Apple Disease Update: Week of August 23, 2021
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As Gala harvest concludes (and now that I’ve been able to do some additional research) this week I’d like to address two main topics. The first is going back to the question posed a few weeks ago: Do we need to continue to spray for Glomerella leaf spot following harvest? In short, my advice would be to continue to spray a protectant fungicide (e.g. Captan) every 14-21 days following harvest on Gala or cultivars harvested within a few weeks of Gala. I’d particularly advocate for this if you have incidence on your trees less than 20% or so. Anything more, my thought is that any overwintering inoculum is likely going to be quite high (prior to interventions such as flail mowing and urea applications) that it’s not likely worth your effort or money.
I promise the reasoning behind this advice is not because I enjoy watching you juggle harvest and additional applications at the same time. During the past week I have gone and sampled leaves with GLS lesions from the leaves and the ground. In both locations, there was evidence of acervuli (the fungal structures that contain the asexual spores of the pathogen-see photo directly below). When I ruptured the acervuli, mature conidia (those asexual spores that appear salmon/orange on bitter rot lesions on fruit) were discovered. This means that pathogen is still out there and able to infect (as evidenced by the lesions on young leaves in the top photo). What we are working to discover is how consequential these GLS lesions are to late season and overwintering inoculum load. In other words, if these late season infections to not result in the formation of acervuli, perithecia (overwintering structure) or conidia, then it may not matter! To answer this our plan is two-fold: we will 1) sample symptomatic young leaves every two weeks and evaluate for the presence of acervuli and spores and 2) remove young symptomatic leaves every two weeks and put them in storage boxes outside of our lab to better understand the development of perithecia (overwintering structure of the pathogen) in regards to late season infections. I will provide updates through these posts on objective one as we get data this season.
The second topic I’d like to address today is your favorite: “What to spray next”? Despite the recent rain, I encourage you to refrain from applying any more than four TOTAL applications of a strobilurin (FRAC 11) fungicide this season. To review, this means do not apply any more than FOUR applications of any combination of the following fungicides registered for apple: Merivon, Luna Sensation, Flint Extra, Sovran, or Pristine. We have increasingly been diagnosing local outbreaks of strobilurin resistance in region and it would be devastating to our industry to lose this product to resistance. Furthermore, resistance to strobilurins has been found to be quite stable. This means that even if you refrain from applying strobilurins for a year or two, the resistance is not likely to go away. If you are at your stroby max for the year and you are harvesting within the week, my suggestion would be to apply a 3/4 rate of captan (e.g. 3.75 lb of Captan 80WDG per acre) PLUS 3-4 pts of ProPhyt (or another phos. acid product). If you are not approaching harvest, consider a captan + ziram (1/2 rate of each) or 3/4 rate of captan by itself. If you still have additional applications of a strobilurin and you’re harvesting this week (and you don’t have resistance to strobilurins) my suggestion would be Merivon plus a half rate of captan.