Fruit update – August 20, 2025

 Article written by Madeline Wimmer, Extension Educator - Fruit Production          

• Grapes:

• Growth stage update: Nearing harvest

• Article topic: Measuring fruit ripeness

• Nutrition: Magnesium deficiency

• Blueberries:

• Nutrition: Iron deficiency

• Plums:

• Mount Royal (European plum) at harvest

Grapes

Growth stage update: Nearing harvest

Images: Itasca and Marquette, two UMN grape cultivars, along with Sabrevois, Swenson White, and Louise Swenson, cultivars bred by Elmer Swenson. Photos taken at Salem Glen Winery, located in Olmsted County (Zone 5a). 

This week I stopped by Salem Glen Winery to check on their wine grape development. Salem Glen has been growing a wide variety of grapes since the early aughts, including a number of French-hybrid grapes (e.g. Foch), UMN bred grapes, as well as cultivars bred by Elmer Swenson and Tom Plocher (e.g. Petite Pearl, Crimson Pearl). 

Many of these cultivars were measuring around 14° Brix out in the field, progressing closer to harvest time, but not quite ready. Some cultivars, like Sabrevois, will be picked around 18° Brix, when it typically maxes out, while others will hang longer until they reach closer to 24° Brix.

Measuring fruit ripeness

Quantifying how ripe grape clusters are can make a big difference to harvesting quality fruit for both fresh eating (table grapes) and wine production. Bringing underripe table grapes to the market can impact consumer perception, as underripe grapes can be overly tart and lack desirable flavor profiles.

As grapes ripen, sugars increase while acid levels go down and the pH rises. When evaluating berries in the vineyard, measuring soluble solids (e.g., representative of sugars; measure in degrees Brix) with a refractometer and the pH (roughly estimated with pH strips, or more precisely with a pH meter) are fairly simple to do, while total acidity is usually best done indoors with a titrator.  

To learn more about this process, check out this week’s article on measuring fruit ripeness:

Article: Monitor grape cluster ripening for a quality harvest this year

Nutrition: Magnesium deficiency

Images: Two different Itasca leaves showing symptoms of Magnesium deficiency. 

Magnesium is at the center of every chlorophyll molecule and is essential for its production. When unavailable, a specific pattern of interveinal chlorosis begins to show up on older leaves. As the condition worsens, brown dead (necrotic) spots begin to show up interveinally as well. 

Epsom salt foliar sprays can relieve symptoms during the current growing season, but ultimately growers should use a combination of tissue analysis and soil testing to determine how to adjust the amount of magnesium in the soil for future growing seasons. This is especially important because magnesium is a (divalent) cation that competes with other cation nutrients in the soil, namely potassium and calcium. 

The optimal time for collecting petiole samples for lab tissue analysis has passed, where optimal magnesium concentrations are between 0.20–0.50% at bloom, and between 0.26–0.80% at early veraison. Growers who are interested in performing a petiole test should wait until the following season, but can still use soil samples to indicate how to address the soil conditions for the future (magnesium can be applied later in the season as it does not stimulate vegetative growth and does not have a high potential to leach). 

Blueberries

Nutrition: Iron deficiency

Image: Interveinal chlorosis with a light yellow coloring is a classic sign of iron deficiency as seen in this blueberry plant. 

Blueberries are well known for their unique requirement for acidic soils (pH ideally between 4.5–5.5) where they originally evolved. When the soil pH is out of balance, which generally means the pH is too high, root growth becomes restricted as well as access to nutrients like iron and zinc. Iron deficiency can show up as interveinal chlorosis with a light, green-yellow background color. Shrubs can also have smaller leaves, and be stunted overall. 

Correcting this condition often involves adjusting the soil pH, but growers should also take note of their water pH, as irrigation water with a high pH can cause the soil pH to creep up eventually. To learn more about acidifying irrigation water, check the resources section below. 

Resources related to this article: 

• Treating irrigation water (Texas A&M)

• Acidifying soil (Oregon State University)

Plums

Mount Royal (European plum) at harvest

Image: Mount Royal variety of European plum hangs in the sunlight, ready for harvest.

Plums are one of the easier types of stone fruits that can be grown in Minnesota without interventions like high tunnels with double plastic layers for overwintering. Despite this, issues with cross pollination, fruit set, russeting, and diseases like plum pocket can still be problematic. 

Two types of plums, European plums and hybrid plums can be grown in Minnesota growing zones. There are more types of hybrid plum varieties than European ones suitable for production, but European plum varieties Mount Royal and Stanley offer the advantage of being self-compatible. This means they do not require a pollenizer (second tree variety) for pollination and fruit set. For other plum varieties that require cross pollination, the variety Toka works well, along with wild plum trees like Prunus americana (American wild plum) and P. nigra (Canadian wild plum). 

Plum varieties show a number of different skin and flesh colors. The plum in the above photo is the European plum variety Mount Royal, which has purple skin and yellow inner flesh. At the time this photo was taken (08/19), these fruits were firm with some give and full of flavor, ready to eat fresh from the tree. Because the fruit set was so successful this year, these plums were weighing down the branches. This is less of an issue than for peach trees, for example, which typically bear larger fruits with heavier loads on younger branches, and can lead to branches breaking. 

For more information on plum and other stone fruit varieties suitable for Minnesota climates, check out the following resources:

• Growing stone fruits in the home garden (UMN Extension)

• University of Minnesota plum varieties (MN Hardy)

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.

This article may be shared for educational purposes with attribution to the University of Minnesota Extension. For other uses, please contact UMN Extension for permission.