Vermont orchard management: petal fall edition

By Terence Bradshaw

Bloom status in Vermont orchards is all over the map. Of course, cooler high elevation and inland sites are generally going into full bloom now while Champlain and Connecticut Valley sites are at petal fall (PF). Honeycrisp (in the Champlain Valley anyway) are lagging behind most other cultivars worse than usual: at the UVM orchard, we just started to see some king blooms yesterday while neighboring Mac and Empire are at full bloom and starting to think about PF. So this information may be more timely for some than others, but we’ll all need to be thinking about some complicated orchard management items in the coming week.

Disease: rain today, more on Tuesday may trigger an apple scab infection period, keep covered. Many Addison County orchards had enough rain and wetness from last Thursday’s (May 19) rain to trigger what was probably the biggest infection of the season. Depending on models, what you input for green tip date, and your trust in the weather forecasting/measuring systems used by NEWA and other sites, we are at about 85-95% apple scab ascospore maturity (with a wide range of deviation). Keep covered against rains for the next 7-10 days anyway. Fire blight infection is still a possibility in orchards with a history of the disease and open blossoms (even those stragglers blossoms, which I’m seeing a lot of). I discussed this last week, but keep protection against fire blight (strep, Serenade only if organic and you have no other option) on within 24 hours of rain or other wetting events in blocks with high-risk cultivars and/or any history of the disease. I don’t think this fire blight season will be a big one for two reasons: one, most orchards will be in full petal fall by the time the risk gets too high, and two, we didn’t have much fire blight at all around last year so inoculum is relatively low. Still, if I had a block of Honeycrisp, Gala, or late-blooming cider varieties I would consider treating. In fact, I’m spraying the UVM farm tomorrow morning and will be including strep In the tank since we have had a history of FB in one block with Gala, Cortland, and Mutsu and adjacent to a block with tall spindle Honeycrisp that I wouldn’t want to lose to save a little extra effort.

Insects: pretty quiet, but the extended bloom and many orchards with no protection may add up to a lot of damage while we’re waiting for all petals to fall to get in with an insecticide. Eric Boire is setting some traps around the Champlain Valley this year and reporting back to me; to date, he’s been clean on codling moth (CM), European apple sawfly (EAS), oriental fruit moth, and oblique banded leafroller in the few orchards he’s been able to deploy traps and check them. We’ll be checking traps tomorrow so my data is a week old, but we have been catching EAS and red banded leafroller in our orchard, some of which of course includes an unsprayed block. If I were spraying tomorrow, I would add a Bt product into the tank to give some protection against lepidopteran larvae (remember, Bt only works against lep larvae, not flying adults) until we can get a full petal fall insecticide on. The warm temperatures of the last couple and next few days will drive plum curculio (PC) into orchards, so a full-orchard insecticide once all of the bees and flowers are gone will be warranted. Organic growers can start applying Surround any time now to get the base coat built up before PC start to oviposit. Remember that Surround and Bt have relatively little effect on codling moth, which is likely only starting its flight now and thus has no larvae in the orchards to worry about. I mention this because a targeted CM spray will likely be needed in orchards with a history of that pest in a few weeks, we’ll discuss that as it comes closer.

Thinning: The $64,000 question this week, and likely worth a lot more than that. The reports I’m getting are that not only are the blossom stages all over the place, but blossom quality has been extremely variable too. Many of us have seen freeze damage likely from that April 5 snap, haggard-looking blooms, missing king blossoms, and ragged spur leaves. I’ve heard one report of open flowers with what appears to be no pollen in them. Many trees were likely stressed by an overabundance of crop last year, those trees and many others may be short on critical nutrients, although you should be applying foliar zinc, boron, and nitrogen and nitrogen, potassium, and whatever else is called for to the soil to keep things in good shape. The good part is that we have had some great bee flying days, although early blooming sites and cultivars were a little cloudy during king bloom but I think weather on May 14 was conducive for good flight.

With all that being said, here’s my take on things. Fruit set is likely to be all over the place, and given that, I would avoid a petal fall thinner on all but the healthiest looking trees (bloom-wise) and even then might think twice. Most thinners are likely to be more active, relatively speaking, than usual this year. The spur and early shoot leaves developed under cool, cloudy weather and thus likely have thin cuticles. Upcoming warm and at least partly cloudy days will increase carbohydrate deficit in trees which enhances thinning activity. And the generally weak-looking buds are likely to produce less set fruit. NEWA has a carbohydrate thinning model calculator component that, in most years, may help growers guide thinning decisions. However, I and most regional Extension professionals am suggesting that it not be relied on to make your final thinning decisions this year given the poor state of bloom and tree condition.

I’m going to be careful on specific recommendations except to say that multiple applications of mild thinners may be best this year. Look at the orchard in the first few days after petal fall. If you have time, tag some clusters and measure fruitlet growth with a micrometer and track the growth of those fruitlets after thinner application; fruit that stop growing in diameter a few days after application will likely thin off. When fruit approach 5-6 mm in size, a mild thinner application such as carbaryl, Maxcel/Exilis, or Amid-Thin (any one of those applied alone) may be your best bet. Another look at 10-12 mm may suggest that another application is needed, but I would be hesitant to apply a big shot all at once now. The 2015 New England Tree Fruit Management Guide has a good synopsis of thing options in Chapter 11, but keep in mind that any options this year should be on the conservative side. That said, no action may lead to overset of fruit, so don’t be afraid to do something in blocks that look healthy and have reasonable fruit set.

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