Article written by Madeline Wimmer, UMN Extension Fruit Production Educator.
Apples
Growth and pest management updates
Apple fruit diameters 6–11 mm, increased vegetative shoot growth
What makes for good conditions with post-bloom thinning?
Apple scab updates from the NEWA station at the HRC
Article: Codling moth: A common insect pest to manage in Minnesota
Watersprout removal on older limbs
Bud grafting at the UMN Horticultural Research Center
Grapes
Growth and pest management updates - Shoots 4–7 inches in length
UMN Extension Fruit Crop Reporting Tool
Apples
Growth and pest management updates - Apple fruit diameters 6–11 mm, increased vegetative shoot growth
Images: Different stages of fruit set for Zestar! (upper left; 11mm), Gala (upper middle; 7mm), First Kiss (upper right; 7mm), McIntosh (lower left; 8mm), Harlson (lower middle; 8mm), and Honeycrisp (lower right; 8-9mm). Photos taken on May 20, 2026 at Sekapp Orchard in Rochester, MN.
During this time period, it becomes more apparent which fruits have been successfully pollinated as they begin to swell with growth. We covered instructions for management during petal fall in last week’s update article, including product options and considerations for post-bloom fruit thinning.
What makes for good conditions with post-bloom thinning?
How effective thinners are, is related to the energy/carbohydrate status of the tree. Trees tend to use more energy and have lower available carbohydrates when temperatures are warm (heat stimulates energy use) and conditions are cloudy (with less sunlight, photosynthesis is lower). This is a good set up for fruit thinning because the thinners have a higher chance of increasing competition for carbohydrates between the fruitlets, which then leads to fruit abscission for the weaker fruitlets.
Thus, ideal application timing for thinning is when fruitlets are smaller and the weather is warm and cloudy. However, you might be in a situation this year where you experienced blossom damage from freezing temperatures earlier this spring, in which case you can read a recent article from the UW Fruit Team on Thinning Apple Trees After a Frost Event: What to Do and When.
Apple scab updates from the NEWA station at the HRC
Ascospore maturity is currently at 85% at the UMN Horticultural Research Center (HRC) as of 05/20, based on the reported green tip date 04/14. With the forecasted rain event later this week, which could lead to a 14% discharge, 90% of spores are expected to release by 05/23. Soon after all ascospores are forecasted to discharge, the chance of infection in orchards that followed good early-management practices dramatically decreases, which is an advantage of adhering to a thorough program early in the season.
Article: Codling moth: A common insect pest to manage in Minnesota
Looking to learn more about best practices for codling moth management? Check out this week’s article, “Codling moth: A common insect pest to manage in Minnesota.”
Watersprout removal on scaffold limbs
Images: A step by step demonstration of hand removal of a newly emerging watersprout from a larger scaffold branch on a freestanding central leader trained apple tree. Image from Sekapp Orchard in Olmsted County, MN (Zone 5a).
Watersprouts are shoots that emerge on scaffold branches on the inside of the tree canopy, and they start to emerge around this time of year. They are often unnecessary for tree health and take up more energy than they can give since they grow on the shaded inner parts of the canopy.
Watersprouts are commonly removed during dormancy, but this can lead to a pattern known as, “cut and regrowth,” where new shoots grow from the location where the watersprouts were pruned off. This cycle can become repetitive.
The other option to manage watersprouts is to remove them as they emerge and are still tender. This can usually be done by hand, rather than shears required for dormant removal, and the resulting wounds tend to be much smaller.
For orchards that have historically dealt with fire blight (Erwinia amylovara), there can be some risk of infection, especially when done during or immediately after a rain event.
Bud grafting at the UMN Horticultural Research Center
Images: The bud grafting process requires good technique and timing for success. Photos taken by Kate Scapanski, UMN Apple Researcher.
Fruit breeding is a huge process that takes years from the first initial cross (taking pollen from one flower to the next) to a final release of a cultivated variety. Just like all commercial apple varieties are propagated by grafting, each unique apple being assessed in the UMN Fruit Breeding Program needs to be propagated by grafting to review its fruit and tree characteristics. (If you’re interested in learning more about the process of judging a quality apple, check out a previous article, “ UMN apple breeding program: Desirable traits and the selection process.”)
There are a few different ways to graft-propagate apple trees, including bud grafting, which UMN uses for its test trees. This week, I talked to Kate Scapanski, UMN Apple Researcher, to learn more about their process.
Kate mentioned that this process starts with cross seedlings after they have grown up in late July. Most of the grafting happens in August and September, and will continue later into the season, although the likelihood that the graft will successfully take can decrease when it’s done right before the first hard frost.
The process is straightforward: A woody shoot of current year’s growth is taken and its leaves removed. What’s left are the buds that have been developing for next year’s growth. Those buds are then removed with a grafting knife with the goal of separating the woody from the inner vascular tissue.
Then, a T-slit is made on the rootstock, just deep enough to peel back the cambium, and large enough to slide the bud graft in, making sure to line up the bud shield and the top of the T on the rootstock. Budding/grafting strips, or budding rubbers, are used to secure the graft and keep the bud in good contact with the rootstock vascular tissue.
Images: Two scion buds are initially grafted onto one rootstock in case one does not take. Photos taken by Kate Scapanski, UMN Apple Researcher.
Two grafts are made on one rootstock to ensure at least one takes. Once the scions begin to establish, the stronger of the two is chosen to become the true scion, while the other bud is eventually removed. During the next spring, after the new grafted trees have overwintered, the rootstock is cut back above where the grafts were made. This typically happens in mid-April to early May to encourage the plants to direct energy to the scion and not the rootstock.
Grapes
Growth and pest management updates - Shoots 4–7 inches in length
Image: Shoots are growing rapidly on this Concord grapevine’s cane at Firefly Berries. Photo taken on May 20, 2026.
Grape shoots are growing rapidly in Southeast Minnesota with pre-bloom clusters visible (stage 12 of the E-L growth stage guide, 5 leaves separated). Instructions for disease management continue under the section: Grape Bud Break to Pre-bloom on page 138 of the Midwest Fruit Pest Management Guide and switch to Grape Prebloom to Shatter (page 142) when shoots are between 10–12 inches tall.
There’s still time to do shoot thinning at this stage, but after shoots reach past 12 inches or so, it will become increasingly difficult, so take time to get out there and thin vines to 6–8 shoots per linear foot of canopy when you can!
UMN Extension Fruit Crop Reporting Tool
Interested in sharing anything about the fruit crops on your farm? Feel free to share observations about diseases, pest management, or other production aspects using our new UMN Extension Fruit Crop Observation Report tool. If you have a question you want addressed in a future update article, you can include that in your report.
Thank you to our farm and ag professional partners for contributions to the UMN Fruit Update series. Non-credited photos in this article were either taken by Madeline Wimmer or within the UMN Extension system.
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