2020 Apple Disease Update: 1/2″ Green and Tight Cluster

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Tight Cluster on Gala

With the exception of ‘Rome Beauty’ and trees recently planted, nearly all cultivars in North Carolina have passed the green tip stage. If you have not yet applied a full rate of copper for protection against fire blight (and apple scab), my suggestion would be to skip it this year-particularly given the phytotoxicity/russet issues reported last year. If you do have ‘Rome Beauty’ still at green tip and haven’t applied copper to that block yet, the next day or two would be ideal to do so. Copper can also safely be applied with oil (for scale control). At the tight cluster stage, apple scab, apple rusts, and powdery mildew should be the primary targets of your disease management programs.
Apple Scab Primary Infection: Ascospores (a fancy name for spore) of the apple scab fungus have been maturing inside of fungal structures called pseudothecia for the past few weeks. If you’ve done a good job of breaking down and destroying your leaf litter during the dormant period then you likely have fewer pseudothecia that are just waiting for the opportunity to blow out their ascospores. While that’s great news since you’ve likely significantly reduced the total primary apple scab spores hanging out in your orchard block, make sure not to let your guard down. Protectant (or kick-back) fungicide applications will be necessary to continue to protect against primary apple scab infection events. Particularly if the rain continues this week, or you didn’t have an opportunity to apply a fungicide prior to yesterday’s or today’s rain.
For an orchard located in Hendersonville NC, I initiated the apple scab maturity model between 50-80% green tip on ‘Gala’ (March 13). As of March 24, ascospore maturity had reached 15% and four apple scab infection events had been predicted (one combined). An additional infection event is predicted for tomorrow (March 25). To check out predicted infection events at a location closer to you follow this link:
Weather Stations in North Carolina

NEWA Primary Scab Infection MOdel

If you are in the hunt for some fungicide options that will provide you good kick-back activity, consider the options in table below. My suggestion is to include mancozeb (3 lb/A) with these applications for resistance management. If you’ve had severe black rot/frog-eye leaf spot in the past, consider substituting captan for mancozeb, but keep in mind annual application limits (you may need it later on for bitter rot/GLS). If a range in rate is provided on the label, if you are applying fungicides as a curative/kick-back application, use the highest rate. Also, for resistance management and greatest benefit, apply the fungicide within 24 hours of the infection event. Kick back fungicide options for apple scab

Apple Rusts: In 2019, I continued to receive several inquiries regarding rust diseases on apples. Fungicide applications targeting cedar, quince, or hawthorn rusts should begin no earlier than tight cluster but no later than pink bud. Rusts are unique because they actually require two hosts to complete their life-cycle. Because of this requirement, the good news is that you don’t need to worry about a secondary cycle (as with scab and powdery mildew) and thus the window for fungicide applications is relatively short. If you have an alternate host (e.g. Eastern red cedar or susceptible junipers) for apple rusts within a couple miles of your orchard, infection is likely to be more severe. In general, FRAC 3 (DMI fungicides) should be your go-to for rust control. You don’t need to break the bank either. Some of the older fungicides like Rally 40WSP will do a fine job against apple rusts. Other options included Topguard, Procure, Inspire Super, Cevya, and Indar 2F.
Cedar apple rust

Powdery Mildew: More on this next post, but just keep powdery mildew on your radar. A few commercial ‘Jonagold’ orchards that I visited last year had an alarming amount of powdery mildew on the foliage, and I’m starting to wonder if some of the severe russeting on fruit we occasionally observe here in Western NC is partially the result of infection by the powdery mildew fungus.
Management of powdery mildew should begin around mid-late tight cluster/early pink. If it’s a warm, humid, but dry spring, powdery mildew infections are likely to be more frequent and severe. For those a little beyond the tight cluster stage, start scouting for primary powdery mildew infections which appear stunted, malformed and have a white-silver color.
Primary Powdery Mildew infection on apple
That was a lot of info for such a short post! To accompany the table on good “kick-back” fungicides for apple scab (above), here’s an additional list that provides efficacy information for all of the diseases discussed here. Of course, always apply with a multi-site protectant fungicide (e.g. mancozeb or captan), for resistance management. I am also not aware of specific resistance issues in your pathogen populations (which again is why tank mixing is a darn good idea)! For additional fungicide options please visit the 2020 Southeastern Apple Production Guide.

Efficacy table tc

Disclaimer: Those contributing to this newsletter have made every effort to provide accurate and up-to-date management recommendations. However, please keep in mind that pesticide regulations are constantly changing and thus recommendations set forth in this publication should not serve as a substitute for an actual pesticide label. In addition, factors such as host phenology, weather, cultivar, and tank mixtures might affect the efficacy and safety of an agrichemical product.