MSU Extension Hops

In the field

For a hop production update from around the state, please see the hop crop report for August 1, 2024.

Weeds

Weeds remain an issue for many growers this year with difficult early season conditions and ample rain and heat in many locations. Ideally, all weeds should be treated when small for optimal control. Some growers use various forms of cultivation to suppress weeds. Refer to the Michigan Hop Management Guide for weed control options.

Diseases

Growers may be observing symptoms of viral infection. Testing is available through Michigan State University (MSU) Plant and Pest Diagnostics.

Growers should be doing their best to keep rapidly expanding tissue protected from fungal diseases. The following section contains information on fungicide-based disease management for hops. In all instances, it is important to manage fungicide resistance and avoid applying similar products back-to-back. This is particularly important with site-specific systemic fungicides. To reduce the development of resistance with systemic fungicides:

• Do not make more than three applications per season of the same Fungicide Resistance Action Committee (FRAC) code, regardless of if it is allowable based on the label.
• Do not make two consecutive applications of the same FRAC code.
• Rotate with unrelated fungicides in a different FRAC code that have efficacy on the target pathogen.

Downy mildew

Covering young, developing bracts before cones close up is critical to protecting against downy mildew when conditions for disease are favorable. Getting adequate coverage on undersides of bracts where infection occurs becomes increasingly difficult as cones mature. Downy mildew is caused by the fungus-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.

Utilize a protectant fungicide management strategy to mitigate the risks of early and severe infections but can also utilize 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 Table 2 in the Field Guide for Integrated Pest Management in Hops).

Apply fungicide treatments regularly on a protectant basis as soon as bines reach 6 inches in the spring regardless of the presence or absence of visible symptoms of downy. Applications should continue season long on a seven-to-10-day reapplication interval. The time between applications may stretch longer when the disease pressure is low, particularly after cone closure. Several periods in the season are particularly critical for disease control: immediately before and after training; when lateral branches begin to develop; bloom; and when cones close up.

Refer to the Michigan Hop Management Guide section on downy mildew for additional management information, including fungicide options.

Halo blight

Halo blight of hop is caused by the fungus Diaporthe humulicola and can cause yield losses and change the chemical composition of the hop cones. Halo blight of hop first appears in late spring to early summer as brown to gray lesions that develop on the hop leaves that are surrounded by a chlorotic (yellow to light green) halo. The lesions may have concentric rings radiating from the center of the leaf lesion, but rings may not be present. The pathogen can also infect hop cones and burrs. Hop burrs become necrotic and desiccated.

The pathogen overwinters on dried hop tissue as small black structures called pycnidia. When conditions are warm (greater than 60 degrees Fahrenheit) and humid, the pycnidia produce spores that are dispersed by wind and water. The pathogen causes lesions on the surface of the hop leaf that give rise to new pycnidia and the cycle continues with burr and cone infections occurring as plants develop. If environmental conditions are right, the hop plant can go through multiple infection cycles per year. In the late summer or early fall, the pathogen seems to colonize hop bines that have been left after harvest, where the pathogen remains dormant over the winter.

To help suppress halo blight, post-harvest dead material and new shoots should be removed in the fall to reduce overwintering inoculum. Post-harvest hop plants can also have the top layer of their crown removed in a process called crown pruning, which can reduce inoculum of the pathogen that can infect the hop crown. Additionally, there appears to be a correlation between the severity of downy mildew symptoms and the severity of halo blight symptoms, therefore early season downy mildew disease management can be a preventive measure against halo blight in hops.

Chemical control of halo blight is often needed. The disease is present throughout the growing season, so multiple chemical applications are needed. There is evidence that fungicides registered for powdery mildew with the active ingredients trifloxystrobin (FRAC 11), flutriafol (FRAC 3), tebuconazole (FRAC 3) and fluopyram (FRAC 7) have been shown to reduce the incidence and severity of halo blight symptoms.

For more information on halo blight management, refer to the MSU Extension publication, Managing Halo Blight of Hops.

Powdery mildew

In addition to downy mildew, be vigilant in scouting for and managing powdery mildew. Powdery mildew of hop (caused by the fungus Podosphaera macularis) is an important but sporadic disease in Michigan hopyards. Seasonal powdery mildew 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.

