Apple Disease Update: June 30, 2020

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Apple showing unknown rot disease

This week serves as a reminder to stay the course and DO NOT let up on 7 to 10 day summer cover applications. Even if we do not get substantial rainfall over this time period the combination of moderate nightly temperatures (65F deg +) and dew formation on leaves creates an ideal microclimate for infection by the GLS/bitter rot fungus and symptom development of these diseases. In our GLS/bitter rot research trial on ‘Gala’, we are seeing few if any symptoms on trees in which we have predominantly applied multi-site protectant fungicides (i.e. captan, mancozeb, captan + ziram) with two applications of Merivon (at 1st and 3rd cover) alternated into the program. I’ve received a few questions regarding spray recommendations and resistance management over the past few weeks and I’ll address those for the rest of this post. If you have any additional question email me directly ( OR post them in the Feedback section and I’ll be sure to address those in next week’s post.

What should I be Spraying this Week?

Let me start off by saying that this is among my LEAST favorite questions to answer for the following reasons:

  • I am not sure of pathogen resistance profiles in your specific block or orchard. Many resistance problems are location-specific, meaning that you could have strobililurin (e.g., Flint) resistance in one block and not in a block a few hundred yards away. If you have reason to suspect resistance, please be in touch.
  • Fungicide programs are highly reflective of weather patterns/the environment. For example, applications of Merivon to control bitter rot and GLS may be best timed prior to an epic rain event or harvest rather than relying on a calendar schedule.
  • Fungicide programs should be tailored for individual grower goals. U-pick, fresh market, processing, organic, or homeowners will have different thresholds for disease severity, spray intervals, russet severity, and input costs
  • I am challenged by not wanting to endorse any specific fungicides-particularly when these fungicides in our trials provide statistically equivalent levels of control. This is why I usually write/speak in terms of FRAC codes while providing some examples in parenthesis.

All of this said I’ve provided a potential spray guide that maintains seasonal application limits for early-, mid-, and late-season harvest cultivars that can be found here: Apple Spray 2020. Again, this is a general guide and reflects a perfect situation with no major rain event and no resistance.

What if strobilurin (aka QoI; aka FRAC 11; aka stroby) resistance has been confirmed in my orchard?

It’s important to keep in mind that stroby resistance will be confirmed for a particular fungal pathogen or pathogen population in your orchard. Thus, just because your “bitter rot population” (e.g., Colletotrichum fructicola) is determined to be resistance to the strobies, it does not necessarily follow that your apple scab population (e.g., Venturia inaequalis) will have resistance. If you have “stroby” resistance to your bitter rot/GLS population the following fungicides SHOULD NOT be controlled to apply the disease:

  • Flint WG, Flint Extra, Sovran, Pristine, Merivon, Luna Sensation

If you have stroby resistance in the fungi causing GLS and bitter rot, you will have to predominantly rely on multi-site protectants (mancozeb, captan, captan + phos acid., captan + ziram). The mutation that causes resistance to these fungicides is quite stable and does not seem to impart a fitness penalty, meaning that resistant dudes can compete equally with sensitive dudes.

Can you simplify these FRAC codes for us?

I was asked by an NC apple grower last week if I could provide “color codes” rather than referring to FRAC groups by name or number. Below is a prototype that does not have all of the fungicide groups listed, but has the ones that I consider are regularly applied here in NC. I grouped the FRAC codes by colors-notice that FRAC 7 + 11 is purple (since red (FRAC 7 coding) + blue (FRAC 11 coding) = purple. Please provide any feedback of what else you’d like to see or how it can be better arranged. My hope is to procure some funding to provide you with laminated posters for next field season.

FRAC Groupings