Michigan hop report – June 9, 2020

Growers are reporting substantial potato leafhopper pressure and the first flight of European corn borer.

Yellow and stunted spring hop spike
Yellow and stunted spring hop spike, systemically infected with hop downy mildew with spore masses on leaf tissue. Photo by Erin Lizotte, MSU Extension.

Warmer than average temperatures over the past couple weeks have helped put much of the state near normal degree day accumulation after starting the season 10-14 days behind average. For a more complete look at weather reports focusing on farmers’ interests, view the most recent agricultural weather forecast given by Michigan State University agricultural meteorologist Jeff Andresen. Be sure to download and refer the 2020 Michigan Hop Management Guide for nutrient management and pesticide related information.

With warm temperatures and regular rainfall across much of the region, hop downy mildew risk is high and management is critical. Use a protectant fungicide strategy to mitigate the risks of early and severe infections in combination with cultural practices to reduce disease. Keep in mind that varieties vary widely in their susceptibility to downy mildew and select the more tolerant varieties when possible. Refer to the article MSU Extension article, “Managing hop downy mildew in Michigan,” for more information on identifying and managing downy mildew.

Hop plants are also at risk of infection by powdery mildew. Spring to mid-June is a critical fungicide application window for powdery mildew. For more information, refer to the MSU Extension article “Managing hop powdery mildew in Michigan in 2020.”

Potato leafhopper numbers are growing as the initial arrivals from the southeast reproduce locally. Some hopyards are reporting high populations and substantial damage already. Like many plants, hops are sensitive to the saliva of potato leafhopper, which is injected by the insect while feeding. Damage to leaf tissue can reduce photosynthesis, which can impact production, quality and cause death in baby plants. To learn more about potato leafhopper, refer to the Hop Potato Leafhopper Factsheet.

Twospotted spider mite
Twospotted spider mite adult along hop midvein under magnification. Photo by Erin Lizotte, MSU Extension.

Twospotted spider mite populations are building due in part to warm temperatures. Twospotted spider mite is a significant pest of hop in Michigan and can cause complete economic crop loss when high numbers occur. Feeding decreases the photosynthetic ability of the leaves and causes direct mechanical damage to the hop cones. Leaves take on a bronzed and white appearance and can defoliate under high pressure. Intense infestations weaken plants, reducing yield and quality.

Dry, hot weather provides ideal conditions for outbreaks. Scout carefully for mites season long and treat while populations are at low levels when mites are most effectively managed. Refer to the Twospotted Spider Mite Factsheet for more information on identification and management.

Rose chafer
Rose chafer beetles mating and feeding on hop. Photo by Erin Lizotte, MSU Extension.

Expect to see rose chafer emerging in the coming week or so. Rose chafers are considered a generalist pest and affect many crops, particularly those found on or near sandy soils or grassy areas conducive to grub development. The adult beetles feed heavily on foliage and blossom parts of numerous horticultural crops and can cause significant damage to hop plants, particularly young plants with limited leaf area.

Rose chafer skeletonize leaf tissue, giving them a fine lace-like appearance. Rose chafers cause simple mechanical damage, so consider that established plants can sustain a significant amount of leaf feeding from rose chafers with no negative implications to the plant or crop. Young plants with limited leaf area may require more aggressive management. Chafer activity has typically subsided by the time burrs are present, so yield reductions are not an issue. For more information, refer to the Rose Chafer Factsheet.

European corn borer eggs
European corn borer eggs with characteristic fish scale appearance. Photo by Frank Peairs, Colorado State University, Bugwoord.org.

European corn borer remains a concern this year, particularly on sites with infestations in 2019. European corn borer has been a minor pest of hop in Michigan over the last decade. Other Midwestern states have reportedly experienced more substantial damage from this introduced moth. Unfortunately, damaging levels of European corn borer larvae were present in some Michigan hopyards last year.

While it is unknown exactly what caused the 2019 outbreak, it is possible that delayed planting of its major crop host (corn) made hop an attractive alternative egglaying site for the moth. Weather-based corn borer models are predicting that first generation flight began as early as June 5 in southern Michigan and growers are reporting adult moths in hopyards. For more information on European corn borer, refer to the MSU Extension article, “Be on the lookout for European corn borer in hops.”

Suspected vine borer
Suspected vine borer in hop bine, June 2, 2020. Photo by anonymous grower.

This week, one farm reported an infestation of what we suspect is hop vine borer (Hydraecia immanis), which feeds on hops during larval development, causing bine wilt. Hop vine borer damage can result in plant death if root and crown feeding is significant, although it is not typically considered a pest of economic importance. Check the base of wilting bines for entrance holes and frass—the damage looks a lot like European corn borer damage but is located lower in the plant. Hop vine borer adults likely emerge before training and lay eggs. Bine collapse from larval damage has been observed in late May or early June.

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This work is supported by the Crop Protection and Pest Management Program 2017-70006-27175 from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.

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