Apple Disease Update: July 14, 2020
In the previous Disease Update I sent a basic spray schedule for Glomerella leaf spot/bitter rot management for early-, mid-, and late- harvest varieties. In this schedule, please take note that in general, Captan + Ziram is rotated every 7 to 10 days with Captan + Prophyt (or equivalent phos. acid fungicide) or Captan + a single site fungicide such as thiophanate-methyl (e.g., Topsin). Of course,this is beyond that of applying a stroby-containing fungicide (blue or purple color code, e.g. Flint or Merivon) which should be incorporated at this point in the season prior to a predicted heavy rain period and in the immediate pre-harvest period.
While we have not observed a benefit in our fungicide trials of adding ProPhyt in tank mixture with a 3/4 rate of captan (on a 7 to 10 day spray interval), in regards to GLS and bitter rot control, the addition of a phos. acid or single-site fungicide with captan at least every other spray may be important for flyspeck/sooty blotch management. In addition to evaluating the efficacy of summer fungicide programs (petal fall to harvest) against GLS and bitter rot, we also take harvest data on flyspeck and sooty blotch incidence (see graph below). Fungicide applications commenced at 50% petal fall and continued on 7 to 10-day intervals until harvest on 21-yr-old ‘Tenroy Gala’ grafted to M7 rootstock. All programs were “non-rotational” meaning that within a treatment, the same fungicide was applied throughout the entire season. Nope, this is not considered a legal commercial program, but we wanted a direct comparison across different fungicide groups.
As you can see from the untreated program, there was high disease pressure as greater than 90% incidence was observed for both flyspeck and sooty blotch (FSSB) Including ProPhyt with captan reduced the incidence of flyspeck and significantly reduced the incidence of sooty blotch compared to captan applied alone. Incorporating a DMI fungicide such as Inspire Super or thiophanate-methyl provided the greatest control against FSSB, whereas other fungicides containing active ingredients in other FRAC groups (e.g. Aprovia, Merivon) tended to reduce the incidence of flyspeck to a much greater extent than sooty blotch.
To simplify things, I’d consider adding “something” to captan every other spray. However, if you’d prefer to be more exact on your timings, NEWA has a FSSB model available for your enjoyment: Apple Diseases. If you want some further info check out this FSSB post by Rachel Kreis from 2018.
Lastly, we’ve been receiving several reports of bitter rot outbreaks on Honeycrisp. Given the cultivar this isn’t necessarily surprising-although with the fungicide programs of apple growers here in NC this shouldn’t be happening. If you do notice control failures please let me know ASAP and we’ll head out and collect some samples for species ID and a fungicide sensitivity profile.