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Weekly Fruit Update - August 25, 2021

MN55 (First Kiss) calyx color compared to ripeness measured by the starch-iodine index (SII) test. Apples with moderate staining (SII 4-6) are ripe, strong staining (SII 1-3) are underripe, and little to no staining (SII 7-8) are overripe. There may be a correlation between calyx color and ripeness.


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


What's being harvested?

For the most part, MN55 (First Kiss) harvest is finishing up.

SweeTango and Zestar! harvest are approaching in the next week. Our team is harvesting SweeTango in Preston and White Bear Lake next Tuesday and Wednesday for our apple hail netting research project. I just sampled some delicious ripe Zestar! in Long Lake, so I imagine that some growers are already harvesting that and most will be very soon.

The starch-iodine test combined with flavor testing may be used to schedule harvest dates for both varieties. The SweeTango contract requires that certain quality and ripeness standards are followed in order for the fruit to be sold as SweeTango. All licensed growers of SweeTango in Minnesota should have access to the list of standards, or can contact Ferguson's Orchard for a copy.

Please see this article for more information: Grading and Sorting Apples for Direct-to-Customer Sales

That much-needed rain

We all really needed that rain that much of Minnesota got (is getting) this week. For the most part, it is welcomed. Just keep an eye out for fruit splitting, which can sometimes happen to ripe fruit if it takes on a lot of water all at once after a dry spell.

Splitting on very ripe crabapples after a large rainfall event. Photo: Annie Klodd


What's being harvested?

Not too much yet, however table grapes and some Swenson varieties are being harvested. I saw a bit of Itasca being harvested at one vineyard. I suspect it is not yet ready at most sites, except in the case of sparkling wine usage. Be sure the pH is in the acceptable range rather than only harvesting based on brix. Itasca does well with a late harvest, so growers need not hurry to harvest Itasca if the metrics are not there yet; especially since the vines have had relatively low disease pressure this year and we are still early in the harvest season. 

We look forward to tasting many high quality Itasca wines from 2021, which will help establish a great reputation of this new variety. Here are some suggested targets for Itasca, based on UMN observations at the Horticultural Research Center in Excelsior:

  • 24.7-28.2 brix
  • 3.04-3.31 pH
  • 8.7-10.8 g/L titratable acidity has been typical at the HRC, but levels as low as 6 g/L have also been observed.

See this fact sheet for more detailed Itasca recommendations. 

What about the rain?

Our growing season has been so dry, but it's time to remember the words of UMN graduate student Davy DeKrey, "When it rains it spores!" 

Watch for active infections of powdery mildew and downy mildew on grapevine leaves and fruit. If visible infections are present, use a curative systemic fungicide with strong efficacy on the relevant disease. For powdery mildew, choose from a variety of systemic products such as Torino, Rally, or Quintec. For downy mildew, some example systemics include Abound and Forum. Unfortunately, many of these systemic products have long pre-harvest intervals (PHI) of up to 14 days; please consider the PHI before choosing a fungicide. If you are planning to harvest in the next few days, it might be too late to apply anything curative.

If no mildew infections are present but you would like to prevent potential infections during the next rainfall, apply a protectant fungicide with good efficacy and a short PHI. Captan is effective on downy mildew. JMS Stylet oil is one option for powdery mildew that is more effective than Captan for this disease. Do not use Ridomil Gold or Ridomil Gold Copper if you have yet to harvest, as the PHI is too long. These can be used after harvest, though.

These product names are just examples. Please refer to the Growing Grapes in Minnesota guide and Midwest Fruit Pest Management Guide for fungicide recommendations.

Berry splitting and bunch rots 

Sour rot introduced via bird pecks. Grower-submitted photo.


Split and injured berries attract fruit flies, which can then introduce sour rot infection. Berries split open for numerous reasons including wasps, birds, hail, and heavy and sudden rainfall. It is also possible that spotted wing drosophila may cause slight berry injury as they attempt to pierce the skin of grapes. The question is, what to do if this injury and infection occurs?

If you spot (or smell) sour rot, Oxidate will help dry up the infection so that infected berries are not accidentally included in the harvest bins. See Don't Let Grape Bunch Rots Spoil Your Grape Harvest.

If the cause of the splitting is something manageable for the future, then identifying the cause is helpful. If it was caused by recent rainfall or hail, then there is obviously nothing we can do to prevent that. However, wasps, birds, and SWD are manageable. Wasp management strategies were covered last week in this article. Spotted wing drosophila management is covered here - see the table below for insecticide comparisons.

Table: Comparison of insecticides labeled in grapes in Minnesota, and their efficacy on SWD, J. beetles, Asian lady beetles, and phylloxera. Click on the table to enlarge it. It is best viewed on a computer screen or tablet.

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