Michigan hop crop report for the week of Aug. 23, 2021

Harvest is underway across Michigan.

Hops being harvested.
Hop harvest in east central Michigan on Aug. 21, 2021. Photos by Sean Trowbridge

Weather

Last week was 4-6 degrees Fahrenheit warmer than normal, especially during nighttime hours. We were also drier than normal across much of the state last week. This followed wetter than normal conditions the week prior.

Soil moisture levels (plant extractable water in top 3 feet) in northern Michigan are slightly above normal and slightly below normal in some locations across southern Michigan.

In terms of degree day totals, most areas of the state are still in surplus (seven to 10 calendar days), going back to beginning of March. It looks like these surpluses will continue based on the 6-10 day outlook.

GDD summary
Accumulated degree day base 50 summary for hop producing areas of Michigan on Aug. 23, 2021, as compared to the five-year average. Data source: MSU Enviroweather. 
Rainfall summary
Accumulated rainfall summary for hop producing areas of Michigan on Aug. 23, 2021, as compared to the five-year average. Data source: MSU Enviroewather.

It will be uncomfortably hot and humid for the next few days with heat index values near 100. There is a 30-40% chance of scattered showers nearly every day of the week through this weekend, which generally occur late in the day. The best chances for rain are Wednesday across the Lower Peninsula.

By the end of the weekend, temperatures should return to normal. However, the 6-10 day outlook (Aug. 29-Sept. 2) suggests above normal temperatures and greater than average precipitation across the entire state.

6-10 day outlook temperature probability
NOAA 6-10 day outlook, Aug. 29-Sept. 2: Temperature probability.
6-10 day outlook precip probability
NOAA 6-10 day outlook, Aug. 29-Sept. 2: Precipitation probability.

Watch the most recent agricultural weather forecast from Michigan State University state climatologist Jeff Andresen.

Stage of production/physiology

Most hops across the state are in Principal Growth Stage 8: Maturity of Cones. For those interested in determining optimum harvest date, refer to these Michigan State University Extension and University of Vermont articles:

Average Michigan hop growth stage chart.

Average Michigan hop growth stage based on date. Botanical drawings courtesy of Dodds, Kevin. 2017. Hops, a guide for new growers. NSW Department of Primary Industries.
Graph
Timing of hop production management activities in northwest Michigan.

Hop stage

Statewide update

Thunderstorms, high winds and high heat and humidity have challenged growers over the last couple weeks. Growers have reported signs of storm damage, mildew and halo blight in more susceptible varieties and are working diligently to harvest hops (see photo below). Mite pressure has been relatively low overall as has European corn borer. Japanese beetle numbers have been high.

Storm damage to hops.
Storm damage in a northern Michigan hopyard, Aug. 21, 2021. Photo by an anonymous grower.

Diseases

As we are approaching harvest, several cone diseases have become apparent in yards. Particularly this year has been challenging to manage fungal diseases that thrive in this high humid weather. So far, we have been observing halo blight, downy mildew and powdery mildew in several hopyards.

Halo blight infections are caused by a fungus and when cones are infected there are a variety of symptoms but primarily, they all lead to some type of cone shatter during harvest. We have also heard reports that infected cones give off some type of "cheese-like" aroma, which is likely unfavorable to the brewing process. This fungus is a Diaporthe sp. and researchers at Michigan State University have been studying management methods for the past two years.

Halo blight can be a serious disease in specific yards and lead to significant cone infections and ultimately shatter during harvest. It appears to have a wider geographic range than previously thought and has been recovered throughout Michigan on both leaves and cones.

How the pathogen survives during the winter, spreads, causes new infections is still being investigated. The goal is to provide management tools that can be used to prevent foliar and cone infections in yards. The best practice that we currently recommend is to manage all other aspects of your yard, reduce humidity and control powdery mildew and weeds within the yard. We observed halo blight on leaves in the state as early as May 28 and we began to observe cone blight as early as mid-July in several locations.

Halo blight
Foliar halo blight. Photo by Tim Miles, MSU.
Cone halo blight.
Cone halo blight. Photo by Tim Miles, MSU.
Healthy cones.
Healthy cones. Photo by Tim Miles, MSU.

Powdery mildew has been particularly bad this year especially in susceptible cultivars like Cashmere. We have been observing it in many yards southwest Michigan the entire season and saw it on some specific hop cultivars as early as May 18. Occasionally we saw flagged shoots but now have been seeing more typical symptoms on the foliage and cone infections.

In the past, we have thought of powdery mildew being very sporadic in Michigan but it appears to be more widespread this season. The best method currently is to use clean planting material when establishing new hop yards and utilize good cultural and chemical management practices.

Please consult the MSU Extension article, “Managing hop powdery mildew in Michigan in 2020,” for more specific details about how to manage powdery mildew using other cultural practices and chemical applications. 

Powdery mildew on hops
Powdery mildew observed on the cultivar Willamette near harvest. Photo by Timothy Miles, MSU.

At this point in the season, downy mildew shouldn't be forgotten but as we are near harvest, it might be OK to stretch intervals unless you have had significant lateral downy mildew infections and have portions of your field that are infected. If you think an application is needed to protect cones, there are several effective foliar fungicides are available and include products in FRAC codes 4, 11, 21, 40, 43, 45 and 49.

