Apple Disease Update: Week of July 13, 2025
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Another warm, humid, and in many areas wet couple of days. Although I wrote a post last week, it doesn’t seem like the system automatically sent it out to those registered to receive updates. There was some good information in the post regarding PHIs and locally systemic fungicide selection which is still relevant for this week. If you missed it, you can check it out here.
In scouting my orchards last week, I didn’t observe anything “new” in regards to diseases popping but, it incidence and disease severity was certainly increasing in our research blocks. Treatments in our Marssonina leaf blotch trial are beginning to display differences. While the Captan 80WDG treatment has been providing ok control, treatments including the DMI fungicides Cevya and difenoconazole (Inspire Super) as well as the SDHI fungicide, Tesaris, continue the multi-year trend of showing the greatest efficacy against MLB. We have included Omega and Axios (UPL) this year, and will provide early results this week or next. While the fungus causing MLB can cause disease on fruit, we have not frequently observed fruit symptoms in NC. On highly susceptible cultivars such as ‘Rome Beauty’, premature defoliation due to MLB should be a concern as this can lead to:
- Compromised photosynthesis leading to poorer fruit quality
- Decreased cold hardiness and possible colonization and symptom expression of trunk and limb pathogens such as Botryosphaeria
- Sunburn on fruit
In our ‘Rome Beauty’ research orchard where there’s lots of inoculum, fungicide applications every two weeks with a rotation of Captan 80 WDG (2.5-3 lbs per acre) with Captan 80WDG (2.5 lb/A) + one of the fungicides mentioned above has resulted in excellent control.
In addition to MLB, keep in mind that symptoms of GLS and bitter rot are becoming fairly severe in orchards with sub-optimal control or where fungicide applications have failed. In the case of Glomerella leaf spot, ascospores are commonly produced on lesions, particularly once leaves have fallen. These spores can be be wind dispersed over long distances. This differs from the orange masses of conidia (spores) that are often observed on fruit which are frequently splash dispersed. If you see fruit on the ground or in the tree with bitter rot infection, I’d highly recommend removing them from the orchard and destroying. I know some growers will bury them outside of the orchard, which should also be fine.
It’s important to keep in mind that GLS and bitter rot infection can still occur without rain. Under ideal temperatures, infection may require less than 3 leaf wetting hours (LWH). In some locations in Henderson County, NC, 7 of 13 days so far in July have exceeded the approx. 3 hr leaf wetting requirement. That said, it’s important to maintain fungicide applications on 7 to 10 day intervals.
Lastly, some important fungicide preharvest intervals that may be approaching on some cultivars:
- Omega: 28 days
- Aprovia: 30 days