Michigan hop crop report for the week of Aug. 1, 2022
Hop cones are at a critical window for disease management.
Much of the state has caught up to normal accumulated rainfall this year, although the Thumb and isolated areas of the Upper Peninsula remain significantly behind normal. Temperatures last week were above average and continue to contribute to higher degree day accumulation compared to average. The forecast looks hot and dry, with higher than normal temperatures moving in this week and persisting over the long term forecast.
Stage of production/physiology
Hops across Michigan are in Principal Growth Stage 6: Flowering (burr stage) and 7: Development of cones.
In the field
Growers are actively managing hop nutrients. Leaf plus petiole sampling for SAP analysis is underway. If growers are not using SAP analysis, Michigan State University Extension recommends taking leaf plus petiole samples for analysis when hops reach 8-10 feet (pull samples from 5-6 feet) and again when they hit the wire (about 18 feet, pull samples from 1 foot below wire) to determine if supplement nutrients, generally foliar applied at this stage, are needed. MSU recently updated their nitrogen fertilization recommendations for Michigan hop producers. Growers should not be applying nitrogen at this time.
Current recommendations are based upon nitrogen fertility and hop cone quality research conducted by Anne Iskra and colleagues at Washington State and Oregon State Universities. Cumulative nitrogen should not exceed 160 pounds/acre/year and most, if not all, nitrogen should be applied June 1 – July 15. Excessive nitrogen does not improve yields and total oil volume and alpha- and beta-acids has been found to decrease with increasing nitrogen rates.
If you are unsure of what is causing symptoms in the field, you can submit a sample to MSU Plant & Pest Diagnostics. Visit the webpage for specific information about how to collect, package, ship and image plant samples for diagnosis. If you have any doubt about what or how to collect a good sample, please contact the lab at 517-432-0988 or firstname.lastname@example.org.
Weeds continue to be a challenge this year, particularly in field without early season control. Weeds are most easily controlled when under 3 inches and using a multi-pronged approach. Refer to the Michigan Hop Management Guide for weed control options. For weed identification, check out Plant & Pest Diagnostics’ Plant and Weed Identification page or send photos to email@example.com.
As burrs and cones develop, Michigan yards are in a critical disease management stage to protect cones from a plethora of diseases, most notable downy mildew, powdery mildew, and halo blight. Downy and powdery mildew levels are high in some yards, particularly those with susceptible cultivars. Cultural practices for both diseases include anything that can reduce the understory weeds (which will decrease humidity) and anything that dries the canopy out more quickly. For an in-depth update on disease management in hop, check out Tim Mile’s presentation on the June 22, 2022, Hop Chat, posted on the Michigan State University Hop News Facebook Page.
Downy mildew is caused by the fungal-like organism Pseudoperonospora humuli and is a significant disease of hop in Michigan, potentially causing substantial yield and quality losses. This disease affects cones, and foliage and can become systemic; in extreme cases the crown may die. Cool and damp weather during the spring provide ideal growth conditions for the pathogen. Disease severity is dependent on cultivar, environmental conditions, and management programs. Growers should focus on proactive management strategies, including 1) sourcing clean planting stock, 2) clean crown management in the spring, 3) scouting regularly and 4) utilizing a preventative fungicide program. Refer to the Michigan Hop Management Guide section on downy mildew for additional management information.
Halo blight, caused by Diaporthe humulicola, causes cone and leaf lesions and cases cones to shatter during the harvesting process. Halo blight is relatively widespread in Michigan with growers reporting up to 50% yield loss due to shatter. Research on the biology and management of halo blight is underway and initial data indicates that fungicides in Fungicide Resistance Action Committee groups 3, 7, and 11 are effective.
Powdery mildew of hop, caused by the fungus Podosphaera macularis, is an emerging disease in Michigan that has serious implications for growers. Powdery mildew was confirmed in Michigan in 2014 and has been a concern on greenhouse/nursery plants for years. Seasonal disease severity is dependent on cultivar, environmental conditions and management programs. Focus on proactive management strategies, including sourcing clean planting stock, scouting regularly and utilizing a preventative fungicide management program.
Powdery mildew resulting from bud infection appears in the spring white stunted shoots called “flag shoots.” Flag shoots are rare, accounting for less than 1 percent of all shoots in a field and making detection at this stage very difficult. Secondary lesions become visible as leaf tissue expands and first appear as raised blisters, which quickly develop into white, round colonies. Infected burrs and cones can also support white fungal colonies or may exhibit a reddish discoloration if infected later in development.
