May 2025 – UVM Fruit Blog

Vermont Apple IPM: Thinning, codling moth, general IPM

Orchard video covering this week’s material is here: YouTube 5/30/2025

Vermont orchard are at petal fall (inland and upland) to generally 10-12 mm fruit size in the lower elevation valleys. Primary apple scab ascospore release is complete for all orchards, so apple scab is not a major management consideration only if you know the disease is managed well in your orchard. That means taking a very careful, methodical walk through the orchard and checking high up in trees and at the ends of rows where coverage may have been spotty to ensure that you don’t have active scab in your orchard. I recommend keeping at least some captan on for the next week or so anyway, and adding a single-site fungicide material if you have questions about your spray efficacy and go into tomorrow’s rains without appropriate coverage.

If you applied full-orchard petal fall insecticide, you are likely between insect pests now. Codling moth still needs some time for mating females to lay eggs and eggs to hatch. For more information on ecological codling moth management, consider attending this Cornell Zoom meeting on the topic next Tuesday June 3 at 2:00 PM. Plum curculio will still be active in orchards for another week or two, but if you applied a block-wide insecticide at petal fall, you can manage PC with boerder row sprays.

The main management consideration that I am getting a lot of calls and emails about is thinning. The crop is all over the place, from well-thinned single fruitlets; to unthinned six-fruit clusters with intact king fruit; to unthinned clusters with no king fruit as king blooms were open during the wettest, coldest, most bee-unfriendly weather of the spring. Fruitlet size also runs the gamut from petal fall (~5 mm) to up to 15-20 mm.

Thinning is complicated, but it needs to be done. Dr. Renae Moran has a good summary of chemical thinning materials and strategies in the New England Tree Fruit Management Guide: Apple Fruit Thinning : New England Tree Fruit Management Guide : UMass Amherst. I’ll post some general thoughts in bullet point:

Fruitlets whose sepals have not closed and/or whose stems have begun to yellow are likely to thin off.
Fruitlets will be most susceptible to the older thinners (NAA, NAD, 6BA) at 10-12 mm in size. Carbaryl at 1 pint per acre increases their activity, and I suggest including it if using those materials.
Once fruitlets are over 18 mm those thinners no longer work sufficiently to warrant their application. A newer material, Accede, is available and labeled in Vermont for thinning apples and peaches. It is much more preferable to the old ‘rescue thinning’ standby, Ethephon (which I would no longer recommend now that there is something netter on the market).
Thinners are more effective when warmer / hotter weather comes before (2 days) or especially after (4 days)n application.

It’s hard to make blanket recommendations across multiple sites, elevations, fruit set levels, cultivar, tree ages, and all the other interacting aspects that we see across the state, so I won’t even try. If you have a specific scenario you’d like to run by me, reach out and I’ll do my best to help. My plan for our UVM orchard is to go in Sunday or Monday with 1 pint per acre of carbaryl and 100 ppm of Maxcel.

The UVM Tree Fruit and Viticulture Program is supported by the University of Vermont Agriculture Experiment Station, UVM Extension, USDA NIFA E-IPM Program, and USDA Risk Management Agency.

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VT Apple IPM: 2025 Petal Fall

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

—

Terence Bradshaw

Associate Professor and Chair

UVM Agriculture, Landscape, and Environment

Sent via mobile app. Please excuse typos.

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
Horticulture Research and Education Center
Message me on Teams

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VT Apple IPM: Fire blight alert

With welcome, warm, sunny weather, we are seeing epiphytic infection potential rise in virtually all orchards around the state. We are expecting by Thursday or Friday to have high enough populations of bacteria to cause fire blight infections on trees with open blossoms. That will include most orchards in the state which may either be just beginning to bloom or starting to end their bloom. Growers should plan to apply streptomycin to all blooming orchards on Thursday or Friday this week. Streptomycin works best when applied with a non ionic surfactant. There is also good potential for apple scab infection this weekend, so application of an appropriate fungicide would also be prudent. Next week we’ll discuss options for insect management and thinning at petal fall.

The UVM Tree Fruit and Viticulture Program is supported by the University of Vermont Agriculture Experiment Station, UVM Extension, USDA NIFA E-IPM Program, and USDA Risk Management Agency.

