Fruit update – July 3, 2024

Madeline Wimmer- UMN Fruit Production Extension Educator

This fruit update contains information about…

• Apples
• Insect pest updates: leafhoppers and apple maggots.
• Using hail nets to mitigate hail damage and as an exclusion tactic for managing insect pests (video included).
• Grapes
• Disease update: powdery mildew.
• Insect pest update: Japanese beetles.

Apples

Insect pest update

Leafhoppers

Images: 1) A collection of shoots tips on an apple tree showing curled leaves, 2) a close up of a shoot tip with curled leaves, and 3) the underside of a curled leaf revealing a leafhopper insect (family Cicadellidae).

Unlike some other foliar symptoms, apple leaf curling can be easily spotted in an orchard. After turning a few leaves over, the culprit pest can usually be found on a leaf’s underside. While insect pests like aphids and mites can cause leaf curling in apples, leafhoppers can also lead to leaf curling. 

Leafhoppers are insects that act primarily as indirect pests in apples by piercing and sucking photosynthates and sap from apple foliage and sometimes the fruit. The species known to affect apple trees include white apple leafhopper (Typhlocube pomaria; WALH), rose leafhopper (Edwardsiana rosae), and potato leafhopper (Empoasca fabae), with potato leafhopper being a less common pest in apple orchards.

Images: 1) White apple leafhopper adult (Typhlocube pomaria), 2) rose leafhopper nymph (Edwardsiana rosae), 3) potato leafhopper nymph (Empoasca fabae), and 4) apple aphids (Aphis pomi). Images retrieved from Invasive.org.

Notes about leafhoppers in apple orchards:

• The first generation of leafhopper adults typically emerge in June.

• WALH is known for having two generations per year and limits its feeding to apple leaves, whereas the rose leafhopper can have three generations per year and will additionally feed on apple fruits.

• Hopper burn is another symptom that can show up as yellowing or browning near apple leaf edges and is caused by the potato leafhopper.

• Rose leafhoppers are unique in that their eggs are laid and overwinter on roses and bramble crops like domesticated and wild raspberries. In contrast, WALH overwinters in apple wood that is between 1-5 years old.

Scouting for leafhopper nymphs—the juvenile life stage—should begin after petal fall. Aim to inspect at least 10 leaves from 10 different apple trees and plan to start treatments when at least 30% of leaves are infected or when there is an average of 1 or more nymphs per two leaves inspected. For more information about which pesticides to use for managing leaf hoppers, refer to the Midwest Fruit Pest Management guide starting on page 55.

Apple maggot

Image: A map of Minnesota on 07/02/2024 showing southern regions where apple maggot (Rhagoletis pomonella) adults have emerged, mid-regions where adults are likely to emerge in 1-2 weeks, and northern regions where too few degree days have accumulated to accurately forecast adult emergence. Image retrieved from the USA National Phenology Network.

The Minnesota Department of Agriculture (MDA) Integrated Pest Management (IPM) newsletter has begun to report some trap catches for apple maggot (Rhagoletis pomonella) in the past week in Minnesota counties: Chisago, Wabasha, Washington, and Wright. Trap catches will continue to increase as adult emergence has been forecasted for many regions in southern Minnesota. In contrast, the very northern Minnesotan regions can anticipate adult emergence in 2-3 weeks or even longer, based on the current forecast.

The UMN Fruit Update- June 12, 2024 newsletter article discussed more about trapping options for apple maggot and recommended beginning an apple maggot spray program after the first trap catch; however, it is also helpful to note that red ball traps that contain lures will attract more apple maggot than non-lured traps. This is why some resources set the action threshold for 1-2 apple maggot catches on yellow cards or non-lured red spheres, while the threshold for lured-spheres is higher, closer to 5 apple maggots per sphere. To learn more about chemical management tactics for apple maggot, refer to the Midwest Fruit Pest Management Guide section, “Apple third and summer covers- insect pests” starting on page 38.


Hail netting

Images: 1) Golf ball sized hail that occurred shortly after a harvest event near Madison, WI (Zone 5b) in 2016. Draped hail netting installed over central leader (1) and high-density apple trees at Pine Tree Apple Orchards, located near White Bear Lake, MN (Zone 5a.)

