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Fruit update – June 5, 2024

Madeline Wimmer-  UMN Extension Educator: Fruit Production 

This fruit update contains information about…
Apples- Growth stage, insect & disease management, and a note about when to remove trunk shields/protectors.
Grapes- Growth stage, information about the grape cultivar Brianna, disease management, and training new vines: grow tubes.
Minnesota Department of Agriculture IPM Fruit Update sign up form.


Image: Minneiska (SweeTango®) fruits ranging from 22-35mm at ApplesRus located in Rochester, Minnesota (Zone 5a).

Growth stage
Many apple fruits in the southern Minnesota regions are ranging between 20-40mm in diameter. At this stage in apple phenology—about 30 days since anthesis/bloom—fruits are finishing up cell division and transitioning into the cell expansion phase (1).

Image: 1) An apple leaf with bronze-colored halo lesions indicative of either alternaria blotch (Alternaria mali) or black rot (Botryosphaeria obtusa). 2) Apple leaves showing classic apple scab (Venturia inaequalis) foliar lesions (photo from Penn State Extension.)

Disease management
Plant disease clinics—including ours at the University of Minnesota—exist to assist in diagnosing disease symptoms and signs. However, being able to differentiate apple foliar lesions visually is still an important step in developing an Integrated Pest Management program for any orchard. If you’ve been following the UMN Fruit Updates, we’ve talked a lot about managing apple scab, caused by Venturia inaequalis, but what do the foliar lesions that lead to secondary scab look like?

Younger apple scab lesions tend to be “velvety brown” to “olive green” in color with unclear margins (image 2) and the leaf surface can begin to look deformed as it ages (2).

Other pathogens can cause infections that lead to foliar lesions, including cedar-apple rust (Gymnosporangium juniperi-virginianae), alternaria blotch (Alternaria mali), and black rot (Botryosphaeria obtusa.) This season, one non-apple scab foliar disease that has shown up in SE Minnesota, exhibits bronze-colored, halo-like foliar lesions that look similar to early infection of either alternaria blotch or black rot .

Images: 1) Black rot foliar (a) and fruit (b) infections showing frogeye leaf spot and concentric rings on fruit. 2) Alternaria blotch lesions on an apple leaf (a) and fruit (b). Photos 1a, 1b, and 2b are from the University of Georgia, and image 2a is from CABI Compendium. 

Comparing alternaria blotch and black rot
Alternaria blotch in its earlier stages can be confused with black rot, which becomes more distinguishable as the diseases progress. The symptoms on the fruit are also very different. For example, black rot can lead to a leaf condition known as frogeye leafspot, where concentric rings or lobes are formed as legions enlarge. 

Alternaria blotch on fruit is not common and does not always lead to fruit rot. In contrast, black rot fruit infections start around the fruit calyx and, similar to the leaves, form concentric rings as it develops and can lead to fruit rot. 

Note: Bear in mind, as you become more familiar with different apple diseases, that symptoms vary in appearance and will not always look like a "textbook image." 

Similar to apple scab, A. mali spores that lead to alternaria blotch are spread by rain and splashed upon lower foliage initially, which can eventually spread up into higher canopy regions. Because its spores overwinter on leaves, a good way to mitigate damage in future years is to mow or mulch apple leaves to help them decompose. This can reduce the inoculum present during the following growing season. 
Good sanitation practices can also help with managing black rot. This includes removing mummified fruits, mulching leaves, and removing branches and other plant parts that are also subject to black rot infection. 

Images: Two pictures showing tree guards on older trees. One is installed on a central leader trained tree (left) and the other on high-density trained tree (right).

When should trunk guards be removed from apple trees?

Trunk guards are important for apple tree establishment and protecting against damage from wildlife and herbicide sprays. When is the appropriate time to remove them and are there consequences for having them on too long? Depending on how vigorous your tree is, the existing environmental conditions, and wildlife pressure, tree guards can typically be removed 3-5 years after planting.

Prolonged use of tree guards can be benign or problematic. For the two images shown above, the tree on the left had moss growth limited to trunk parts located within the trunk guard. This is a sign that the guard was creating a very moist environment that could simultaneously be conducive to secondary pest and disease issues. Other problems that may result from prolonged tree guard use include girdling, for which some tree guard designs are more likely to cause than others. Similar to so many other choices growers face in production, the risks of keeping the guards on should be weighed against the potential risks associated with their removal, like wildlife damage on trees with less wood development.


Images: 1) La Crescent in full bloom at the Horticulture Research Center near Chaska, MN (Zone 5a; photo taken by John Thull UMN HRC Vineyard Manager) and 2) Itasca nearing bloom in SE Minnesota near Eyota (Zone 5a.) Both photos were taken on 06/05/2024 in Zone 5a regions.