Vermont Apple IPM: Extended apple scab infection event edition

As I write this just after noon on Saturday, May 10, it looks like this long stretch of rain we have been experiencing for the past week is wrapping up a bit earlier than expected. Over the past seven to ten days around the state we have seen rainfall totals from less than two to over five inches, and rain or at least drizzly conditions every day. Even when there were gaps in the rain, apple scab spore germination can continue in dry periods under 24 hours, so for most orchards there has been essentially a continuous scab infection since May 1 or 2. Many growers have sprayed twice during this infection, but we can expect for most orchards that fungicide residue sprays applied even yesterday morning may be considered gone or at least substantially compromised. I recommend that growers go in with an appropriate kickback material as soon as possible now that the rain has largely stopped, looks like it will stay stopped, and wind conditions for spraying look good through Monday. Post infection activity starts from the time of application, so delaying application for a day or two will reduce the time period that the material will be active on spores that germinated in the past couple of days. The materials that have kickback activity fall in the FRAC codes 3, 7, 9, and 11. The strobilurin materials in FRAC code 11 tend to have less kickback activity and are more useful for summer diseases so I do not recommend those be used at this time. The DMI fungicides in FRAC code 3 tend to have longer kickback activity as well as have excellent activity against powdery mildew and cedar apple rust which are also active at this time. As always, I recommend tank mixing single site fungicides with a protectant fungicide like mancozebs or captan. We are approaching the end of the season for using mancozeb fungicides if applying at the full label rate of 6 lbs per acre. Captan is another effective fungicide that many growers tend to switch to at this time of year.

As orchards in the warmer parts of the state are in full bloom, and insect activity has been very low statewide with little to no European apple sawfly observed on traps in multiple orchards, I do not recommend application of insecticides at this time. Warm weather is expected to return on Monday and with it we should finally see substantial pollinator activity. There is a lot of concern around the state right now that the weather conditions in the past week have been inadequate for appropriate pollination of fruit crops, but I expect that a warm sunny day on Monday will provide adequate pollination for most orchards that are in bloom.

When we think of a blooming orchard we also think about fire blight. Conditions have been so cool during and prior to bloom that the epiphytic infection potential of the bacteria that causes the disease has been very low to such a point that treating blossoms has not been recommended. At the far end of the NEWA fire blight model five days out some orchards are showing an EIP of over 100, which could cause infection if all other conditions are met. You will need to revisit this later in the week, and I recommend that all orchards have a treatment of streptomycin on hand to use should an infection period be predicted.

Speaking of bloom, we have observed in multiple orchards relatively low blossom density this year. The frost that affected most orchards in 2023 allowed trees to save resources they would have put into growing crops that year which produced a large bloom last year. That heavy bloom event and resulting large fruit crop taxed the resources on many trees especially on varieties that lend themselves toward biennialism like Honeycrisp, which resulted in a lower density of fruit buds this year. There is nothing you can do to put fruit buds on a tree that doesn’t have them for this season, the best you can do is thin aggressively in those heavy blooming years. I guess most orchards will have to think about that next year. That doesn’t mean that I don’t recommend thinning crops this year, we’ll discuss that in the next newsletter.

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
Horticulture Research and Education Center
Message me on Teams

VT Apple IPM: Twilight meetings this week and Prebloom pest management

Reminder: TREE FRUIT TWILIGHT MEETINGS
Thursday and Friday, May 8 and 9, 4:00-6:00 pm

Join the Vermont Tree Fruit Growers Association and UVM Extension on May 8 at Yates Family Orchard, 1074 Davis Road, Hinesburg, VT or May 9 at Wellwood Orchard at 529 Wellwood Orchards Road, Springfield, VT. Topics discussed include tree fruit management practices, value added products, marketing, pest and disease updates, pollinators, and funding opportunities and challenges. The events are free and geared to commercial growers and people that work with them, and offer one VT pesticide recertification training credit for categories 1A, 1, 2, 3, 4, 5, and 6. Registration is not required, but helpful for any unexpected announcements regarding the meetings. To let us know if you plan to attend, contact Laura Johnson at laura.o.johnson or 802-656-4827.

We are in full scab management season with ascospore maturity creeping up, and significant, extended wetting that can discharge a lot of spores. Hours required for infection to occur are relatively low with the temperatures in the 50s and 60s that we are seeing this week. Fungicide coverage should be maintained during this wet week, and a material with kick-back activity (FRAC codes 3, 7, 9, 11- be sure to rotate among these!) applied should you have any question on coverage of protectant fungicides. Use of these materials should help with management of other diseases, including powdery mildew and cedar apple rust, as well.

For the time being, I’m fairly confident that we can consider fire blight a non-issue heading into bloom. That can change quickly should things warm up, and trees are susceptible to blossom blight any time blooms are open. The number to watch for in the NEWA fire blight model is the value in the “Infection Potential EIP value” column (data shown for UVM Horticulture Research and Education Center in South Burlington, VT):

That number refers to the Epiphytic Infection Potential, a relative value that predicts the size of the population of the Erwinia amylovera bacteria that causes fire blight infection. When that number reaches 100 the population is considered large enough to cause disease infection. That number is affected by accumulated heat units, so cool conditions cause it to decrease, whereas hot weather can rapidly cause bacteria to multiply rapidly, The other conditions that are included in the model include wetting events, temperature during bloom, and presence of open blossoms. The EIP values predicted for the next five days are just too low for fire blight to be of concern. This carries across all parts of the state, although southern Vermont orchards are seeing or expecting warmer temperatures that check the “average 60°F during bloom” box which makes the relative risk higher (i.e., yellow flagged). But keep an eye on that EIP number – if it’s well under 100, you’re likely in good shape.

Insect management should be on your mind, though. Many growers know that I am generally not in favor of insecticide sprays at pink unless scouting indicates a need because of the residues that would be present for pollinators to be exposed to at bloom, but this is a great time to be monitoring pest populations to be prepared to manage them when the time comes. I checked white sticky traps hung on 4/24 for European apple sawfly and found none, same for codling moth traps hung last week. For those who are following along, we use this monitoring summary to plan our trapping. In addition to codling mothy and obliquebanded leafroller listed there, we are also trapping for Oriental fruit mothy and dogwood borer, both traps can go up now. For the latter pest, my M.S. student Eli Wilson wrote a summary on them a couple of years ago:

Here is what you need to know about the dogwood borer (DWB):

Identification:

The adult form of the dogwood borer is a small, clearwing moth that is shown in the image below. They have approximately a 1-inch wingspan and have a black and yellow color pattern making them resemble wasp. They have mostly clear wings with circular windows at the tips. The larvae of the dogwood borer (DWB) are about ½ inch long and can range from white to light pink with a large brown head.

This is the time of year that the adult dogwood borers begin laying their eggs in the crevices of tree bark, so it is important to be monitoring for their presence in your orchard.

Damage:

The dogwood borer larvae are the ones that cause the damage, feeding on the phloem and cambium layers of the tree. Signs of an infestation may first appear as a pile brownish red frass on the outside of an entry hole in the tree trunk. While a few borers will not cause significant damage a population can build over the years and lead to reduce tree vigor and even girdling. An image of the larval form of the dogwood borer (DWB) is also shown below.

Trapping:

You will be using two orange TRÉCÉ PHEROCON VI DELTA TRAPs to monitor the dogwood borer in conjunction with lures labeled “DWB”. The lures should be replaced every 4 weeks and you can replace the stick liners every week if you choose. If you do not replace the liners each week, be sure to remove all the trapped dogwood borers after recording that week’s catch total. The orange Delta Traps should be hung within the tree canopy at approximately 4-feet off the ground as shown in this instructional video: Hanging Delta Traps

I hope this information is helpful, if you have any questions, comments, or concerns please feel free to reach out!

Happy trapping!

Best,

Eli Wilson

The UVM Tree Fruit and Viticulture Program is supported by the University of Vermont Agriculture Experiment Station, UVM Extension, USDA NIFA E-IPM Program, and USDA Risk Management Agency.

PAT credit listed 2025-spring-tree-fruit-twilight-meeting.pdf

Vermont Grape IPM: Bud burst and early season disease management

This edition of our newsletter was written by UVM PhD student Bethany Pelletier. You can expect more from her this season. While I have you, I’d like to promote the Cold Climate Viticulture course Bethany and I will be teaching at UVM this summer. We haven’t finalized this year’s syllabus, but you can see the 2023 syllabus here. This course is only offered in alternate years, so will not be available again until 2027. Please reach out to me if you have any specific questions about it.-TB

Spring has sprung in Burlington Vermont! Here in South Burlington, some of our earliest varieties like Marquette and Foch are looking pretty close to bud burst. Here are some things we should all be thinking about around this exciting time of year;

Our first line of defense for ALL growers is going to be good pruning and sanitation techniques. We should all be just about pruned up by now, but it’s never a bad idea to do another vineyard walk and make sure you didn’t miss a vine or two. Taking the extra time to clear out any dead wood or at least flail mow to break it apart will make a HUGE difference in the presence of our early season fungal pathogens like black rot and Phomopsis. Be sure to discard this material far away from the vineyard, not just at the end of a row.

