of using Cueva by itself. With any copper there is a risk for fruit russeting, so use copper with caution. (Anecdotally, it seems the risk of russeting from using Cueva is minimal, especially when tank mixed with Double Nickel; however, research is ongoing.) I am evaluating several alternative options this season and, hopefully, there will be a few more tools added to your tool box for fire blight management.

In case you need a refresher:

Remember: blossom sprays protect only flowers that are open and only protect blossoms prior the infection event. Since blossoms do not open all at once, it is necessary to apply several sprays when infection conditions are frequent during bloom. It is important to be vigilant in monitoring weather conditions: average temperatures >60°F and wetting events (rain, heavy dew). Unfortunately, applying fungicides or plant growth regulators during bloom using high volumes of water can provide a wetting event necessary for infection when all other conditions for blossom blight are present.

Options available to protect blossoms and considerations to keep in mind:

Apply antibiotics as complete sprays and add an adjuvant or surfactant. Antibiotic sprays are most effective when they are applied the day before or the day after an infection event (within 24 hours).

Streptomycin is still the best option since it kills the bacteria and has partial systemic activity. Note: the systemic activity does not persist like fungicides and you have about a 48 hour window. Streptomycin still works in the Mid-Atlantic.

Kasugamycin is new to the market this year. It is different from streptomycin in that it reduces bacterial growth and reproduction, rather than killing it directly. Research in Michigan has shown this product has helped regions where streptomycin resistance is a big problem.

Oxytetracycline is an antibiotic that functions similarly to kasugamycin in reducing bacterial growth.

There is a 4 spray maximum when applying antibiotics and do not apply antibiotics after bloom. This is necessary for resistance management. Please do not think that just because 3 antibiotics are available you are able to apply 12 antibiotic sprays. Not only is it expensive, it is unnecessary and generally not a good idea.

Blossom Protect is a live yeast product that colonizes the flower and prevents the bad fire blight bacteria from entering the nectaries. Research on the West Coast indicates this is a very successful product for controlling fire blight. HOWEVER, this product is not as effective for our conditions on the East Coast at the present time. This is most likely due to the natural flower microbial community, which seems to prevent good colonization of the Blossom Protect. I tested this product last year during very high pressure conditions and only achieved ~40% control, whereas I had 76% control using streptomycin. In addition, the strep treated trees had significantly few instances of shoot strikes compared to the trees treated with Blossom Protect. Research is currently underway in Michigan to see what measures can be taken to make Blossom Protect work better in our conditions.

Although applying copper at bloom will kill bacteria, copper can cause fruit russetting and should be used with caution.

Be mindful of rattail bloom. All blossoms are susceptible to infection if the bacteria and conditions are present.

Kari Peter, Penn State University

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