Major Apple Scab Infection with Rain Events 28 – 30 April, Along With Cedar-Apple Rust Infections
Apples are at Petal Fall or beyond on early cultivars at in Winchester. From 28 – 30 April 2023, we are expecting multiple rain events followed by the slow drying conditions as per the NWS forecast visible at this moment. This is predicted to lead to major scab infection in all of Virginia. This will also be a significant rust infection event. At petal fall, I would recommend using Rally, Sonoma, Topguard, Rhyme or Procure plus mancozeb (stop mancozeb at 77 days before harvest for early maturing cultivars), but if powdery mildew was not an issue last year, you can use Inspire Super or Cevya plus mancozeb instead which is best option for scab and rust control. Another tank mix consideration is Fontelis, Sercadis, Miravis or Excalia plus mancozeb for scab and powdery mildew control at the same time. Juniper rusts can have extended infection periods up until 1st or 2nd cover, or longer. We are in a year with dry spring when cedar galls have not fully come out yet, at least in Winchester area. Warm wet weather at petal fall can also allow Botrytis to infect the dying petals and move from petals into sepals where it can remain quiescent until fruit are moved into storage where the Botrytis can cause gray mold storage decay. Using Inspire Super at petal fall and/or first cover is be the best option because it contains cyprodinil, a fungicide that is effective against Botrytis (if Botrytis is not resistant to this fungicide). 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 powdery mildew was not an issue last year, Inspire Super plus mancozeb would also be my recommendation for the first cover (10 days after petal fall). If mildew was more of a problem last year for you, use Aprovia, Fontelis, Sercadis, Miravis or Excalia plus mancozeb (3 lb/A).
Keep in mind that the systemic fungicides here are preferred for severe scab events like this because with warmer temperatures the green tissue will increase its surface area intensely from 28-30 April (1), and the DMI will enter your leaves while they expand and protect them through the multiple rains (2). The idea is to have enough fungicide residue cover to stay on the leaves through the oncoming infection periods. If rust is not of a concern for your location, as maybe no cedars are close to you, mancozeb (3 lb/A) plus Fontelis, or Sercadis, or Miravis, or Excalia, would be a better choice for scab control (all these single-site SDHI fungicides will not be effective on juniper rusts). This blog is based on the outputs from the NEWA apple scab model available at https://newa.cornell.edu/apple-scab:
