This was meant to be sent out with some field pictures yesterday morning, but my teaching job and administrative duties all collided into a backup of worktodo this week. Hopefully it’s timely enough to be useful. -TB

After a few decent days of pollination during bloom last week in between the cold, cloudy, showery weather, I saw good fruit set on most cultivars at the UVM orchard over the weekend. Short of measuring and tracking the diameters of individual fruitlets to those which are continuing to grow, a very effective way to see if a fruit has been set is to look at the sepals on the calyx end of the fruit which were at the base of the blossoms when it was a flower. These five pointed, modified leaves will close up a bit on a blossom that was pollinated, fertilized, and is developing into a fruit.

Management considerations at petal fall are a bit more difficult than usual given the relatively low blossom density we had this year after substantial crops around most of the state last fall, questionable pollination weather for much of the bloom period, and continued cloudy, cool wet conditions. All of these would suggest that growers should be cautious with applying thinner applications this year. However, in most situations aside from an off-year on biennial cultivars or orchards with very poor fruit set, I recommend application thinning this year (and really every year). The weather this week is not conducive to application thinners that increase cell division, e.g. MaxCel, Exilis, etc. Those materials should be left on the shelf until temperatures after application will hit 75 to 80°F. That leaves the auxin-based thinners (Fruitone, Refine, etc.) as the best option for thinning the crop at petal fall. That said, even those materials do not work well in these 50° temperatures. Fruit are also growing extremely slowly, and will remain at the ~5mm “petal fall” window through the weekend. This will allow us to wait a bit until warmer weather is expected and will also allow late blooms to drop and minimize pollinator exposure to sprays. Dr. Terence Robinson from Cornell recently recommended relying a bit more on cabaryl to increase thinner activity this year. Where there is complete pedal drop and wildflowers have been mowed or otherwise controls, I recommend addition of carbaryl (Sevin XLR alone or generic carbaryl plus a surfactant) at one pint per acre, which will help to increase the effectiveness of thinning materials. Carbaryl is a relatively mild insecticide, but will provide some protection against insect pests that have had free reign of the orchard in the last few weeks if used at two pints per acre. I do recommend getting in with a more effective fall insecticide as soon as you are able for control of plum curculio, European apple sawfly, and some of the generalist lepidopteran pests that I have observed in various orchards. Codling moth have just begun to be trapped in area orchards, which means they are still flying and mating, and therefore have not laid eggs nor those eggs hatched into a stage where a targeted pesticide would be most effective. We will likely manage those pets next couple of weeks.

For most Vermont orchards, the apple scab ascospore maturity model predicts that all overwintering inoculum has been released and if the disease adequately controlled during the primary phase of the disease, then management for that best may be finished for the 2025 year. However, I strongly recommend maintaining fungicide coverage for the next 10 to 14 days. That is because apple scab is not the only disease of concern. Some of the summer fruit rots can get established early in the season, cedar apple is likely still a consideration, and in the case of low spray orchards or orchards where protection completely washed off, Marssonina leaf spot could become a serious disease given the constant wet weather we are seeing. We also cannot know that we have fully controlled for apple scab until we give enough time to see any lesions that may have broken through during these long infection periods.

Fire blight may not be a huge consideration this year, unless you had open blossoms that were not covered with streptomycin during the potential infection that occurred on Friday and / or Saturday of last week. The temperatures are just too cool now the bacteria to build to an infective population, and most orchards are at petal fall anyway which means that blossoms are no longer susceptible to the disease. Keep an eye out in the coming weeks for the development of any potential blossom or shoot blight symptoms. In orchards that have had a history of fire blight, I recommend maintaining low dose prohexadione calcium (Apogee, Kudos, etc.) applications for the next several weeks to help stiffen up cell walls in developing shoes and which will reduce the trees susceptibility to fireplace shoot infection.

Here’s what we’re doing for pest management and thinning in the UVM orchards this week: fungicide (captan plus Rally) tomorrow Wednesday morning to provide coverage through the next round of wetting on Thursday and Friday, then a weekend application of carbaryl plus eight ounces of NAA per acre for thinning, Avaunt insecticide for curculio and other petal fall pests, and possibly another fungicide depending on how much rain fell since the last application. That thinning application will be timed to coincide with the necessary low wind conditions and hopefully just ahead of any expected warm up we get early next week.

Good luck and let me know if you have any questions.

Terry

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Terence Bradshaw

Associate Professor and Chair

UVM Agriculture, Landscape, and Environment

Sent via mobile app. Please excuse typos.

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Terence Bradshaw (he/him)
Associate Professor, Specialty Crops

Chair, Dept of Agriculture, Landscape, and Environment
(formerly Plant and Soil Science)
College of Agriculture and Life Sciences

University of Vermont
117/210 – Jeffords Hall | 63 Carrigan Dr
Burlington, VT 05405

(802) 922-2591 | tbradsha
https://go.uvm.edu/pssbradshaw

UVM Commercial Horticulture | UVM Fruit Blog
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