Fall Glomerella and Bitter Rot Update

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Symptoms of Glomerella Leaf Spot later in the season. Left Photo: Leaf Happy October and end of harvest to all apple growers reading this post. I wanted to take a few moments today to address the Glomerella leaf spot and bitter rot concerns growers throughout the region have been experiencing this season. The first thing I want to remind everyone of is the Disease Triangle. Remember, for a disease like GLS and bitter rot to occur, three things need to happen:

  1. The pathogen needs to be present. The Colletotrichum pathogen(s) causing bitter rot and GLS are typically always present and active during the spring, summer, and fall in Western NC. It’s important to realize that the pathogen hangs out and is present on hosts other than apples: weeds and grasses, ornamental and plants along wooded borders, other fruit and vegetable crops, etc. It even hides asymptomatically in the leaves or apple cultivars that do not show symptoms of GLS (e.g. Honeycrisp and Rome Beauty). So, even if you don’t see any bitter rot or GLS symptoms on your apples, spores may be coming from somewhere else.
  2. Increasing acreage of susceptible hosts (e.g. cultivars). Besides the normal GLS-susceptible cultivars historically grown in the region, growers are increasingly becoming daredevils and planting popular “new” varieties such as ‘Honeycrisp’ and ‘Evercrisp’ to accommodate consumer demand. We are beginning to discover that some of these newer cultivars tend to be infected by more aggressive Colletotrichum species which also are less susceptible to strobilurin fungicides. They may even have a shorter incubation/latent period which may further effect fungicide timing.
  3. A conducive environment for disease development must be present. This year we seemed to have the trifecta: Early freezes resulted in a lost crop, which resulted in a more relaxed fungicide schedule on the affected trees, which resulted in more inoculum available for trees with fruit. Furthermore, the constant rain, heat, and humidity did not prevent favorable conditions. While it’s normal to consider the weather only, I urge you to also consider the microclimate of your tree. To a fungus, leaf wetness and heat from dew is largely similar to that of rainfall (with the exception that rainfall facilitates spore dispersal more than dew). In a practical management sense: Even though it’s “dry” this does not mean you can wait 14, 17, or 20 days between fungicide applications. Even in June, when we had relatively little rain, conditions were still ideal for infection to occur.

While this was a challenging season in many respects, the bottom line is what can be done in future seasons to avoid catastrophic losses. I’m still confident that a 7-10 day application interval using mancozeb, captan, and strobilurin fungicides such as Merivon beginning at petal fall will provide excellent protection through at least the end of September. Certainly, if a tropical storm is coming through the area or days of rain are predicted, additional attention to fungicide decision and application interval will have to be considered. Many of the failures I responded to this year were associated with a lapse in spray coverage: 14-20 day intervals earlier in the season, flooding of fields due to our tropical storms, etc. At the MHCREC, we maintained 7 to 10 day fungicide applications all season and little to no disease was observed in blocks with highly susceptible cultivars that were planted adjacent to my “Glomerella Gala” research block. Next year, my hope is to conduct demonstration fungicide application interval trials throughout the county.

It’s also important to “phone a friend(s)” to share research ideas and management recommendations. My colleagues throughout the east coast are recommending the same or highly similar programs as I do here in NC. Basically, multisite protectant fungicides and strobilurins are reliable when applied on a tight interval and initiated at or around the petal fall period. There is certainly room for improvement though in our recommendations and as such we have a team applying for both state and federal funding to i) Determine when spores are released from different overwintering locations. This will help recommendations as to when you should initiate tightening up spray intervals and ii) Monitor captan residues under different environmental conditions to help you better gauge how frequently to spray.

In the meantime, I obviously am here to assist you as much as possible through the winter and in regards to your spray schedule next year. Starting in November I will be sending out a spreadsheet where you can sign up for a meeting with me (Zoom or in person) where we can evaluate your GLS/bitter rot management plan from this previous season and identify areas where things may be improved. Of course, I’m happy to also chat anytime I myself am not in a Zoom meeting:).

Hang in there!