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Fruit update- 05/15/2024

This fruit update contains information about…
  • Apples - growth stages, crop load management, and insect & disease management.
  • Grapes - growth stages, canopy management, and insect & disease management.
  • Day-neutral strawberries- University of Minnesota strawberry planting considerations.
  • Fruit growth stages from NE Minnesota.

Image: Apple blossoms finishing bloom on a central leader trained tree at Northwoods Apple Orchard in SE Minnesota near Oronoco (Zone 4b).

Growth stage: As the lilacs come into full bloom in SE Minnesota, most commercial apple varieties are at the tail end of bloom in the growth stage “petal fall”.
Fruit thinning: This year in Minnesota the winter was overall mild, apple chilling requirements were fulfilled faster than average, and there was little frost risk during bloom, which may lead to increases in fruit set for the growing season. For those who are considering chemical thinning to balance fruit set this year, plan to make applications of products or combinations of products like NAA (e.g., Fruitone N, PoMaxa, or Refine), Carbaryl (e.g., Sevin and Sevin XLR+), or 6-BA (e.g., Maxcel) before fruits reach 15mm.
The University of Minnesota recommends the following combinations of products be used when apples are between 11-15mm:
  • NAA + Sevin
  • Maxcel + Sevin
  • Maxcel + NAA
Note: NAA becomes less effective after fruits pass 15mm in diameter. An alternative to NAA after 15mm includes horticultural oil; however, this can lead to increased risk of phototoxicity. If using an oil to enhance thinning, apply it during slow-drying conditions such as early morning or late evening and avoid applications on days with high temperatures, especially during periods of low rainfall.
The NEWA apple scab tool is forecasting a high likelihood for an infection event near Chaska between May 15th and 16th this week, based on the UMN Horticultural Research Center’s weather station. Continue following a fungicide spray program that meets your production goals and refer to the Midwest Fruit Pest Management guide for more information on fungicides for apple scab management.
Insect pests
Plum curculio: During petal fall, plum curculio adults can begin entering the orchard when temperatures are consistently above 60F and start ovipositing, or laying eggs on fruit as soon as fruits reach 5mm. Growers can scout for plum curculio by looking for slit-like scars on fruits, especially on orchard perimeter rows. Refer to the Midwest Fruit Pest Management guide for options in regards to management (pyganic is often used for management in organic production). Plum curculio adults are active and management is typically needed until 308 DD (baseline 50F) have accumulated post petal fall.
Note on apple maggot forecast tool: Growers who are new at tracking apple maggot may want to check out the National Phenology Network’s Apple Maggot Forecast. Forecasting using this tool can give growers an idea of when apple maggot adults emerge and disperse their eggs, which will eventually hatch and lead to unsaleable or aborted fruits. The forecast tool shows when apple maggot adults are expected to emerge in 2-3 weeks, 1-2 weeks, seven days or fewer, when the adults are fully emerged, and when their activity has ended. Right now, Minnesota has not accumulated enough growing degree days to forecast the date in which adults are expected to emerge at this point in time.
Other insect pests: The UMN Horticulture Research Center reported its first trapping of oriental fruit moth on 05/07/2024, but has not reported any catches of first generation codling moth. The first generation of oriental fruit moth tends to affect apple fruit stems and managing this generation can reduce the risk of fruit damage from subsequent generations that emerge. If both oriental fruit moth and plum curculio are present, broad spectrum insecticides may be used for management.


Grape cultivar (left), Itasca (middle) and Marquette (right) at 4in shoots showing clusters at the bud, pre-bloom growth stage. Photo (left) taken at Four Daughters Winery near Spring Valley, MN (Zone 5a, photo taken by Emma Christianson) and photos (middle, right) taken at the UMN Horticulture Research Center near Chaska, MN (Zone 5a, photos taken by John Thull- UMN HRC Vineyard Manager).
Growth stage
Grapes in the southern half of Minnesota are moving along quickly, many ranging from 4-6in tall showing clusters at the bud stage.
Canopy management
At this growth stage, shoots are still tender enough to be removed by hand. Cold climate grapes are typically thinned to 6-8 shoots per linear foot of canopy. Secondary shoots can also be thinned. Secondary shoots are visible when there are two shoots emerging out of the same node (see picture below), in which their emergence is more common in some cultivars than others.