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% 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.

Conditions that favor powdery mildew are reported to include low light levels resulting from cloud cover, canopy density, excessive fertility and high soil moisture. Leaf wetness from dew or rain does not directly impact powdery mildew infection, but results from high humidity and cloud cover, which favors disease. Temperatures from 46 to 82 F allow powdery mildew to develop, but disease is favored by temperatures of 64 to 70 F; disease risk decreases when temperatures consistently exceed 86 F for six hours or more.

Regular fungicide applications are needed to prevent infection and are applied regardless of visible symptoms. Different fungicides are utilized for powdery mildew control during three distinct periods of the season: emergence to mid-June; mid-June to bloom; and bloom to preharvest. The Fungicide Resistance Action Committee (FRAC) mode of action classification codes are included to help growers make resistance management decisions.

Emergence to mid-June (current conditions)

Consider a combination of applications of sulfur (FRAC M2), oils, Flint (trifloxystrobin, FRAC 11), Rhyme (flutriafol, FRAC 3), Procure 480 SC (triflumizonle, FRAC 3) or Unicorn DF (tebuconazole + sulfur, FRAC 3 + M2). Under high pressure, tank mix with oils and integrate copper (FRAC M1) into your downy mildew programs when possible. Avoid tank mixes of copper and sulfur, as phytotoxicity may occur.

Mid-June to bloom

Consider Rhyme (flutriafol, FRAC 3), Procure 480 SC (triflumizole, FRAC 3), Luna Experience (fluopyram + tebuconazole, FRAC 7 + 3), Vivando (metrafenone, FRAC 50), Gatten (flutianil, FRAC U13) and Torino (cyflufenamid, FRAC U6). Under high pressure, tank mix with oils and integrate copper into your downy mildew programs when possible.

Bloom to preharvest

Use a combination of Quintec (quinoxyfen, FRAC 13, 21-day preharvest interval), Pristine (pyraclostrobin + boscalid, FRAC 11 + 7, 14-day preharvest interval), Luna Sensation (fluopyram + trifloxystrobin, FRAC 7 + 11, 14-day preharvest interval), Vivando (metrafenone, FRAC 50, 14-day preharvest interval), Gatten (flutianil, FRAC U13) and Torino (cyflufenamid, FRAC U6, six-day preharvest interval).

Many fungicide programs can give adequate disease control on leaves when applied preventively under low disease pressure. On cones, however, differences among fungicides are substantial. Mid-July through early August is an essential disease management period. The fungicide Quintec (quinoxyfen), Luna Sensation (fluopyram + trifloxystrobin), Vivando (metrafenone), Gatten (flutianil, FRAC U13) and Torino (cyflufenamid) are especially effective during this time and should be utilized in regular rotation when burrs and cones are present.

Fungicide applications alone are not sufficient to manage the disease. Under high disease pressure, mid-season removal of diseased basal foliage delays disease development on leaves and cones by lowering inoculum and increasing airflow. Do not apply desiccant herbicides until bines have grown far enough up the string so that the growing tip will not be damaged and bark has developed—typically 10 feet or more. In trials in Washington, removing basal foliage three times with a desiccant herbicide (e.g., AIM) provided more control of powdery mildew than removing it once or twice. Established yards can tolerate some removal of basal foliage without reducing yield. This practice is not advisable in baby plantings (less than 3 years) and may need to be considered cautiously in some situations with sensitive varieties such as Willamette. The potential for quality defects and yield loss increases with later harvests when powdery mildew is present on cones.

For additional information on powdery mildew of hop, refer to the MSU Extension article, “Managing hop powdery mildew in Michigan.”

Insects

There is a lot going on with insects this week! European corn borer is still active, mite levels are building, potato leafhopper populations are high in some locations, particularly in southern Michigan. Rose chafer is out and active and Japanese beetle will emerge any day.  

Japanese beetle adults will be emerging any 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 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. 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, consider more aggressive management as yield and quality are directly affected.

For more information on biology and management, refer to the Japanese Beetle factsheet.   

European corn borer is active in many areas of the state. To estimate adult flight in your region, refer to the Enviroweather European Corn Borer Model. European corn borer overwinters as larvae inside the host plant where it pupates (matures) in response to warming temperatures in spring. First generation flight of moths is expected from 450–950 growing degree days (GDD) base 50, based on a March 1 start date for GDD accumulation.

