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Weekly Fruit Update - June 2, 2021

Author: Annie Klodd, Extension Educator - Fruit and Vegetable Production

This week's fruit crop update includes: 

  • Blueberries - leaf reddening, freeze impacts
  • Strawberries - freeze mitigation and impacts, fungicide application
  • Apples - Late thinning options
  • Grapes - Tasks to do during and after bloom


Red discoloration on blueberry leaves - At least 2 blueberry growers have shared photos of red coloring on blueberry leaves between Friday and today (see photo below). In both cases the symptom is widespread over the field and not particular to a variety. This is most likely a response to the cold temperatures the morning of Friday, May 28. According to Mark Longstroth, emeritus blueberry Extension specialist at Michigan State University, cold temperatures can cause leaves to redden temporarily. The leaves typically return to a normal color quickly. One grower reports that the red color has mostly disappeared as of today (June 2). If you are experiencing this but the leaves remain red for over a week, submit foliar samples to a lab such as the UMN Soil Testing Laboratory to check for nutrient deficiencies. Phosphorus and magnesium deficiency both cause leaf reddening. 


Red leaves on blueberry plants observed for several days after a cold night. Photo: Aaron Wills, used with permission

Freeze damage - Blueberries were in full bloom and past-bloom during last week's freeze. The critical temperature for blueberries at bloom is 28F. The freeze mainly impacted northern Minnesota, including farms around Brainerd, Grand Rapids, Fergus Falls, and others. Farms in impacted areas used overhead sprinkler irrigation or row covers to protect the tender blossoms from the freeze. Some losses are still expected, but these measures were critical at saving much of the crop. All Minnesota berry growers should have some type of freeze protection tools in place in case of weather events; overhead sprinkler systems and row covers are the two primary options. There are no spray products or fertilizers that will protect berry plants from freeze.


Row covers protecting strawberries from a freeze event, May 28, 2021. Photo: Pleasant Valley Orchard, used with permission


June-bearing strawberries are in full bloom across Minnesota, and harvest will begin in mid to late June depending on location and variety. Bloom-time fungicides can still be applied to help prevent botrytis and anthracnose-related losses later on. Consult the Midwest Fruit Pest Management Guide for recommendations.

Freeze damage - Strawberries were in bloom during last week's freeze. Some freeze damage has been reported from growers in northern Minnesota, west central Minnesota, and in the Willmar area. I have not received any reports of damage in the Alexandria area, and spoke with one grower who said temperatures did not get low enough there to cause damage. The critical temperature for strawberries at bloom is 30F. Injured blossoms have dark brown or black centers that are visible 1-2 days after the freeze. 

As with blueberries (above) the use of row covers and overhead sprinkler irrigation drastically reduced losses. All berry growers in Minnesota should have some type of freeze protection tools in place, whether it be overhead irrigation or row covers. While row covers are undoubtedly a lot of work to apply, it can be done. One Minnesota grower (pictured above) covered several acres of strawberries with row covers last week, over a 12-hour work day. They report that it was, indeed, a lot of work. But then again, so is staying up all night running overhead sprinklers. We will revisit this topic again next spring.


SweeTango fruitlets after successful thinning - undeveloped fruitlets will fall off. June 2, 2021.

Apples are at or near the end of the chemical thinning period now, depending on location and variety. Assess your fruit load, to see how effective your first and second thinning applications were. If more thinning is needed, "rescue thinning" is possible until 25 mm. When fruitlets are between 18-20 mm, thinning becomes very difficult and spray options are limited to carbaryl and ethephon. The efficacy of these products at this large fruitlet stage is harder to predict. 

After the fruitlets reach 25 mm, hand thinning is the only remaining option to control fruit load.

For mild rescue thinning, apply carbaryl at 1 pint to 1 quart per 100 gallons of water between 18-25 mm. For more aggressive thinning, consider using ethephon (i.e. Ethrel). Ethephon can result in over-thinning or complete fruit loss, so it should only be used if absolutely necessary. Recommendations have been developed to help growers apply it successfully, like avoiding very high temperatures and tank mixing with carbaryl and a surfactant - these recommendations are detailed in the Wisconsin Fruit News article linked below.

The last 2 weekly fruit update articles discussed earlier thinning options, as it is best to have a proactive approach with thinning.

Resources for late thinning:


Grapes are nearing bloom, with bloom already beginning to occur in certain varieties and locations. This is a good time to prepare your spray program for Japanese beetles, which will begin emerging in about 3 weeks. Additionally, look forward to post-bloom leaf removal.

Do not apply insecticides during bloom. Spraying insecticides during bloom is dangerous to pollinators. Additionally, bloom-time applications are largely unnecessary because there are no significant grape insect pests active during bloom except for phylloxera; and, phylloxera can be controlled either before or after bloom.  

Likewise, a fungicide application should be done prior to bloom and ideally not during bloom unless absolutely necessary due to rain events. Due to ongoing dry weather, growers may be able to spread out fungicide applications, spraying every 12-14 days rather than every week. In preparation for bloom, look at the 10-day weather forecast. Protectant fungicides like mancozeb should be sprayed before a rain, and systemic fungicides (of which there are many including Rally) should be sprayed after a rain; after a fungal infection event.

Our webinar recording from June 17, 2020 describes post-bloom activities: Shoot thinning, hedging and skirting, and Japanese beetle management. 


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