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Fruit update - June 12, 2024

Madeline Wimmer- Fruit Production Extension Educator

This fruit update contains information about…
  • Apples- Growth stage, apple maggot monitoring and management, codling moth update, and Honeycrisp leaf mottling disorder.
  • Grapes- Growth stage and a note about when to start basal leaf removal.
  • June-bearing strawberries- Growth stage and diseases to watch out for during rainy harvest seasons, and fruit damage from insects and insect-like pests.
  • Minnesota Department of Agriculture IPM Fruit Update sign up form.


Image: Connell Red apples ranging from 3-4 cm in diameter at Northwoods Apple Orchard located in Oronoco, Minnesota (Zone 5a.)

Growth stage

Apples in SE Minnesota are ranging from 2.5-4.5 cm in diameter this week in SE Minnesota.

Insect pests

Apple maggot

Image: A map of Minnesota on 06/12/2024 showing southern regions where apple maggot adults are expected to emerge in 2-3 weeks. Image retrieved from the USA National Phenology Network.

The USA National Phenology Network maintains an apple maggot forecasting tool on its website displaying data for the current day and a six-day forecast. At this time, some southern regions in Minnesota can anticipate adult emergence around 2-3 weeks, which will vary based on upcoming temperatures and degree day accumulations. Similarly, the Cornell NEWA program connected to the UMN Horticulture Research Center (HRC) weather station indicates apple maggot is in the overwintering pupae stage and predicts emergence around 796 to 1072 accumulated degree day (DD, sometimes referred to as growing degree days or GDD) units. To this day, the UMN HRC has accumulated 644 degree days, indicating that their orchards are getting closer to the predicted emergence date.

Monitoring and managing apple maggot:
Now is a great time to start hanging traps throughout the orchard to track apple maggot presence. There exist a few different traps for monitoring apple maggot, which include yellow cards and round red balls, each requiring an adhesive to help catch insect pests. A lure that mimics ripening fruit can additionally be added to round red balls to more effectively attract apple maggot. These traps can be found for purchase on commercial Integrated Pest Management (IPM) websites like Great Lakes IPM, but can also be found on online marketplaces or even some garden centers. Insecticides for apple maggot can be applied 7 to 10 days after the first trap catch as a best practice. To learn more about appropriate pesticides for managing apple maggot, refer to the Midwest Fruit Pest Management guide section, “Apple third and summer covers- insect pests” starting on page 38.

Codling moth update

As of today, the UMN HRC has accumulated 252 degree days (DD) since the first codling moth catch on 05/25/2024. Around 250 DD is when the first generation of codling moths begin to emerge and management becomes a priority. For Minnesota orchards experiencing more rainfall at this time, there is some hopeful news that codling moths may be less successful in egg laying since hot and dry weather are its optimal egg-laying conditions. Refer to the Midwest Fruit Pest Management guide starting on page 34 to learn more about codling moth management.

Honeycrisp leaf mottling disorder

Images: Apple leaves showing mid-season mottled chlorosis characteristic of Honeycrisp. Photo (right) taken by Shane Bugeja, UMN Local Extension Educator for Blue Earth County.

Beginning last week in southern Minnesota and other Upper Midwest regions, growers may notice a specific type of mid-season chlorosis sometimes referred to as Honeycrisp leaf mottling disorder or zonal chlorosis. Symptoms look different than chlorosis from a nutrient deficiency, being more random in its appearance and showing blotches of yellowing. It tends to be more prevalent in younger Honeycrisp trees and is not known to affect fruit quality. The physiological cause of this issue is due to excessive carbohydrates that build up in the leaves. As trees mature and bear more fruit, the fruits will act as a carbohydrate sink, drawing excess sugars away from the leaves resulting in less zonal chlorosis.


Image: 1) A Concord grapevine cluster showing early signs of bloom at Firefly Berries located near Rochester, Minnesota (Zone 5a) and 2a,b) berries beginning to set on an early-blooming grape cultivar located at the UMN Horticulture Research Center (Zone 5a; photo taken by John Thull UMN HRC Vineyard Manager.)

Growth stage

Throughout Minnesota, most grapes are between early bloom (i.e., 10% flower caps fallen off) to early fruit set. Refer to last week’s newsletter publication for more information about disease management during this growth stage.

Optimal timing for basal leaf and lateral shoot removal

Basal, or cluster/fruit zone leaf and lateral shoot removal is a simple, low-input practice that can enhance fruit quality by increasing air flow and sunlight on developing fruit clusters and the basal buds that will give way to next year's shoots and clusters. Beyond that, basal leaf and lateral shoot removal gives way to more thorough spray penetration and fungicide coverage on fruit clusters because of their enhanced exposure.

