Apple Disease Update: Week of August 2, 2021

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Bitter Rot on AppleAlthough temperatures this week are predicted to be below average, I would encourage you to not let your guard down with summer disease management. Even today, temps in the low 70s and the misty rain conditions provide an opportunity for the bitter rot/GLS pathogen(s) to germinate and cause infection. Last week was a fairly long post-my plan this week is to quickly provide some fungicide options and then go into a bit more detail as to why I suggest Merivon + Captan vs. a straight-up captan application if you are near harvest…..

Fungicides Game Plan for this Week

  • If you are not harvesting within the next two weeks, consider a Captan (1/2 rate) + Ziram (3 lb/A), a Captan (1/2 rate) + a FRAC 3 fungicide such as Inspire Super or Cevya (for flyspeck sooty blotch control) OR Captan (3/4 rate) plus ProPhyt (3 to 4 pt/A for flyspeck/sooty blotch control). These options will have good to excellent efficacy against GLS, bitter rot, and Marssonina leaf blotch. If you applied a fungicide specifically for flyspeck/sooty blotch control last week, you should be fine to leave out an additional fungicide this week.
  • If you are harvesting within the next 10 days or sooner, consider a Merivon (full rate) + Captan (1/2 rate) application. The Merivon will provide extended protection during storage (see below). Note-other FRAC 11 (stroby) fungicides will also have excellent efficacy against bitter rot and GLS but there is a 14 day PHI.

So why am I suggesting you open up your wallets and opt to include Merivon in your final 1 to 2 fungicide applications prior to harvest? It is known that the active ingredients in Merivon-pyraclostrobin and fluxapyroxad- have a longer half life than that of captan, meaning that they will break down more slowly. Certainly, cold storage will slow the breakdown of both captan and Merivon (think about why dead bodies are stored in the cold prior to embalming and burial). If you were to harvest all apples of a particular cultivar the day after spraying and chill them immediately, then differences may be negligible. Unfortunately, harvests rarely work that way and factors such as labor availability, crop size, fruit maturity, weather conditions, and the nature of CA storage cause fruit (and thus fungicides) exposed to the outdoor environment for an extended period of time. This provides an opportunity for common agents of fungicide breakdown-alkaline hydrolysis (high pH + water), non-pathogenic microbes, uv radiation, and heat- to more quickly reduce the availability of the applied active ingredient. Furthermore, many of these conditions (rain, dew, heat) can also expedite the appearance of symptoms.

In 2017 and 2018 we evaluated the efficacy of pre-harvest fungicide applications for post-harvest bitter rot control. To speed the trial up, we selected fruit that had early bitter rot symptoms (the small depressed spot) and kept the apples at room temperature for 14 days. In 2017, applications were made on a 14 to 21 day interval beginning at petal fall and continuing until two days prior to harvest. In 2018, applications were made on a 7 to 10 day interval beginning at petal fall and continuing until two days prior to harvest.

2017 Post Harvest Bitter Rot Incidence

Incidence of bitter rot postharvest 2017

2018 Post Harvest Bitter Rot Incidence

Post harvest incidence of bitter rot, 2018

Over this two year study we learned the following:

  • Of the fungicides evaluated, Merivon provided a period of protection against bitter rot that was greater than captan or captan + phos. acid
  • If applied on extended intervals, the addition of ProPhyt in tank mixture with captan may provide greater control of bitter rot in storage vs. captan applied by itself
  • Captan residues may stick around longer than we thought and have an additive effect. In 2018, more applications of captan were made during the season in comparison to 2017 and the incidence of post-harvest bitter rot was lower than the previous year.

As always, please feel free to email me ( with any additional questions!