(I) Major Apple Scab and Cedar-Apple Rust Infections 25-28 April; (II) If Any Trees Are Still in Blooom, Add Streptomycin

As we were getting closer to 48 h away from rain forecast for 25-28 April, the model predictions for apple scab infection stabilized and are now destined to occur. With these rains cedar apple rust infections will also occur. This seems to be a uniform front of weather coming from west to east across all Virginia. If you have any apple trees still in bloom add streptomycin to your tank, before the rains start. This is needed as these rains will trigger significant fire blight infection events for any apple tress in bloom. Most apple cultivars are at Petal Fall but young trees are still in bloom.

At petal fall, I would recommend using Cevya, or Inspire Super, or Rally, or Sonoma, or Topguard, or Rhyme, or Procure plus mancozeb (stop mancozeb at 77 days before harvest for early maturing cultivars). This will cover you for scab rust and powdery mildew infections. In regions where you think Rally is not that effective add Microthiol Disperss to your spray tank in addition to DMI fungicide and mancozeb. Using DMI fungicides at petal fall and/or first cover is be the best option because they are systemic fungicides. Powdery mildew can have an extended infection periods in highly susceptible cultivars until first or second cover, so reserving DMI fungicides for use by second cover in these cases is necessary. If you had issues last year with powdery mildew you should have started DMI fungicides from tight cluster and continue applying them until second cover.

If powdery mildew was not an issue last year, Inspire Super or other DMI plus mancozeb would still be my recommendation for the first cover (10 days after petal fall). If mildew was a bigger problem last year and not cedar apple rust, use SDHIs: either Aprovia, Fontelis, Sercadis, Miravis or Excalia plus mancozeb (3 lb/A). Add a surfactant (e.g., LI700) to these SDHI fungicides as it allows their longer residual activity. SDHIs are not effective for rust.

For scab, consider using Axios this year – it is a new fungicide in FRAC 52 group and is very effective. Only three applications per year are allowed. Incorporating Axios into spray program will help prevent fungicide resistance to FRAC 3 (DMIs) and 7 (SDHIs). At this time of the year systemic fungicides here are much needed for severe scab events like we expect 25-28 April because with warmer temperatures the green tissue will increase its surface area intensely, and the DMIs and SDHIs (if you add attractant) will enter your leaves systemically, while they expand, and protect them through the multiple rains. The idea is to have enough fungicide residue cover to stay on the leaves through the oncoming infection periods. This blog is based on the outputs from the NEWA apple scab model available at https://newa.cornell.edu/apple-scab

I decided not to post screen shots this time as I want you to get used to using these models yourself – it is imperative that you use them and get comfortable interpreting them. If you have any questions you can contact me, using usual channels.