See the MSU Extension article, “Managing hop downy mildew in Michigan,” for more information on management methods for downy mildew and refer to the Michigan Hop Management Guide.

Now is a great time of year to look for viral disease symptoms on new growth. On June 1, we observed a Centennial plant that is known to be infected with four viruses: 1) American hop latent virus, 2) hop latent virus, 3) hop mosaic virus and 4) hop stunt viroid. The best way to control viruses is to utilize sanitation practices prior to planting (e.g., using virus tested stock, destroy heavily symptomatic plants). For more information about hop viruses, please check this article, "Virus visible in Michigan hopyards."

Sclerotinia wilt (also known as white mold) is being observed at this time. According to the Hop IPM Field Guide, Sclerotinia wilt affects nearly 400 weed and crop plant species, including green bean, pea, lima bean, canola, carrot, lettuce, potato, sunflower and squash. The disease is caused by a fungus, Sclerotinia sclerotiorum, and is only an occasional problem on hop in wet, cool climates. Sclerotinia wilt can cause damage when soil and plants remain continuously wet and temperatures are mild.

Disease symptoms generally appear in late spring or early summer as soft, watersoaked lesions on bines just below or near the soil surface at the crown. The infected tissue collapses, creating a light brown to grayish lesion approximately 1 to 4 inches long. In some instances, the fungus can colonize stems higher on the plant. During wet weather, fluffy white growth of the fungus may form on the infected tissue.

Small (about 1/16 to 1/8 inch), hardened, black overwintering structures (sclerotia) form on and in diseased bines. As the disease progresses, the lesions expand and may girdle the bine, causing a wilt. Bines with smaller diameter seem more likely to fully wilt than larger diameter bines. Leaves generally remain green until the bine is girdled completely. Disease symptoms may appear similar to those caused by Fusarium canker or Verticillium wilt, however, the presence of fluffy white mycelia and sclerotia are diagnostic for Sclerotinia wilt.

Control measures for Sclerotinia wilt of hop usually are not needed. Cultural practices that reduce the duration of wetness on plants and the soil surface can reduce disease incidence. These practices may include limiting nitrogen fertilization, removing excess basal shoots and leaves, stripping leaves from lower bines, delaying the first irrigation as long as possible, and timing irrigations to allow the top two inches of the soil to dry completely between irrigations.

For more information, refer to the Hop IPM Field Guide.

Sclerotinia wilt
Sclerotinia wilt. Photo by anonymous.

Insects

Twospotted spider mite activity continues around the state and mites found during scouting should be treated. 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.

Potato leafhopper numbers have continued to build in untreated areas. 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. Refer to the Hop Potato Leafhopper Factsheet for more information on identification and management.

European corn borer remains a concern this year, particularly on sites with infestations in 2019-20. European corn borer has been a minor pest of hop in Michigan over the last decade. Other Midwestern states have experienced more substantial damage from this introduced moth. Unfortunately, damaging levels of European corn borer larvae were present in some Michigan hopyards for the last two years. The Enviroweather corn borer model has predicted adult moth emergence in all major hop production regions in Michigan at this time.

Regardless of location, scout for adults, eggs and larvae now. For more information on European corn borer, refer to the MSU Extension article, “Be on the lookout for European corn borer in hops.”

Japanese beetles are being widely observed at this time. Adult Japanese beetles aggregate, feed and mate in large groups after emergence, often causing severe and localized damage. They feed on the top surface of leaves, skeletonizing the tissue between the primary leaf veins. If populations are high, they can remove all the green leaf material from plants. Japanese beetle may also feed on developing flowers, burrs and cones.

For more information on identification and management, check out this Japanese beetle factsheet and refer to the Michigan Hop Management Guide.

Japanese beetle on leaf
Japanese beetle feeding on hop. Photo by Erin Lizotte, MSU Extension.

Weeds

Grass weed pressure remains substantial this season, especially with recent heavy precipitation. Growers commonly use a selective grass herbicide containing the active ingredient clethodim to manage annual and perennial grasses within the row. Growers utilizing Aim will also get some control of grasses and broadleaf weeds in the row. Aim should not be used on baby hops. As a reminder, pay attention to pre-harvest intervals (PHI) for all pesticides. For more information on registered herbicides, refer to the Michigan Hop Management Guide.

Virus testing

MSU Plant & Pest Diagnostics is now providing testing for hop viruses and viroids! Please refer to the recent MSU Extension article, “MSU Plant & Pest Diagnostics provides testing for hop viruses and viroids,” for information on costs and protocol for submitting samples.

Fertility

Soil testing labs

Plant-tissue labs

Comprehensive soil health testing labs

Stay connected

For more information on hop production, visit the MSU Extension Hops website. Also, Michigan State University Extension is hosting a series of interactive Hop Chat Zoom meetings this 2021 season to allow easy communication between producers and MSU faculty. These informal weekly sessions run every Wednesday at noon from May 4 through Sept. 7 and include crop and pest updates from MSU Extension’s Rob Sirrine and Erin Lizotte. In addition, MSU faculty will drop in to address timely issues and provide research project updates. Bring your field notes, too! We want to hear what’s going on in your hopyard. Registration is free but required. Sessions will not be recorded. Register here!

This material is based upon work supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2017-70006-27175. 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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