2020 Apple Disease Update: Petal Fall
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Congratulations and celebrate because primary apple scab season ended on April 12th in Western NC! However, please do not get lazy on managing this disease. The lesson of letting up on scab sprays too early was taught to myself and growers in NY back when I was working as a research technician in Kerik Cox’s program at Cornell. Like this year in NC, the primary scab season wasn’t too eventful-in NC we have had only had 7 predicted primary infection events so far in 2020. Because of this, spray intervals were extended more than normal. However, around petal fall that season in NY, rains started to come on heavy (similar to what we experienced this past weekend), releasing several spores (ascospores) with little fungicide residues remaining on the foliage. Combined with a lack of hot temperatures that summer in upstate NY, we saw several late infection events. It’s hard to believe, but even just a few missed early scab lesions can quickly lead to problems. If you hadn’t applied a single-site scab fungicide (e.g. Inspire Super, Aprovia, Flint) or you only applied a multi-site protectant chemistry (e.g. mancozeb) prior to the storms this weekend the final release of primary spores may cause you a headache down the road. Fortunately, if you continue to intensively manage scab and then follow up with normal GLS/bitter rot programs for the rest of the summer, you should be able to rest a bit easier.
Since the weather has been relatively cool, I’m reluctant to suggest adding any GLS/bitter rot fungicides (i.e. the strobilurins) to the mix just yet. DMI fungicides (FRAC 3) such as Inspire Super, Cevya, or Indar 2F should provide good to excellent control against apple scab, will provide excellent control against cedar apple rust, and are decent against powdery mildew-all three of which your apple trees are highly susceptible to at this time of year. Another option would be application of an SDHI fungicide (FRAC 7) such as Aprovia or Miravis. Both of these fungicides perform similarly to the aforementioned DMIs for powdery mildew management, but do not provide the bang that the DMIs do for rusts. Unlike the DMIs, we’ve not yet seen resistance to Aprovia or Miravis in the apple scab fungus. However, I’d still recommend adding a half rate of mancozeb (i.e. 3 lb/A) to your tank mix.
Next week we’ll cover GLS and bitter rot in depth! Happy Growing!!