The average first generation adult flight and egg laying period in Michigan occurs in June (early June in southern Michigan and mid- to late June in northern Michigan). In single generation locations, the larvae generated from this first flight of adult moths will overwinter. In warmer or multigenerational locations, larvae from the first generation pupate and a second adult moth flight and egglaying cycle occurs between 1,450 and 1,950 GDD base 50 (mid-July to mid-August on average). In areas where both one and two generation corn borer co-exist, emergence and development may overlap or occur in succession, and requiring consistent scouting and monitoring.

Refer to the Enviroweather European corn borer model for the current European corn borer status. Be sure to select the weather station closest to your hopyard for more accurate information.

European corn borer 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. 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. In hop, European corn borer larvae can be found in leaf petioles, sidearms, cone petioles (strigs) and bines. Their location and prevalence in the plant dictates the severity of damage. The most severe damage observed in Michigan hops occurs when hopyards are infested by first generation flight in June during bine elongation and subsequent sidearm and cone development stages. This early infestation greatly reduces yield and leads to variable cone maturity dates.

Growers should focus on scouting for adult moths and eggs to allow for corrective management before larvae enter the bine. European corn borer eggs are smaller than the head of a pin but are laid in visible groupings. Eggs are white when first laid but change to yellow and then develop a black spot (the larval head capsule) just before hatching. Eggs are likely deposited on the underside of hop leaves in masses of 20 to 30 and covered with a waxy film. If available, growers may have better luck scouting and spotting eggs in adjacent corn fields.

European corn borer larvae are light gray to faint pink caterpillars with a dark head and have dark spots along the sides of each segment and a pale stripe along the back. They grow to about 1 inch but start out very small at hatch and feed briefly on leaf tissue before boring into hop bines and even hop leaf petioles.

European corn borer pupae are smooth, reddish brown, cylindrical and about a half-inch long and found inside bines. The European corn borer moth is about 1 inch long and light brown with wavy bands across the wings. The male is slightly smaller and darker. The tip of the body protrudes beyond the wings. Adult moths are most active in grassy areas before dawn.

Small European corn borer larvae are the intended target of insecticides, so monitoring for adult moth flight is critical to predicting the start of egglaying and the subsequent window of egg hatch. Begin monitoring and trapping well before predicted flight (450 GDD50) to avoid missing the beginning of flight.

For information on European corn borer management, refer to the MSU Extension article, “Be on the lookout for European corn borer in hops.”

Potato leafhopper levels have been elevated in some locations. 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. The most common classes of insecticides recommended for control of potato leafhopper include the pyrethroids, neonicotinoids and organophosphates. Organophosphates and pyrethroids are broad spectrum and disrupt natural enemies often leading to very serious mite issues. These should be used with extreme caution and only when absolute to target another pest at the same time—for example, Japanese beetle on baby hops. Neonicotinoids are, by comparison, longer lasting and narrow spectrum, making them a solid choice for potato leafhopper management.

To learn more about potato leafhopper, refer to the hop potato leafhopper factsheet and the Michigan Hop Management Guide.

Twospotted spider mite pressure is high with the continued high temperatures. Growers may find they need to apply miticide more often this season as their reproductive rates are temperature driven. Carefully monitor mite pressure as twospotted spider mite thrive under hot and dry conditions.

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 and the Michigan Hop Management Guide for information on available miticides.

Stay connected!

For more information on hop production practices, please sign up for the hop MSU Extension Newsletter, the FREE MSU Hop Chat Series and continue to visit Michigan State University Extension’s Hops webpage or the MSU Hops News Facebook.

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 pestid@msu.edu.

Become a licensed pesticide applicator

All growers utilizing pesticide can benefit from getting their license, even if not legally required. Understanding pesticides and the associated regulations can help growers protect themselves, others, and the environment. Michigan pesticide applicator licenses are administered by the Michigan Department of Agriculture and Rural Development. You can read all about the process by visiting the Pesticide FAQ webpage. Michigan State University offers a number of resources to assist people pursuing their license, including an online study/continuing ed course and study manuals.

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.