Monday afternoon, May 12, 2014: Warm weather and predicted showers continue to favor fire blight. A strep spray should have been applied sometime on Saturday or Sunday to all orchards that are at high risk for fire blight (i.e., trees <6 years old, active fire blight in the orchard or adjacent orchards in either of the last two years, apple cultivars that are highly-susceptible to blight, all pear cultivars).
Unfortunately, if we get any rain at all in the next few days, orchards that were sprayed with strep this past Saturday or Sunday (May 10 or 11) will need a second strep application either tomorrow (Tues.) or within 24 hours after the start of any rain. Orchards sprayed with strep today (Monday) will probably need a second application on Wednesday or Thursday. The need for a second application was verified using both the MaryBlyt and the Cougar Blight (NEWA) models for predicting fire blight.
It may seem counter-intuitive to apply strep a second time, just two or three days after the first spray. The rationale for the second spray is that, with warm weather, many new blossoms have opened since the first spray was applied. Bacteria deposited on those new flowers by pollinators or by splashing rain reach disease-causing thresholds very quickly in warm weather. The only at-risk orchards where a second spray may NOT be needed are orchards where all flowers were already open when the first spray was applied.
For orchards where fire blight has been causing persistent losses, or in younger orchards that had blight last year, Apogee can be tank-mixed with streptomycin to further reduce risks from fire blight. Apogee does not control blossom blight, but orchards treated with Apogee when shoots are one to four inches long (i.e., right now) are much more resistant to secondary spread of fire blight. Thus, Apogee provides a second level of defense for situations where the streptomycin sprays are not 100% effective. Anyone with at-risk orchards who has not yet applied a strep spray this year should definitely consider applying Apogee along with streptomycin prior to the next rain event. Remember that Apogee must be applied with a water conditioner if you have hard water, and remember that Apogee should NOT be applied to Empire (see the product label!).
Finally, remember to omit Regulaid (or other adjuvant) that is commonly used to enhance uptake of strep if your strep spray will be tank-mixed with chemical thinners during late bloom or at petal fall.