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Weekly Fruit Update - August 4, 2021

 Annie Klodd, Extension Educator - Fruit and Vegetable Production

In this week's fruit update:

  • Apples - First Kiss stop-drop, testing fruit for maturity, MNAGA field day
  • Grapes - Re-cap of Wednesday's pre-harvest webinar
  • Raspberries - distinguishing abiotic & biotic problems
  • A note about blueberries 
  • No strawberry update this week
To see trapping data for spotted wing drosophila (SWD) and Japanese beetles throughout Minnesota, please visit UMN FruitEdge and click on the tabs for each insect.


Buckle up, it's August!

The annual Minnesota Apple Growers Association orchard tour is this Friday, August 6, starting at 9:00am at Pine Tree Apple Orchard. The field day will include a tour of the orchard, a variety of topics, and lunch. It will continue for an hour or two after lunch. Visit to register.

Time to apple a stop-drop spray for MN55 (First Kiss): University of Minnesota recommends applying a stop-drop spray (NAA/Fruitone or ReTain) 2 weeks before your anticipated first harvest of MN55/First Kiss. This variety is usually harvested between Aug. 17-26; therefore, it is time to apply a stop-drop spray. 

MN55 (First Kiss) apples ready for harvest. Photo: Annie Klodd

Fruitone (NAA) recommendations: Apply according to label directions 7-10 days before anticipated harvest. It starts being effective two days after application, and remains effective for 7-10 days. A second application closer to harvest may be needed if drop is observed or if more than 8-10 days have passed and not all of the fruit have been harvested yet. Since it can stimulate ripening, do not use it on overripe fruit.

ReTain recommendations: Apply according to label directions, in one application. Apply a 1/2 rate two weeks before the anticipated harvest. No second application should be needed.

Using Fruitone and ReTain together: Apply a tank mix containing a 1/2 rate of Retain and 20 ppm of Fruitone 7-10 days before harvest. Only one application is needed. This application may slow down fruit ripening and color development.

Reminder of MN55 (First Kiss) harvest quality standards: The licensing agreement for First Kiss helps ensure that all MN55 apples sold as First Kiss meet excellent quality standards. As you prepare for First Kiss harvest in a couple of weeks, refer to this article from 2020 which lists the quality and harvest targets for First Kiss. Do not harvest MN55 that has a green undertone. The fruit is unripe and overly tart when the green undertone is present; this does not meet quality standards in the grower agreement and will diminish the reputation of the variety.


Testing apples for ripeness: Our taste buds and our desire to harvest "ahead of the curve" can trick us into picking apples before they are ripe.

While taste testing and skin color observations are helpful, they should not be the only test for ripeness, because they are subjective for each grower. Furthermore, some of our key varieties color up long before they are ripe. For example, in Preston yesterday, some SweeTango apples were already at 50% red with a yellow background color, but harvesting them today would be at least a month too early and would violate SweeTango quality standards. "Color picking" is most appropriate once the grower has determined that at least some of the fruit is ripe in terms of flavor and sugar:starch content, and should not be used as a general standard.

Compare the staining on your test apples with the Cornell Starch-Iodine Index chart.


The starch iodine test is a simple, objective, and quick tool for growers to help ensure fruit are picked ripe. It is a sure-fire way to avoid putting overly-starchy apples on the shelf, because it shows how much starch has been converted to sugar. It can be used alongside taste tests, brix measurements, and visual observations.

Starch iodine testing involves spraying an iodine solution onto the flesh of the apple. As the iodine binds to starch in the apple, it stains the flesh surface blue. If most of the starch in an apple has been converted to sugar, then less staining will occur. Therefore, the riper the apple, the less blue staining you will observe. Compare the staining on the apple with the Cornell Starch-Iodine Index Chart to determine ripeness. Cindy Tong wrote an article about how to do starch iodine testing in 2018. 

First Kiss should be harvested when the starch-iodine index is 5-6 on the Cornell scale. The SweeTango agreement states that fruit should be harvested between 3-6; 3 is most appropriate for apples that will be stored long-term, and apples going for direct sales should be picked closer to 5-6.

Iodine solution is commonly used in commercial orchards west and east of us, so it is available through orchard suppliers such as Orchard Valley Supply.


As fruit ripens, look for signs of bunch rot diseases including sour rot and botrytis bunch rot. Both diseases can still be addressed through pesticide applications, reducing their impact on the crop. 

Know how to identify these diseases: Botrytis involves "fuzzy" gray fungal growth. Sour rot does not, but it does develop a vinegar smell after some time. Sour rot is not a fungal disease, so it requires different products than botrytis, which is fungal. Sour rot is caused when fruit flies contact damaged fruit, introducing the bacteria that leads to the disease. Therefore, sour rot management programs are more successful when they include insecticides effective on fruit flies. 

We discussed this topic more in-depth in Don't Let Grape Bunch Rots Spoil Your 2020 Harvest.

Other diseases visible right now are powdery mildew, downy mildew, black rot, phomopsis, and anthracnose. It is too late to control for phomopsis. A combination of preventative and curative fungicides are recommended to stop the spread of powdery mildew if infections are observed at this time. Grapevines generally become resistant to black rot infection 4-6 weeks after bloom. Therefore, black rot observed now occurred earlier in the season.

Read about all of these diseases in the Growing Grapes in Minnesota Guide and the Midwest Fruit Pest Management Guide (MWFPM guide). The MWFPM guide is the best place to look for pesticide recommendations, because it is updated every 2 years.


This hot weather may cause heat damage on raspberries, particularly in some high tunnels. Watch the temperature in the tunnel, and use whatever ventilation tools you have at hand to moderate the temperatures and air flow. Tunnel ventilation also depends somewhat on the orientation of the tunnel and what structures are around it that may be limiting air flow.

Heat damage can be hard to distinguish from diseases like cane blight and phytophthora. It is important to know whether damage observed is heat- or disease-related, or a mix. If necessary, mail samples to the UMN Plant Disease Clinic for diagnosis. Samples should include the crown of the plant and affected canes. The crown is important for testing for phytophthora.


Feel free to reach out and let me know what information would be helpful to have regarding blueberries during the growing season. There are relatively few blueberry growers in Minnesota compared to the other major fruits I work with. Therefore I receive fewer blueberry questions. Growers are welcome to contact me any time at

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