Michigan hop pest report week of June 1, 2023
Michigan hopyards are entering a critical pest management period for key insect pests and diseases. Scout regularly and thoroughly.
For a hop production update from around the state, please see the June 1 Hop Crop Report.
Grass weeds need to 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.
Downy mildew infection symptoms are visible in some yards. Downy mildew is caused by the fungus-like organism Pseudoperonospora humuli. It 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. 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.
Scouting for downy mildew involves monitoring the crop for signs and symptoms of disease to evaluate the efficacy of the control program being utilized and gauge the level of disease pressure throughout the season. Growers should keep records of their scouting, including maps of their fields, a record of sampling and disease pressure, as well as the control measures utilized. Scouting should begin as soon as plants begin to grow and should continue until the crop is dormant.
To begin scouting, section your farm off into manageable portions based on location, yard 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 hotpots where you have historically encountered high mildew pressure. Weekly scouting is recommended at a minimum.
Unfortunately, even when best management practices are followed, downy mildew can gain a foothold in Michigan yards due to high disease pressure, challenges with fungicide application timing, suboptimal spray coverage, fungicide wash-off, cultivar susceptibility or a combination of these factors. In addition, fungicide resistance and infected nursery plants may play a role in some disease control failures. Recent research indicates that fungicide resistance in hop downy mildew is not widespread in Michigan for popularly used fungicides with FRAC code 40 (Higgins et al., unpublished). See the list of recommended fungicides below for more information on FRAC code 40 fungicides.
Clean planting material should be used when establishing new hopyards, since many insect and disease pests are readily spread via nursery stock. Consider purchasing a few plants from prospective nurseries and have them tested for diseases including mildews and viruses before committing to a large numbers of plants. Additionally, any other signs of poor handling at the propagator level may be used as an indicator of plant quality. Other signs of poor handling would include mite or aphid infestations, spray damage, or poor root development and would be grounds for rejecting a delivery of plants.
Growers should 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).
On mature plants, removal of the first flush of growth can help suppress disease development if disease is already present in the yard from the previous season. The early growth should be completely removed using mechanical or chemical pruning. As bines develop (8-10 feet), the removal of superfluous basal foliage and lower leaves to promote air movement in the canopy and to reduce the duration of wetting periods is recommended. This is commonly achieved through multiple applications of Aim herbicide or concentrated nitrogen fertilizer solutions. Aim will also control smaller weeds within the row. The use of Aim, pruning, and/or crowning should not be performed on baby hop plants (less than 3 years old). If there is a cover crop, it should be mowed close to the ground. If yards have no cover crop, cultivation can help to dry the soil and minimize humidity. Keep nitrogen applications moderate. For more information on pruning, refer to the Michigan State University Extension article, “Pruning hops for disease management and yield benefits”.
Apply fungicide treatments 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. If you are planning to remove the first flush of growth by pruning, the first fungicide application should occur only after regrowth. 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 are closing up. 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.
Refer to the Michigan Hop Management Guide’s section on downy mildew for additional management information.
In addition to downy mildew, growers should also be vigilant in scouting for and managing powdery mildew, which may be a bigger issue this year as conditions are hot and dry. 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.
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% 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% 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 degrees Fahrenheit 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.
Limiting powdery mildew is best approached by integrating resistant varieties, sourcing clean planting material, following crop sanitation practices, limiting early season disease establishment, optimizing fertilization and irrigation, and well-timed fungicide applications. While you may not be able to select resistant varieties because of market factors, some resistant varieties are available. The reaction of a hop variety to powdery mildew varies depending on where it is grown and which isolates of the fungus are present. Generally, Columbus, Cashmere and Galena are considered susceptible; Centennial and Chinook are intermediate; Nugget, Newport and Cascade are resistant, although races of the fungus present in Pacific Northwestern U.S. can overcome the resistance in the varieties as well.
Begin managing powdery mildew in early spring by thoroughly removing all green tissue during pruning. The goal of this early pruning is to remove the hard-to-locate flag shoots and delay or prevent infection. Eliminating flag shoots and early season disease requires removing all shoots, including those closest to the ground, on sides of hills and around poles or anchors. Mechanical pruning has been shown to be more effective than chemical pruning in eliminating flag shoots.
Pruning has not been widely adopted in Michigan, but should be considered to minimize risk, particularly if you are experiencing intense powdery or active infections are reported in the region. Avoid pruning and removing basal foliage on baby hop plants (less than 3 years old). For more information on pruning, refer to the MSU Extension article, “Pruning hops for disease management and yield benefits.”
Regular fungicide applications are needed to prevent infection and are applied regardless of visible symptoms. Appropriate timing of the first fungicide application after pruning is important to keep disease pressure at manageable levels. This application should be made as soon as possible after shoots reach 6-10 inches or regrow in pruned yards. 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
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, 6 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. 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."
There is a lot going on with insects this week! European corn borer adult flight has begun, mite levels are building, potato leafhopper arrival is imminent and rose chafer will begin emerging soon.
European corn borer
First flight of European corn borer (ECB) has begun in the lower half of the Lower Peninsula and is imminent in the northern half. 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 in response to warming temperatures in spring. First generation flight of moths is expected at 450 GDD base 50, based on a March 1 start date for GDD accumulation. Currently, GDD50 accumulation in the Lower Peninsula ranges from 193-328 with 450 GDD50 falling outside of the forecast data range currently. Historically, first flight has occurred in early June. First generation moth emergence continues for 500 GDD (through 950 GDD base 50) with females laying 200-500 eggs over a period of two to three weeks.
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.
Focus on scouting for adults 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, you may have better luck 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 subsequent window of egg hatch. Begin monitoring and trapping well before predicted flight (450 GDD50) to avoid missing the beginning of flight.
Growers should be on the lookout for potato leafhopper. 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 populations are building, due in part to hot dry conditions across much of the state. 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.
Some growers may be seeing adult rose chafer activity. 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 chafer cause simple mechanical damage, so consider that established plants can sustain a significant amount of leaf feeding from rose chafer 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.
Sincere thanks to the Michigan hop producers who provided timely input for the Michigan hop crop report.
For more information on hop production practices, please sign up for the MSU Extension Hop & Barley Production Newsletter, the free MSU Hop Chat Series and continue to visit Michigan State University Extension’s Hops website 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 firstname.lastname@example.org.
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 educational 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.