When it rains, it pours-figuratively and literally as tropical storm conditions are anticipated for the end of the week. I also say figuratively as we have some early survey results in regarding strobilurin resistance in the area. But first, some thoughts on managing the upcoming rains:

• I’m assuming that most if not all of you have ‘Evercrisp’, ‘Pink Lady’, and likely Fuji still to harvest. Two very different Colletotrichum fungal complexes cause bitter rot on these apples-one that’s associated with Glomerella leaf spot (you may even call it Glomerella fruit rot), and the other that leads to the more spectacular “old school” bitter rot lesions like in the above photo-often you’ll see salmon/orange colored spores in concentric rings. In western NC, bitter rot on ‘Pink Lady’ is often caused by a different species or fungal complex of Colletotrichum than what you’d observe on ‘Evercrisp’ or ‘Fuji’. I’m not just telling you this because I enjoy typing, but because in our surveys so far, we’ve identified several of the Glomerella associated Colletotrichum fungi with resistance to the strobilurins (FRAC 11, Sovran, Flint, Luna Sensation, Merivon, Pristine….). If you have time to make a fungicide application before the rains, I would strongly advise applying a full rate of Captan with either Merivon, Luna Sensation, Flint, or Pristine on cultivars that get Glomerella. If you have 30 days until harvest, I would consider applying Aprovia with a full rate of Captan on these cultivars until we can screen for resistance.
• On cultivars where you see the ‘old school’ bitter rot lesions, we haven’t identified strobilurin resistance in multiple orchards. That said, we haven’t done extensive surveying of apples with these symptoms simply because there hasn’t been as much widespread concern in western NC. Other apple production regions do have resistance to the strobilurins so make sure you apply at least a half rate of captan with the strobilurin to these trees for resistance management.
• Don’t rely on captan alone for this upcoming tropical storm. 3-6″ of rain that is predicted can even overwhelm the most efficacious single-site fungicides that are locally systemic. As a strictly protectant fungicide, captan is a sitting duck for washing off. Our research is still ongoing with spreader/stickers, but consider putting your favorite in the tank for some added stickage.
• Lastly I would consider making this a dilute fungicide application rather than spraying concentrate. In some of our experiments this summer, captan was not getting redistributed as much as we’d anticipated it would on our fruit-particularly on the back side facing away from the row middles….

Ok-now for resistance issues….I’m hesitant to make too many recommendations or conclusions as we are still in the process of surveying and screening and don’t want to create any panic. That said, being vigilant and proactive to reduce selection pressure towards resistance is critical. Back when we did our original surveys in 2017 to identify the Colletotrichum species causing GLS, we identified a fairly recently characterized species called Colletotrichum chrysophilum causing the majority of GLS in western NC. Of the nearly 400 isolates we investigated, a single isolate had the mutation within the strobilurin target gene that causes resistance to this fungicide class.

With the GLS severity this year, the easy out would have been to blame it on the weather and certainly that’s partially the case. Or to blame your apple pathologist for suggesting a spray program that couldn’t keep up with the July rain. Certainly that’s also partially true. However, things are more complicated than the “easy answers”. We’ve been isolating Colletotrichum from orchards with GLS since early June. Unlike our original surveys we have found resistance to strobilurins (through DNA sequencing) in nearly all isolates sequenced. Furthermore, in the samples we’ve screened so far, the predominant species causing GLS has changed to Colletotrichum fructicola. So, not only was resistance to strobilurins selected for, but also a new dominant causal species. Again, I stress this is only in the 10 or so orchards that we have so far surveyed.

Tomorrow (Sept. 25) we were supposed to head out with Dr. Terry Kelly (Director of Henderson Cty Extension) to begin a wider survey of fruit and leaves with bitter rot/GLS.Given the rain forecast, we are postponing that until Wednesday October 2. Some of you have already been in touch with me about us surveying your orchard. Others please continue to get in touch if you’d like a visit. Any results that link a particular farm/grower back to strobilurin resistance (or sensitivity) will be kept anonymous.

Thanks again for your support and hang in there!