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Weekly Fruit Update 6/11/2020

Author: Annie Klodd, Extension Educator for Fruit Production

In this article: 

  • Herbicide injury and fungicide phytotoxicity
  • Plum curculio in apples
  • Strawberry frost damage vs. tarnished plant bug
  • Cherry brown rot
  • Spotted wing Drosophila status

If you're seeing interesting things in your fields, insects and diseases, or just want to share photos, we'd love to hear from you! As always, don't hesitate to reach out with questions and pictures. We're still here for technical assistance over the phone, via text, or via email.

Fruit questions go to Annie:
Vegetable questions go to me Natalie:
Food safety questions go to Annalisa:

Spotting herbicide injury and fungicide phytotoxicity on fruit leaves

Dicamba damage on a grapevine leaf. Photo: Annie Klodd

This week, I have received numerous emails and text messages with photos of deformed and discolored leaves. Diagnosing these symptoms can be truly challenging, and we do not always come to a definite conclusion without lab testing. Identification is learned through experience. But even with experience, it is hard to be 100% sure what caused a particular injury symptom. We try our best to help growers pinpoint the cause based on the symptoms and circumstances. When herbicide drift is suspected, growers should report it to the Minnesota Department of Agriculture.

Glyphosate herbicide injury: In this first example of grapevine leaf injury, the most likely cause is glyphosate drift based on the symptoms and circumstances. The leaves took on a deformed, bubbly appearance that is consistent with glyphosate injury. However, it is important to note that glyphosate injury does not look identical in every case.

Glyphosate was recently sprayed on cornfields adjacent to this vineyard during windy conditions. Roundup, like other herbicides, can drift through the wind to nearby off-target crops. In this case, the vineyard is located in the same valley as the neighboring corn fields. Glufosinate was also sprayed in the vineyard recently, but the symptoms on the leaves are not consistent with glufosinate injury. Laboratory testing would be needed to more definitively confirm the cause.

Photos used with the permission of the grower. Permission must be granted before re-using these photos.

Pristine fungicide phytotoxicity: Phytotoxicity means "a toxic effect by a compound on plant growth." Phytotoxicity is sometimes referred to "spray toxicity" or "spray injury" and is relatively rare. Pesticide applications to the crop, even if the product is labeled for use on that crop, can unintentionally turn phytotoxic under very specific situations.  This article is a great resource to learn about phytotoxicity.

In this second example, we see curling at the leaf edges and discoloration and deformation on some leaves. According to weed management experts at Purdue University and Cornell University, these symptoms are not consistent with herbicide injury.

Rather, symptoms like this are much more likely phytotoxicity from an application of Pristine or Captan during hot temperatures. The grower sprayed Pristine and carbaryl, but no Captan. Therefore, Captan was ruled out as a potential cause. Between Pristine and carbaryl, Pristine is more likely to cause phytotoxicity in grapevines.
Phytotoxicity injury on grapevine leaves, likely from Pristine. Photo permission was granted by the grower. 

What causes phytotoxicity? Certain combinations of fungicides and adjuvants can lead to injury. Spraying during hot or cool temperatures can also increase risk with some products. Additionally, combining Captan with oil can be phytotoxic. It is important to read the label of each individual product before applying or tank mixing them, to be aware of potential phytotoxicity risks and how to prevent them.

Plum curculio in apples

Suspected plum curculio feeding injury on an apple fruitlet on June 8, 2020. Photo: Annie Klodd

Apple growers should anticipate some level of plum curculio damage during harvest this year. The hot temperatures we experienced during petal fall were ideal for plum curculio populations. A solid control program throughout the orchard is necessary; perimeter control will not likely be sufficient due to heightened activity.

More information on managing plum curculio can be found here.

Tarnished plant bug vs. frost damage on strawberries

On immature strawberry fruit, feeding damage from tarnished plant bug (TPB) can easily be mistaken with frost damage, and vice versa. Both involve deformation of the fruit and a concentration of seeds on the affected portion of the fruit. Feeding by TPB nymphs is often characterized as a "cat facing" or "button berries." Frost damage is has a less predictable pattern.

A strawberry with damage that is likely due to frost during bloom. However, the damage is also similar to feeding by tarnished plant bug nymphs. It is difficult to definitively distinguish these symptoms. Photo: Annie Klodd.

I observed damage on a few immature Annapolis berries in White Bear Lake on Monday. The damage in this case is more likely to be frost damage than TPB. Reasoning: This early variety did have a few blossoms already emerging during the mid-May frost, the grower had controlled well for TPB, and the injury was occurring on the king fruit rather than multiple berries per cluster. 

Tips for distinguishing frost damage from TPB:
1) In general, TPB damage by nymphs is more concentrated toward the tip of the berry if the feeding was done during petal fall. Frost damage, on the other hand, may appear more randomly over the fruit. 
2) TPB will often feed on multiple fruit on the same cluster. If only one fruit on a cluster is showing symptoms, particularly the king fruit, then it is more likely to be caused by frost damage.
3) Scout for tarnished plant bug from early bloom to first picking, and consider whether a strong insecticide program was done during petal fall and bloom to minimize populations. 
4) Consider the bud stage of the plants during the last freeze/frost event. The more developed the blossoms were, the higher the likelihood of frost damage. Additionally, consider whether damage to blossoms was observed during bloom (center of the flower is black).

This article has a more thorough description of how to distinguish strawberry frost damage from TPB feeding. 

Cherry brown rot

While brown rot is most obvious on ripe cherries, it can also occur on immature fruit. Unripe tart cherry fruit infected with brown rot will turn brown and shrivel. Brown rot on immature fruit has been identified this year by Shane Bugeja, UMN Extension Educator for Blue Earth/Le Sueur counties, who wrote an article on brown rot last week. Refer to the article for information on management.

Immature tart cherry fruit infected with brown rot, June 2020. Photo: Shane Bugeja

Spotted Wing Drosophila status

The SWD Trapping Network in Minnesota has caught Spotted wing drosophila (SWD) in traps in Houston County (June-bearing strawberries), Rosemount (fall-bearing raspberries), and Chanhassen (wine grapes) as of June 9, 2020. Average number of SWD caught have not yet exceeded 2 per trap. 

Average number of SWD adults/trap per sample date in 6 locations across Minnesota. Source:
More weekly graphs from the SWD Trapping Network are available at:

For specific SWD management guides for berry crops, see the SWD Management page at: 

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