Vermont Apple IPM: Big disease events ahead

I’ll be relatively brief, since everyone needs to do the same thing. With orchards at tight cluster to early pink bud stage (I appreciate everyone who is reporting their stages at this link), more significant heat, and a rain event expected late this weekend, we are looking at a double whammy of apple scab and fire blight infection events. Here’s me take, and it has a lot to do with your conditions and how quickly you can cover your orchards.

I don’t like spraying, especially fungicides, especially captan or sulfur fungicides, in the heat. If you can get a good coverage on in the next couple of days in the cooler parts of day, that would be helpful. Tissue is expanding rapidly now, so contact material that redistribute with rain would be useful tools to keep some coverage on. That would be captan or maybe sulfur (if organic), which may be a little too close to an oil (within 7-10 days) application to use. Next preference would be mancozeb, which tends to stick to plant tissues a little better. I would consider adding one of the more advanced single-site products in this spray to help with rust and powdery mildew as well- consider a strobilurin, SDHI, or DMI material, tank mixed with that contact material I just referenced. None of those are available to use in organic systems, so sulfur it is.

The plant growth regulator prohexadione-calcium (Apogee, Kudos) helps to reduce tree vigor at full rate, but even at ½ rate applied pre-bloom can help to thicken plant cell walls, which reduces susceptibility of growing shoots to fire blight. It would not be a bad idea to include this in the fungicide spray.

Fire blight protection needs to be applied to open blossoms 24 hours before or after a wetting event. Assume that you are in a full red alert infection potential after Friday, so any open blossom should be treated within a day of wetting. That means being ready likely Saturday or Sunday to apply first streptomycin (only choice I recommend if not organic) spray, if organic, I would consider alternating a ‘sanitizer’ like low-rate copper (Cueva, Badge, etc.) if growing for cider or russeted fruit are not a concern, oxidate, or low-rate (0.5%) lime sulfur followed up within a day with a biological like Blossom Protect, Serenade, or Double Nickel. Strep sprays should contain Regulaid or another wetting agent- that may exacerbate heat-related phytotoxicity from captan.

All of this is dependent on when your blossoms open vs when the rain comes. Some growers may want to put everything- fungicide(s), bactericide, wetting agent, PGR, into one tank. That’s doable but can be risky, especially in regards to phytotoxicity. Then again, that may be the best tactic at this point. Notice I did not mention insecticides nor foliar nutrients in these sprays. I don’t recommend either. For most retail-oriented growers, a little tarnished plant bug won’t affect you fruit value. For the few wholesale growers, you know if TPB has been a problem and have already been ready with a pink spray regardless of what I say. Given the state of pollinators and that the blossoms are right around the corner, I’d steer clear of insecticides unless you know you need one, and even then, do notapply if anything so much as a dandelion is blooming.

Start thinking about your thinning needs soon, but it’s too late for me to make any suggestion there, yet.

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