Burrs and young cones are very susceptible to infection, which can lead to cone distortion, substantial yield reduction, diminished alpha-acids content, color defects, premature ripening, off-aromas and complete crop loss. Cones become somewhat less susceptible to powdery mildew with maturity, although they never become fully immune to the disease. Infection during the later stages of cone development can lead to browning and hastened maturity. Alpha-acids typically are not influenced greatly by late-season infections, but yield can be reduced by 20 percent or more due to shattering of overly dry cones during harvest resulting from accelerated maturity.
Late-season powdery mildew can be easily confused with other diseases such as Alternaria cone disorder, gray mold, other cone diseases or spider mite damage. Several weak pathogens and secondary organisms can be found on cones infected by powdery mildew; limiting powdery mildew can reduce these secondary infections.
Scouting for powdery mildew involves monitoring the crop for signs and symptoms of disease and evaluating the efficacy of the control program being utilized. Keep records of your scouting, including maps of fields, a record of sampling and disease pressure, as well as the control measures utilized. Section your farm off into manageable portions based on location, size and variety and scout these areas separately. It is more practical to deal with blocks that are of the same variety, age and spacing. Walk diagonally across the yard and along an edge row to ensure you view plants from both the edge and inner portion of the block. Change the path you walk each time you scout to inspect new areas. Reexamine hotspots where you have historically encountered high mildew pressure. Due to increased humidity, hotspots may occur more often near tree lines or in low-lying wet areas of the yard. Scout as soon as plant or pests become active and continue until the crop or pest is dormant. Weekly scouting is recommended at a minimum.
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.
Second generation flight of European corn borer continues around much of the state. European corn borer (ECB) overwinters as larvae inside the host plant where it pupates in response to warming temperatures in spring. First generation flight of adult moths runs from 450-950 growing degree days (GDD) base 50, based on a March 1 start date for GDD accumulation. Second generation flight runs from 1450-1950 growing degree days (GDD) base 50, based on a March 1 start date for GDD accumulation. As adults emerge, they mate, and females lay eggs. Growers are advised to carefully scout for adult moths and eggs on the underside of hop leaves. Growers can use the Enviroweather European Corn Borer Model, to better predict flight and egg-laying in their area.
Egg development is driven primarily by temperature, but generally eggs hatch in approximately 12 days. Newly hatched larvae then feed externally on leaves for approximately seven days before boring into stems and petioles where they continue to feed and grow. This larval feeding period is the critical window for control.
Once inside the plant, observations in hop indicate that European corn borer larvae damage vascular tissue, disrupt the flow of nutrients and water and impede plant development. Since 2019, European corn borer larvae have found in leaf petioles, sidearms, cone petioles (strigs) and bines. There are a number of effective insecticides for managing European corn borer and they should be applied to control larvae before they enter the protection of the bine. Growers with substantial infestations should also carefully dispose of crop waste including chopped bines at the picking line. Consider burying or hot composting crop debris both from the processing line and the field to limit carryover into the next season.
Managing for European corn borer takes a multipronged approach including careful monitoring and targeted pesticide applications. A regular schedule of insecticide applications based on insecticidal residual activity should be maintained from the beginning of egg hatch until two weeks before harvest or when larvae are no longer detected. For more information on European corn borer, refer to the Iowa State University publication, “European Corn Borer – Ecology and Management and Association with other Corn Pests.”
For more information on European corn borer management, refer to the MSU Extension article, “Be on the lookout for European corn borer in hops.”
Potato leafhopper (PLH) numbers are low to moderate across the state. Like many plants, hops are sensitive to the saliva of PLH, 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.
Two-spotted spider mite (TSSM) populations continue building, due in part to warm temperatures. Growers should carefully monitor mite pressure as TSSM thrive under hot and dry conditions. TSSM 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.
Growers are encouraged to scout carefully for mites season-long and treat while populations are at low levels, when mites are most effectively managed. Refer to the Two spotted spider mite factsheet for more information on identification and management.
Japanese beetles activity continues around the state. 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 of the green leaf material from entire plants. Japanese beetle may feed on other plants parts, including developing flowers, burrs and cones.
Currently, there is no established treatment threshold for Japanese beetles in hop. Growers should consider that established, unstressed and robust plants can likely tolerate a substantial amount of leaf feeding before any negative effects occur. Those managing hopyards with small, newly established, or stressed plants should take a more aggressive approach to Japanese beetle management, as plants with limited leaf area and those already under stress will be more susceptible to damage. It is also important to carefully observe beetle behavior in the hopyard; if flowers, burrs or cones are present and being damaged, growers should consider more aggressive management as yield and quality are directly affected. For more information on biology and management, refer to the Japanese Beetle Factsheet.
MSU Extension sincerely thanks the Michigan hop producers who provide timely input for the Michigan hop crop report.
This work is supported by the Crop Protection and Pest Management Program [grant no 2021- 70006-35450] from the USDA National Institute of Food and Agriculture and the North Central IPM Center. 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.