Photo showing primary (P) and secondary (S) buds emerging from a single node on a Concord grapevine cane. The arrows indicate the direction each shoot will grow. Often times, the secondary shoot will be at a 45 degree angle, but this is not always the case. Photo taken at Firefly Berries on 05/07/2024 (Zone 4a). Note: the woody shoot branching from the cane at the same node is a lateral shoot that can be pruned.
Growers can continue routine conventional or organic fungicide spray programs to proactively manage fungal pathogens at this time. To learn more about grape diseases and appropriate fungicides to apply, refer to the Midwest Fruit Pest Management guide.

A grape leaf showing early signs of Grape phylloxera (Dactylosphaera vitifoliae) galls, circled in yellow. Photo taken at the UMN Horticultural Research Center, near Chaska, Minnesota (Zone 5a; photo taken by John Thull- UMN HRC Vineyard Manager).
Insect pests
Grape phylloxera: The UMN Horticultural Research Center reported some early signs of Grape phylloxera (Dactylosphaera vitifoliae) on grape foliage.
  • Grape phylloxera is an insect pest that can affect both the roots and foliage of grapes, but most cold climate interspecific hybrid grapes grown in Minnesota are mostly susceptible to foliar damage.
  • Grape phylloxera exhibits multiple generations per growing season and the first generation, known as "crawlers", emerge in the spring from mother eggs on the trunk and cordons and migrate to the newest grape foliage, potentially leading to 40-50 galls per leaf. Thus, the University of Minnesota recommends managing the crawlers as the most critical management stage, especially when growing more vulnerable cultivars like Frontenac.
  • For vineyards with an extensive history dealing with Grape phylloxera, time insecticide applications during early gall detection. UMN Extension recommends conventional growers use Danitol (pyrethroid) or Movento (systemic) for conventional grape phylloxera management. To learn more about grape phylloxera and management, visit this UMN webpage and refer to the Midwest Fruit Pest Management guide.

Day-neutral strawberries

Day-neutral strawberries growing in planters at Our Farm Fresh, near Hayfield Minnesota (Zone 5a).

Last week we covered some information regarding June-bearing strawberries, which have a shorter harvest window than day-neutral strawberries. Day-neutral strawberries also differ from June-bearing strawberries as they are planted annually in the Upper Midwest. They are often grown in mounded soil rows and covered with a type of mulchtypically plastic mulch for organic productionbut they are also suited for growing in a table-top, hydroponic setting.
The Roger's lab at the University of Minnesota continues to research various aspects of organic, day-neutral strawberry management. This week, I joined Matthew Gullickson of the Roger's lab and two others to help plant over 1,000 bareroot plants for their research plot. For a short overview of the planting experience, click the picture link below to check out this video from our fruit Instagram account @umn_fruittalk, or watch it on our Small Farms YouTube channel here.

To learn more about day-neutral strawberry planting check out this week’s article “Day-neutral strawberry planting key points” and the University of Minnesota Extension webpage: Day-neutral strawberries.

NE Minnesota fruit display
While many of our fruit updates pertain to the lower parts of Minnesota where the majority of fruit production takes place, production of various perennial fruits is still possible in northern regions even though growth stages will generally be further behind warmer regions. Here on display are some images showing current tart cherry, honeyberry, apple, and pear growth stages as of 05/13/2024.

Image: 1)Tart cherry in full bloom, 2) honeyberry in full bloom, 3) apples in "open cluster" growth stage, and 4) pears reaching full bloom in NE Minnesota, at Clover Valley Farm near Duluth (photos taken by Cindy Hale on 05/13/2024).

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The University of Minnesota Extension fruit production program would like to extend a thank-you to the growers who make these updates possible.

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