The timing in which basal leaf removal is completed matters and research done in the Upper Midwest on cold climate wine grapes has shown positive impact for removal basal leaves preveraison around E-L stage 29, about two weeks post fruit set when berries are peppercorn in size (1). This specific stage of basal leaf removal led to an increase in total phenolic content for red and white wine grapes, as well as improved wine color stability. Other research has shown decreased titratable acidity and a reduction in vegetative flavors with increased exposure (2) . This is in part due to the increase in berry temperature that occurs with increases in sunlight.

Best practices for preveraison basal leaf removal:
Basal leaf and lateral shoot removal involves removing 1-3 leaves per shoot in the cluster zone and taking out any lateral shoots growing from the first three node positions.

How many basal leaves are removed depends on which ways the vineyard rows are oriented. For example, rows that run east to west give way to grapevines with a northside and southside canopy, and it is often best to remove more leaves on the canopy northside where less intense sun exposure exists. Vineyard rows that run north to south have canopies with east and westside exposure and can have more comparably equal leaf removal on each canopy side. In contrast, the extent of basal lateral shoot growth tends to be site and cultivar dependent and instructions for removal are independent of row orientation.

June-bearing strawberries

Images: 1) A ripe June-bearing strawberry and 2) strawberry fruit damage. Both pictures were taken in SE Minnesota (Zone 5a.)

Growth stage and diseases

As June-bearing strawberries start to ripen, frequent rain incidences have continued to impact fruit production in various parts of Minnesota. While sufficient water is important for high quality strawberries and yield, excess water from rain can impact fruit quality and consumer flavor perception. Not only can a cooler, rainy growing season lead to “watery” berries, the risk for fruit rots like botrytis fruit rot (i.e., gray mold; Botrytis cinerea) and anthracnose fruit rot (Colletotrichum acutatum) also increases. Visit our UMN webpage, “Strawberry diseases in Minnesota” to learn more about strawberry fruit and foliar diseases.

Fruit damage from insect and insect-like pests

There are many insects and insect-like pests that can cause strawberry fruit damage. Image 2 shown above was found in the field with the injury face-down. Slugs—technically classified as gastropod molluscs—are one pest that can cause open wounds in strawberry fruits. This type of damage is much different than other pests like tarnished plant bugs or sap beetles, in which sap beetles only affect over-ripened fruits and tarnished plant bugs can affect strawberry flowers and fruit.

Slug monitoring and management:
Slugs can vary in size from ¼ inches to several inches long and typically feed at night. Non-chemical management tactics include encouraging natural predators and avoiding broad-spectrum insecticides that could diminish insect predators, weeding and mowing tall grass that can provide shelter for slugs, and trapping and removing slugs using plank boards. Chemical means for controlling slugs do exist including products that contain iron phosphate and metaldehyde. Growers should note that metaldehyde can be toxic to animals when choosing a chemical product for slug management.

What solutions can table top strawberry production offer?

Table top strawberry production is a more viable option for day-neutral strawberry production, but is often grown under a sheltered structure like a caterpillar tunnel and offers the ability for more control over inputs like water and nutrients. Additionally, it adds the advantage of lifting fruit off the ground, increasing the chance of avoiding some diseases and pest issues. For growers who are interested in learning more about this process, check out our recent publication, “Growing day-neutral strawberries in high tunnel table-top systems.”

Minnesota Department of Agriculture IPM Fruit Update sign-up form.

If you are interested in subscribing to the MDA IPM Fruit Update series, follow the link below to sign up.
MDA IPM Fruit Update sign up form.

1. Jacob Scharfetter, Beth Ann Workmaster, Amaya Atucha. Preveraison Leaf Removal Changes Fruit Zone Microclimate and Phenolics in Cold Climate Interspecific Hybrid Grapes Grown under Cool Climate Conditions. Am J Enol Vitic. 2019 70:297-307 ; DOI: 10.5344/ajev.2019.18052

2. Jean Riesterer-Loper, Beth Ann Workmaster, Amaya Atucha.Restricted accessImpact of Fruit Zone Sunlight Exposure on Ripening Profiles of Cold Climate Interspecific Hybrid Winegrapes. Am J Enol Vitic. 2019 70:286-296 ; DOI: 10.5344/ajev.2019.18080

This week’s fruit update was reviewed by Soon Li Teh, Extension Specialist and Assistant Professor in Grape Breeding and Enology.

Non-credited photos in these publications were taken by the author, Madeline Kay Wimmer, M.S.

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

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