Fortunately we are not yet seeing any symptoms of GLS like those in the photo throughout western NC. While temperatures in Henderson County, NC are not as high as originally predicted for this upcoming week, given the rainfall and daytime highs in the 70s, it would be a good time to begin initiating GLS-specific applications this week. What does that mean and why am I suggesting it? Well through our years of GLS research in the NC State Tree Fruit Pathology Lab we’ve learned the following:

• Mancozeb and captan, even at half rates, work well for GLS control
• Mancozeb should be at its annual maximum allowed limits for the 3 lb/A rate (21 lb/A per season) within 77 day PHI
• Finish allowed manxcozeb applications prior to switching over to captan for the season (an exception may be for ‘Red Delicious’ where captan may be needed earlier for Alternaria leaf blotch management)
• FRAC 11 fungicides are most efficacious GLS and bitter rot control in North Carolina. Fungicides containing the FRAC 11 pyraclostrobin (Merivon and Pristine) performed better in our fungicide trials than fungicides containing the FRAC 11 trifloxystrobin (Flint Extra, Luna Sensation) for GLS control. For cultivars that have bitter rot, but no GLS symptoms, these fungicides performed similarly. The FRAC 11 fungicide, Sovran, in general has not shown as high of efficacy for GLS or bitter rot control in the southeast.
• Some orchard blocks in western NC have populations of Colletotrichum fructicola that are resistant to the FRAC 11 (strobilurin fungicides). This means that NO FRAC 11 fungicides will be effective against bitter rot or GLS caused by this fungal species. In 2024, we only found resistance to strobilurins in bitter rot and Glomerella leaf spot caused by C. fructicola. In blocks that don’t get the leaf spot, but have the more typical bitter rot symptoms only, the disease was caused by predominantly by Colletotrichum fioriniae. As of April 2025, we have not detected resistance in this species in NC. That said, this species can develop resistance to the strobilurins as can other species that cause Glomerella. The main exception to this was ‘Honeycrisp’ where we’ve seen bitter rot caused by C. fructicola and it has been resistant.
• Aprovia is the only SDHI fungicide that we have evaluated to have efficacy against the Colletotrichum species causing GLS and bitter rot on apple in NC
• Omega fungicide at its highest rate has good to excellent efficacy against bitter rot. Results have been variable in GLS trials in the southeast.
• FRAC 3 (DMI fungicides such as Inspire Super, Cevya, Rally, etc) do not control GLS and bitter rot in NC
• Starting around petal fall, applications must be made on a 7 to 10 day interval even if there is no rain. Leaf wetness matters when it comes to infection by the GLS fungus and the development of disease symptoms. When leaf wetness and conducive temps align, disease can occur.

Taken these observations into account plus the fact that the forecast is calling for heavy rainfall which will likely remove mancozeb residues faster than locally systemic fungicides, I’d consider the following for fungicide sprays this week:

• Option 1: For cultivars that get Glomerella and if you don’t suspect resistance in the block (from 2024): Merivon 5.5 fl oz/A + mancozeb 3 lb/A
• Option 2: For cultivars that get Glomerella and you either suspect resistance or had resistance confirmed in 2023 or 2024: Aprovia 7 fl oz/A + mancozeb 3 lb/A OR Omega 13.8 fl oz/A + mancozeb 3 lb/A
• If you suspect or know you had strobilurin resistance confirmed in 2023 or 2024 in blocks with GLS or had bitter rot in ‘Honeycrisp’ that was resistant to strobilurins: Consider applying “Option 2” to all nearby ‘Honeycrisp’ and to all nearby cultivars that get Glomerella. Consider “Option 1” for everything else since it’s still the best program when populations are sensitive to strobilurins.

Lastly, please review the blossom blight infection alert for this week. If you still trees with open flowers, they are susceptible. EIP values are well above the 100 threshold value for the week, so getting an application of streptomycin on these trees only before the rain or before a spray would be a good idea. It’s also time for Kudos applications to protect against shoot blight infections. Dr. Annie Vogel provided a fantastic recap of her research with Kudos in young trees last week, and it’s certainly worth a read. If you haven’t gotten an application of Kudos (prohexadione calcium) out yet, I’d make an application sometime this week on nearly all cultivars.