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Weekly Fruit Update - 7/16/2020

Blueberry season is off to a good start in Minnesota. Photo: Annie Klodd

In this week's update:

  • SWD and Japanese beetle trap counts increase
  • Blueberry scale
  • Wet weather causing raspberry cane and root diseases
  • Grape crop estimation

SWD and Japanese beetle trap counts increase

The amount of spotted wing Drosophila in our orchards continues to increase. The UMN trapping network counted 60-90 SWD/trap at several locations this week.

All susceptible crops, especially summer raspberries and blueberries need to be protected from SWD at this time. 

We reviewed conventional, organic, and non-chemical control options for SWD in this June article: 

Even if growers are using conventional insecticides, they should still also be using non-chemical "cultural" control tactics including daily or 2-day harvesting, disposal of all infested fruit, and regular mowing to minimize SWD habitat. 

Japanese beetle numbers leveled off some this past week, but are still high at >2000/trap/week at Rosemount, and areas with clay/loam soils. Fewer Japanese beetle numbers are being found at sandy soil sites (Hastings and Forest Lake). Click here for Japanese beetle recommendations.

Follow these links for more details on UMN trapping data:

Blueberry Scale

Scale shells on a blueberry bush, 7/16/2020. Photo: Annie Klodd

While scouting blueberries today, I noticed shells from scale insects. There are 3 main types of scale insects of blueberries in the US: Lecanium scale, Putnam scale, and Terrapin scale. Scales become pests during their "crawler" stage during which they use their sucking mouth parts to feed on shoots, foliage and fruit during the growing season. They can potentially reduce yield if populations are high, because they secrete a sticky "honeydew" substance that attracts fungi and sooty molds. 

Mid-summer, eggs hatch under the protective shell of the scale shown in the photo, and then they disperse to feed on the bush.

Natural predators such as ladybugs and green lacewings are often sufficient to keep these pests under control. However, if large numbers are observed then a control regimen may be considered. The best times to control scale are in the early spring with horticultural oils, or during the growing season once the crawlers have emerged from the protective shells. 

At this point, the crawlers have emerged and are active (although I was not able to look for them today). They are hard to see with the naked eye and require hand lenses. Since the blueberry season is in full force now, growers may prefer to wait until early spring next year to control the overwintered scale with horticultural oils, rather than applying an insecticide for them now. Other insecticides being applied at this time for SWD and Japanese beetle may be controlling scale as well, but may also be killing beneficial insects that target scale.

For more information on scale insects:

Lecanium scale crawlers. Photo: SD Frank, via North Carolina State University

Wet weather stimulating raspberry cane and root diseases

The wet, warm weather happening now means it's time to watch out for raspberry cane and root diseases that can cause longer term decline in the orchard. 

Brown cankers caused by Phytophthora root rot on a raspberry plant, 7/10/2020. Photo: Annie Klodd
Last week, we confirmed phytophthora root rot as the cause of decline in a fall-bearing raspberry field. The symptoms have been spreading the last few years, but have really started increasing in recent weeks in conjunction with hot, humid weather and regular irrigation. Phytophtora is a disease that requires wet soil conditions to spread and infect.

Phytophthora root rot is difficult to control once it becomes established. Raspberry growers should scout for symptoms like dried up leaves, dead canes, and brown cankers on the canes. Specifically, look for brown discoloration at the base of the canes, and additional browning in the crowns and roots when they are dug up. Growers should also make sure they are not over-irrigating, and plant on sites with good drainage.

Purple cane blight cankers appear along the primocanes. They require wounds to enter, and wet conditions. Photo: Annie Klodd
Cane blight spores are spread through splashing water, and require wet weather or overheat watering that causes water droplets to splash. The spores then infect the canes through small wounds. Cane blight also causes cane dieback and dead (brown, crispy) leaves, but look for purple or brown discoloration along the cane rather than just at the base. 

The fungi responsible for fruit diseases require the right combination of factors to actually infect and damage a plant. The environment and condition of the host plant have to be considered in addition to the pathogen itself. Disease pathogens can lay dormant in the orchard, waiting for the right weather or irrigation conditions, or plant host conditions before attacking and causing symptoms. Numerous diseases favor the hot, humid, wet weather we are currently experiencing, while others require cool, wet weather.

Growers who see these symptoms should submit a sample to the UMN Plant Disease Clinic for diagnosis before pursuing treatment options. Treatment will depend on the disease, and sometimes even comes down to knowing the fungal species present. Treating without accurately diagnosing the problem can prove very costly. I plan to publish another article soon that details management of phytophthora and cane blight in raspberries, but the meantime, visit these two articles:

Grape crop estimation

Grape harvest will be here before we know it in Minnesota. Get ready to estimate crop load in preparation for harvest. Photo: Matt Jergensen. 

Grape growers should estimate their predicted crop yield in order to give an accurate estimate to the winery purchasing the grapes. Yield can be predicted in two main ways: Calculations based on "Lag phase" cluster weights, and comparison to historical data. We are 1-3 weeks out from lag phase, depending on the site and the variety. Lag phase is the period right before veraison where clusters temporarily stop growing in size. Clusters will double in size between lag phase and harvest, so measuring the cluster weights during lag phase allows growers to easily predict the final yield. 

More information on predicting crop yield based on lag phase and historical data can be found here, in a recent article by Joe Fiola at University of Maryland.

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

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