Source: NCSU COOPERATIVE EXTENSION, 10/11/21


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.

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