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Weekly Fruit Update - July 21, 2021

UMN Extension Educators toured Country Blossom Farm and Berry Ridge Farm on Wednesday, July 21 as part of a professional development program funded by SARE. Here, Colleen Carlson walks rows of SweeTango apples at CBF. Photo: Annie Klodd. 

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

In this week's fruit update:

  • Apples: Irrigation & return bloom, insect pests and diseases to watch for
  • Raspberries: Between harvest periods, spider mite leaf symptoms
  • Spotted wing drosophila - when to spray and when not to spray
  • Grapes: Veraison, foliar nutrient testing, and diseases
  • Herbicide drift injury


Irrigation continues to be very important in this dry season, not only for 2021 fruit development but also for return bloom. Next year's fruiting buds are developing in June and July of the current year, and excessively dry conditions can impact return bloom and the success of return bloom sprays. Irrigation is important even if the trees do not look visibly stressed. As discussed in previous weekly updates, growers without drip irrigation should use watering tanks now and seriously consider investing in drip irrigation for future years as our climate change causes our weather to be less predictable from year to year.

Information on using return bloom sprays can be found here

The second hatch of codling moth is currently active. Use traps to monitor for codling moths and apply effective insecticides if trap counts exceed 2 CM for 2 consecutive weeks (assuming use of 1 trap per 2.5 acres; according to Washington State University recommendations). If not using traps, monitor for CM damage on the fruit calyx and apply insecticides once new damage has occurred. 

Additional insect pests active at this time include apple maggot, Japanese beetle, leafhoppers, leafrollers, mites, woolly apple aphid, and spotted teniform leafminor.

Summer rots observed on ripening fruit actually infected the fruit when it was very young earlier in the season. Summer cover sprays can include a fungicide for fruit rots, but consider the economic cost of this spray and whether it is worth it considering our dry season and low disease infection rate.

See the Midwest Fruit Pest Management Guide - Apple Spray Program section for summer cover spray insecticide and fungicide options.


Shane Bugeja holds a raspberry leaf with spider mite damage. See the stippling and subtle white webbing at the top of the leaf. Photo: Annie Klodd

Summer-bearing raspberries are finished for the season, at least in most locations in Minnesota. Fall-bearing raspberries will begin ripening in the coming weeks. The several high tunnel raspberry locations I have visited have very strong fall-bearing fruit set, and it should be a good season.

We received several questions this week about bronze speckling (stippling) and subtle webbing on raspberry leaves, particularly in high tunnels. In all three cases, these symptoms were due to spider mites. 

Spider mites are more common in high tunnels than field environments, and more common under dry conditions. Their populations develop faster on drought-stressed plants. They cause damage to leaves of many plant species by sucking liquid from the leaves and reducing photosynthesis.

Organic growers can reduce spider mite populations by releasing predatory mites and/or spraying an insecticidal soap (i.e. M-Pede, Kopa). However, the insecticidal soap will also kill the predatory mites, so do not use both tools simultaneously.

Conventional growers can use the options above, and/or choose from a variety of miticides.

This article from Penn State provides more information including a list of predatory mites and miticides.

Spotted wing drosophila (SWD): Remember that SWD only infests ripe fruit. Even though SWD is active now, do not spray for this pest until it is observed on your farm when your fruit is ripe. Spraying while fruit is unripe is a waste of an expensive insecticide, to put it directly. Visit FruitEdge to see SWD activity across the state.


Grapes are beginning veraison across the state. For new growers: "Veraison" is the grape development stage where the berries begin to turn color. The will continue to accumulate sugars and water from now until they are ripe.

Testing for nutrient deficiencies: Veraison is the final opportunity to do foliar nutrient testing. The other opportunity was at bloom, which we discussed in a weekly update a few weeks ago. Foliar testing is important to know how well the vines are taking up nutrients; it not only helps diagnose nutrient deficiencies, but is the best way to decide what fertilizers to apply and at what rates. Test reports should be compared to Minnesota-based data for optimal nutrient ranges. All of the necessary information can be found in this article. Extension Educators are available to help interpret reports.

Grapevine leaves exhibiting interveinal chlorosis, possibly due to iron deficiency. This can only be reliably diagnosed with foliar nutrient testing either at bloom or veraison. Photo: Annie Klodd

Diseases: Growers may see powdery mildew and downy mildew on grape leaves at this time. Additionally, black or brown spots on leaves may be apparent and can be due to various diseases including black rot, phomopsis, and anthracnose. It is too late to spray for black rot now. Despite the presence of leaf spots, grapevine clusters become resistant to black rot 4-6 weeks after bloom. The most important time to spray for all of these diseases was between bud break and post-bloom.

Herbicide Drift Injury

Suspecting herbicide injury: Injury symptoms from herbicide drift that occurred in June (such as drift of 2,4-D and dicamba from row crop fields) may just be noticed on grapevine and fruit tree leaves now. If you suspect you have been drifted on, you may contact the Minnesota Department of Agriculture (MDA), which will come to your property to collect samples and investigate the case. You also have the option of independently testing your samples by sending them to a private laboratory and paying out of pocket for testing. However, if growers don't report cases to the MDA, there is no way for them to know the full severity of the herbicide drift problem for our industry.

You can find valuable information on 2,4-D and dicamba drift in this series of fact sheets from Ohio State University.

What is UMN doing to address herbicide drift? Our role as Extension Educators is to provide objective, research-based education about drift and direct growers to reliable resources to learn more. While specialty crop growers may be unaware of programs targeted at herbicide applicators, UMN Extension develops and delivers extensive educational materials and trainings for row crop herbicide applicators to help them understand how to minimize herbicide drift. For example, Crops Extension Specialists did a drift demonstration just this week at the annual Field School for Ag Professionals on the St. Paul campus. Several of us also participated in a national effort to survey specialty crop growers about herbicide drift; you probably received this survey back in February and March from Ohio State University Extension. Please remember that while Extension Educators strive to help in an educational capacity, we have no influence over state or federal regulations regarding herbicide drift.

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