Apple scab management- It’s getting real out there

By Terence Bradshaw

Last week, apple buds in the traditional growing regions in the Champlain and Connecticut Valleys and Bennington County moved into the green tip stage which triggers the beginning of primary infection season for apple scab. Many weren’t ready to protect from infection during the rainy window of Wednesday through now, and in orchards with little scab last year and where little tissue is out, you’re probably fine if you missed getting some fungicide protection on. It’s looking like this rain will wring itself out sometime tomorrow Monday April 30 and we’ll have a dry slot during Tuesday (with low winds, too) and Wednesday to get covered. Wednesday we’re expecting temperatures in the upper 70s, which means that buds will open pretty rapidly. Then, more rains are expected later in the week. All along the way, apple scab ascospores are maturing and will be ready for the next wetting event to infect this rapidly expanding tissue.

Growers at green tip or beyond as of now should get covered with a protectant fungicide early this week ahead of the rains. Copper should not be used at full field rates (4-12 pounds per acre, depending on the product) after the half-inch green bud stage, so if your buds are pushing fast and/ or/ you’ve already applied copper this season, put it away. But many orchards can probably get by with it applied Monday through Tuesday. Otherwise, a protectant fungicide like mancozeb or sulfur (latter if organic) should be used ahead of the rains. If you had substantial scab last year and missed coverage prior to this current wetting event, and you had significant green tissue showing by Friday April 27, you may want to add an anilinopyrimidine fungicide like Scala or Vangard (FRAC class 9) to this mix, but know that any infection that started late last week is too advanced for the fungicide to work really well against when applied five days later. Therefore, I recommend that tactic in only the highest inoculum orchards, the AP fungicide would have best been used no later than Friday or Saturday. I do not recommend the use of lime sulfur to burn out an infection I this low-risk scenario- it just isn’t worth it. But really, if you missed this last one, know that the risk is relatively low and you’d be best to put your efforts into protecting against the next ones.

Oil can be applied at any time now, either alone or in your fungicide (but not with or within ~~7 days of a captan or sulfur application) to help manage mites and scale. If applying with your fungicide, know that oil needs to be applied in as dilute a spray as possible to fully soak overwintering insects and eggs, so your calibration needs to be adjusted so as to not over apply fungicide at the same time. Oil rate of 2% by volume is fine now, but should be decreased by 1/2% for each advance in bud stage until; tight cluster, after which it shouldn’t be used save for summer rescue applications.

For orchards in the cooler upland regions where buds are still closed, this is a great time to get copper on your trees. It’s also a great time for everyone to flail mow or apply urea to leaf litter to aid in decomposition which will reduce scab inoculum.

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