As buds reach 1-2” height it will be time to shoot thin. We want to take it down to 3-6 shoots per foot of canopy, ideally keeping the primary shoots where possible. Many of our cold climate varieties like to push out their secondary shoots alongside the primaries (as pictured below, the primary is the larger shoot coming out at an angle). We should try to favor the primary shoots as they will have more flowers and generally develop into a stronger cane. This is also the best time to flick off any buds coming up on the trunk that we don’t want for future replacements, as these can REALLY gum up the airflow of the canopy. Shoot thinning is another IPM tactic that will make a huge difference in your disease defense regardless of your pesticide strategy. A cramped and crowded canopy will make a dark and humid microclimate, encouraging more mildews later in the season. In addition, more shoots will mean far more labor for you down the road when it comes time to shoot thin; much more pleasant to knock some shoots down while they’re still young and small!

UW Fruit Program

While proper pruning, sanitation, and shoot thinning will always be your first line of defense in the vineyard, we should all still be thinking about our pesticide management plan for the season. Pesticide orders should be placed by now, and spray equipment cleaned and calibrated. For all growers, but in particular those that use live biopesticides, be sure to check expiration and efficacy dates on your materials. Biopesticides and many organic pesticides often should be replaced yearly for maximum efficacy.

For the first time in my years as a student at UVM I am seeing some notable numbers of flea beetles. I’ll attach a picture here, they may look familiar to you already!

Tom Zabadal. MSU College of Agriculture

I am not too concerned about these at the moment, given that bud swell is progressing fairly rapidly so these little guys will only have a few more days to get any bites in. But if for whatever reason our bud development stalls out, we may want to consider some sort of control. If this happens to any of y’all please poke our fruit team for some management options.

Now that pruning is over and the vineyard is looking nice and neat, it can be easy to forget just how essential these early weeks are for disease prevention. Cold climate cultivars are typically not in need of pesticide intervention until about 5-8” of growth. However, if you are an organic grower or had bad early season disease last year you may want to start putting down some protection sooner! In the UVM vineyard we have both conventional areas and ‘organic’, so I am already planning my future sprays. Here are some tried and true suggestions from our own Dr. Bradshaw;

“The primary disease of concern at this point is phomopsis, as rachis infection at this point in the season may cause significant fruit loss at harvest. Anthracnose may also be active at this point , given the warm/hot weather are expecting later this week. Vineyards that have had recent problems with those diseases or organic growers using copper or other less-effective materials may consider treating this week; if you haven’t had major problems with those diseases, treatment can wait until the 5-8” growth stage as long as you are using a highly effective contact fungicide like mancozeb or captan. Organic growers are in for a bit more work. The standard fungicides, copper and sulfur, have only fair efficacy against this disease at best.

It is worth noting that both copper and sulfur (including lime sulfur) can cause phytotoxicity on certain cultivars. Dr. Patty McManus summarized her research on copper and sulfur sensitivity in cold-hardy grapes in the 2/8/16 Northern Grapes newsletter, and I’ll summarize it to say that Brianna should receive no copper; and Frontenac (all types), La Crescent, Leon Millot, Marechal Foch, Marquette, and St. Croix should receive no more than 2-3 copper sprays per season. Save those for later when black rot and downy mildew become bigger concerns. Sulfur sensitivity was observed on several cultivars, and its use (including lime sulfur) is discouraged on Foch, Millot, Brianna, and Louise Swenson; with limited (2-3) applications suggested on LaCrescent and St. Croix.” -T. Bradshaw, 2024

I believe that’s all for now folks! I wish you all the best of luck this growing season, and fingers crossed that this lovely warm weather stays with us through bud break! I’ll be back again soon with disease outlooks and funny stories from our own vineyard.

Best,
Bethany Pelletier & UVM Fruit Lab

The UVM Tree Fruit and Viticulture Program is supported by the University of Vermont Agriculture Experiment Station, UVM Extension, USDA NIFA E-IPM Program, and USDA Risk Management Agency.