Apple Disease Update: Week of May 24, 2021

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Ooze on apple shoot

First take a moment to enjoy the beauty of the ooze droplets on the ‘Honeycrisp’ shoot above and then after you’re done admiring “nature” think about the millions or billions of sticky Erwinia amylovora cells ready to spread fire blight to other trees nearby. This photo was taken on our “pot lot” at MHCREC where we are growing ‘Honeycrisp’ and ‘Evercrisp’ in containers for research on ambrosia beetles and rapid apple decline. Keep in mind, this plot is fairly isolated from other apple trees and receives drip irrigation. We went to remove some trees from the pot lot last week and realized a local fire blight epidemic had broken out in the Honeycrisp. Sure, Honeycrisp are quite susceptible to fire blight, but I was puzzled as to the source of inoculum-especially since it has been relatively dry over the past few weeks. My grad student, Sean Gresham than reminded me that we had inoculated six of the hundred or so trees with fire blight last year as part of an ambrosia beetle study and more than likely overwintering populations from those infections were the source of the outbreak. A good but painful reminder for us to clean up any cankers in the winter and ensure that green tip applications of copper are made annually!

ooze on honeycrisp

Why am I spending so much time on this story? While out in Henderson Cty last week, we drove past a number of young Honeycrisp and Gala trees with fire blight strikes. Likely the infections came from delayed open blossoms and/or oozing cankers-the latter of which was likely delayed this year due to the uncharacteristically cool temperatures this spring. Given the paucity of fruit on highly susceptible cultivars, vigor management will be critical throughout this season. Although the fire blight pathogen is a poor competitor on the surface of leaves during the hot summer months, the warm humid weather this week combined with the continual flush of new growth provides ideal conditions for shoot blight development.

For the past few years, we’ve been pushing two applications of prohexadione calcium (pro-Ca) around petal fall and 10 days or so after that at a rate of 12 oz/100 gal. Younger trees in general we’ve recommended applying 25 to 50% that rate (3-6 oz/100 gal). Research out of George Sundin’s program at MSU has observed excellent control of shoot blight when applying 2 oz of Apogee plus 1 to 2 oz of Actigard, without the retardation of shoot growth that higher rates of Apogee were giving. This year, Annie Vogel, a Ph.D. student in Tom Kon’s program will be teaming up with my program to evaluate these treatment combos in North Carolina. For now I’d continue with our current recs until we get data out of North Carolina, but if you are feeling daring, go ahead and try out the Apogee/Kudos plus Actigard combo. Also to read more about there study check out these two links:

Living After Midnight With Fire Blight

Applying Apogee and Actigard to Young Apple Trees

If you do have an infection that has gotten out of hand my advice would be the following:

  1. Spray immediately with a 12 oz/100 gal rate of Kudos or Apogee
  2. Begin shoot blight removal by pruning at a minimum of 12 inches below the advancing infection margin
  3. After pruning, consider an application of streptomycin mixed with Regulaid.

In other news this week, I’ve been seeing quite a bit of primary powdery mildew infections. This, like shoot blight is a disease that needs to be managed regardless if you have fruit this year. Sulfur (e.g., Microthiol Disperss 10-20 lb/A) is a fairly cheap option and should get the job done. However, take caution when applying it when temps exceed 85 F. Another fairly inexpensive options that will do a job on mildew include Rally or Procure 480.

Primary Powdery Mildew

Although mildew may seem somewhat inconsequential, especially this year when fruit are not as plentiful as other years, my advice is to continue to stay on top of it. Not only can premature defoliation occur due to the disease, but the pathogen will overwinter in buds and create headaches for you in subsequent seasons.

Other than that, my recommendations for fungicide applications are similar to last week. See our website and scroll down to review and of course don’t hesitate to contact me with any questions!