Hail storms are an extreme weather occurrence that may occur infrequently, but when they do happen, it can deliver incredible damage to both apple fruits and trees and affect fruit marketability. Hail can range in severity and size, from less than pea-sized to as large as golf balls. Insurance options exist to cover hail damage, but the cost-to-recovery ratio may not always be adequate to make up for damage.

That’s where hail netting comes in. Hail netting is a physical way to mitigate hail damage. It typically comes in two forms: overhead and draped hail netting. Overhead hail netting is installed over an orchard block, whereas draped hail netting is applied more similar to bird netting, where it is wrapped around individual rows of high density or smaller central leader apple trees. Beyond the differences in installation, draped hail netting has been researched at the University of Minnesota to determine its potential to exclude insect pests. Through this research, draped hail netting alone showed over 90% decrease in apple maggot and codling moth population within apple rows without affecting fruit marketability, making it a promising new approach to insect management. To learn more about overhead and draped hail netting, check out the University of Minnesota’s webpage, “Hail netting for apple orchards,” and the short video below.

Hail netting in apple orchards short video:

Grapes

Disease update: powdery mildew

Image: Powdery mildew foliar symptoms (Erysiphe necator) found in the no-spray vineyard block at the UMN Horticulture Research Center (HRC; Zone 5a.) Photo taken by John Thull, UMN HRC Vineyard Manager.

Powdery mildew (Erysiphe necator) symptoms have begun to show up within the no-spray vineyard block at the University of Minnesota Horticulture Research Center (UMN HRC). When powdery mildew begins to affect fruit clusters, it can lead to wine taint at thresholds as low as 3% infection on fruits, making it a significant disease to prioritize in management (1). Similar to other fungal pathogens, grape powdery mildew produces fruiting bodies that help it overwinter on grapevine bark, canes, leftover fruit clusters, and ground foliage. Overwintering spores are released with rainfall and infection events are most likely to occur with prolonged leaf wetness. When leaves or other grapevine parts are infected by overwintering spores, it can lead to a cascade of secondary infections from asexual spores originating from infected leaves. Secondary powdery mildew infections are spread by wind and shady conditions are more conducive to its overall growth. 

Cultural control for powdery mildew includes growing powdery mildew resistant grape cultivars and thoroughly managing the grapevine canopy initially by pruning, followed by adequate shoot thinning, and removing basal leaf and lateral shoots, a management task covered in the June 12th, 2024 Fruit Update. Chemical management options for powdery mildew are discussed in the Midwest Fruit Pest Management Guide, starting on page 155.

Powdery mildew is a global issue for grape production and is a major focus of Vitis Gen3, which is a USDA-funded grape breeding project happening at the University of Minnesota and at various research institutions throughout the United States. With this project emerges the possibility for improved endogenous powdery mildew resistance in new grape cultivar releases.

Insect pests update: Japanese beetles

Image: A map from the Minnesota Department of Agriculture showing an increase in confirmed reports for Japanese beetle (Popillia japonica) sightings in Minnesota.

In our June 27th, 2024 Fruit Update, I noted the first Japanese beetle (Popillia japonica) sighting at the UMN HRC vineyard, and this week, more incidences have been reported, including in Fillmore County. In recent years, there have been more confirmed sightings of Japanese beetles in northern Minnesota. Growers in these regions who have yet to be concerned about Japanese beetles as a pest may want to be on the lookout. At the same time, researchers throughout the U.S. are studying natural ecological disruptions to Japanese beetle populations, including pathogenic fungi like Ovavesicula popilliae, the winsome fly (Istocheta aldrichi), and other parasitic insects. Currently, the University of Minnesota is looking for Minnesotan growers to participate in a 2024 survey to understand more about the winsome fly distribution. If you’re interested in participating in this survey, find out more information here.

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The University of Minnesota Extension fruit production program would like to extend a thank-you to our fruit grower partners who make these reports possible.

Non-credited photos in these publications were taken by the author, Madeline Kay Wimmer, M.S.

References:

British Columbia Ministry of Agriculture. 2015. Grape powdery mildew.