14 June 2014: Fire blight is causing problems across a broad swath of the mid-Atlantic and Northeast. Dealing with fire blight at this time of year is always difficult. Penn State Extension just published an excellent article by Dr. Kari Peter that is worth reading (click here). Of particular interest is her comment that Keith Yoder found that applications of Double Nickel plus Cueva (a copper product) helped to arrest spread of fire blight during summer in a test he conducted last year and reported in Plant Disease Management Reports. Keith applied Double Nickel at 8 fl oz/100 gal of dilute spray plus Cueva at 2 qt/100 gal, and he made those applications at roughly monthly intervals.
I’m still a bit leery of applying any copper during summer to fresh market apples because of the risk of fruit injury, and frankly I don’t understand how one can mix copper (i.e., Cueva) with a biocontrol (Double Nickel) because one would assume that the copper (a biocide) would kill the bacteria in the biocontrol as soon as they are mixed in the spray tank. Nevertheless, we have few options for slowing the spread of fire blight during summer, so this option may be worth considering if one is willing to risk some fruit injury (even though fruit injury was not a problem in the published test).
Remember that to minimize risks of fruit injury with copper sprays, applications should be made to dry leaves under rapid-drying conditions.