Growth stage
Grapevines are ranging in growth from pre-bloom to full bloom in southern Minnesota regions. Both photos shown above were taken from Zone 5a regions on the same date, which exemplifies how much impact mesoclimate (i.e., the conditions within a vineyard location) and other factors can have on plant growth beyond what USDA hardiness zones can indicate, which are based on average extreme winter temperatures.

Image: Brianna cluster beginning to bloom as a few flower caps begin to fall off. Photo taken near Eyota, Minnesota (Zone 5a).

Information about the grape cultivar Brianna

Brianna is a cold climate grape cultivar with Vitis labrusca heritage—more similar to Concord grapes—is beginning to show early signs of bloom with a few visible flowers in SE Minnesota. Because of its Labrusca heritage, Brianna differs from other cold climate wine grapes in a few different ways. Its shoots tend to be more rope-like in texture making it difficult, but not unachievable, to train into a Vertical Shoot Positioning (VSP) system, in which VSP is more adaptable for grapes exhibiting upright shoot growth. Its fruit can also develop “foxy” flavors if left to ripen too long on the vine and many wine-makers recommend harvesting it early for a brighter wine that avoids the foxy flavor profile. However, late stage Brianna, which also reputedly has flavors of pineapple and cotton candy can do well in other value-added products like pasteurized juice and jam.

Images: 1) Brianna grapes and 2) representative fruit cluster at harvest from a 2020 vintage at Cambridge Winery located in Cambridge, Wisconsin.

Disease management

For vineyards in pre-bloom, this window of time is a good opportunity for growers to apply a pre-bloom fungicide to proactively protect against various diseases, including downy mildew, black rot, foliar anthracnose, and phomopsis blight. Fungicide applications applied right before bloom through four-five weeks post bloom is known as a critical period for preventing fruit infections, especially for black rot, and downy and powdery mildew. Keep in mind that many grape clusters will be more resistant to both powder and downy mildew, however, the rachis remains susceptible, which can affect fruit development. For more information on effective spray programs and pesticides for managing grape diseases, refer to the pre-bloom through shatter grape section of the Midwest Fruit Pest Management guide, starting on page 163.

Note on applying fungicides during raining growing seasons:
Adjuvants are a category of additives that effectively assist in pesticide application and product longevity. Some examples are surfactants, spreader stickers, crop oils, and anti-foaming materials. Surfactants specifically help emulsify, spread or modify a pesticide’s physical properties. Whereas stickers allow for a pesticide product to adhere better to the application surface and helps mitigate pesticides from washing off due to rain. There are multiple products available that are classified as stickers, and growers should always refer to a pesticide label before mixing in stickers and other adjuvants to their spray tank, and consider testing the mixture in a small batch for compatibility.

If you are new to mixing multiple products for application, consider the following Tank Mixing Order recommended by the Midwest Fruit Pest Management guide:

Tank Mixing Order
1. Fill tank 1/4 to 1/2 full with carrier (water)
2. Begin agitation
3. Utility agents (if needed)
4. Suspension products a. Dry (Pre-mix): WP, DF, WDG, b. Wet F, FL, ME 5. Emulsifiable products (EC)
6. Solution products (S, SP)
7. Spray modifiers (if needed)
8. Finish filling the tank with carrier

And remember, always follow the label on any product you are using. The label is the law.

Images: Young grapevines being established using grow tubes in SE Minnesota (Zone 5a) where considerable rainfall has occurred, leading to pooling water around the vine trunk.

Grapevine establishment: grow tubes
Similar to using tree guards for apples, grow tubes also provide protection against wildlife and herbicide damage for young grapevines as they establish. Other positive results from using grow tubes can be an increase in grapevine shoot and root growth, however, some research has shown that grow tubes can negatively impact wood development. Most importantly, if you choose to use grow tubes during establishment for whatever reason, they should be removed before the fall to help grapes acclimate properly into the dormant season.

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.

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

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

  1. Janssen, Bart & Thodey, Kate & Schaffer, Robert & Alba, Rob & Balakrishnan, Lena & Bishop, Rebecca & Bowen, Judith & Crowhurst, Ross & Gleave, Andrew & Ledger, Susan & McArtney, Steve & Pichler, Franz & Snowden, Kimberley & Ward, Shayna. (2008). Global gene expression analysis of apple fruit development from the floral bud to ripe fruit. BMC plant biology. 8. 16. 10.1186/1471-2229-8-16.
  2. Sutton, T.B., Aldwinckle, H.S., Agnello, A.M., and Walgenback, J.F. 2014. Compendium of Apple and Pear Diseases and Pests, Second Edition. APS Publications.

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