Apple Disease Update: Week of July 6, 2025
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It appears as though we’ve reached the afternoon rainy season here in western NC. While the rain directly aids in the disease infection process by bouncing conidia (asexual spores) to a new location, it’s the more indirect effects of extended leaf wetness periods that are particularly worrisome in regards, particularly in the case of Glomerella leaf spot. With temperatures often hovering in the mid-70s during afternoon rains in the Mountains, these extended leaf wetting events can lead to infection events by the Colletotrichum fungi causing Glomerella leaf spot and bitter rot.
That said, it may be worth including a single site fungicide in your tank with Captan 80WDG this week. If you don’t have resistance to strobilurins (FRAC 11), Merivon (5.5 floz/A), Luna Sensation (5.8 fl oz/A) or Flint Extra (2.9 fl oz/A) would be good options. However, I always prefer to save one strobilurin application (Merivon or Pristine) for the week prior to harvest as the activity of these fungicides last longer in storage. Therefore, if you’ve made two or fewer strobilurin applications this season, go with that. If you’ve already applied more than two strobilurin applications this year, or you have suspected or confirmed resistance to strobilurins (FRAC 11), consider Aprovia (7 fl oz) this week. However, please keep in mind that Aprovia has a 30 day PHI.
Since Ziram has been playing hard to get, I’ve been pondering a new alternative to captan for resistance management. Especially in late harvest cultivars where you may run out of your annual allowance of captan. Initially I was thinking that Omega could be a reasonable option in tank mixture with captan, but more recently given it’s decent efficacy in our GLS field trials and that it is a multi-site fungicide with a low resistance risk, I’m thinking Omega may be a reasonable alternative to Ziram, if you need to rotate away from captan for a spray. In our bitter rot and GLS field trials this year, Captan 80WDG applied weekly from bloom-today has so far provided the best control of GLS (97% control), whereas Omega applied weekly beginning at petal fall has provided 82% control. While not as efficacious as captan, Omega might have place when applied with a FRAC 11 (strobilurin) or Aprovia for resistance management. However, please keep in mind that Omega has a 28 day PHI.
Lastly, if you are concerned about strobilurin resistance in your particular Colletotrichum populations causing GLS and bitter rot, please get in touch with me, your consultants, or your extension agents. We are happy to screen for free (to you).