Controlling powdery mildew

The big question for many Michigan growers this season has been how to manage the risk of powdery mildew. With harvest underway or completed for many Michigan growers, the focus now shifts to how to best prepare yards with existing powdery mildew infections.

Powdery mildew (Podosphaera macularis) is an emerging disease in Michigan hopyards that has serious implications for growers. Hop powdery mildew is completely unrelated to hop downy mildew (Pseudoperonospora humuli) and was first detected in Michigan in 2014.

Powdery mildew has been a concern on greenhouse/baby plants, but was not a significant issue in the field until 2017 when severe outbreaks occurred at multiple Michigan farms, likely from introduction on planting material and highly conducive environmental conditions.

Powdery mildew may overwinter in several ways. The fungus may persist as mycelia (fungal threads) on live tissue of the host, most important in and on crown buds. Shoots emerging from infected buds form “flag shoots,” which are covered with spore masses, appearing white, stunted and with distorted leaves. Flag shoots are rare (less than 1 percent of total shoots) and healthy shoots quickly outgrow infected shoots, making detection difficult. The spore masses produced on flag shoots spread to adjacent healthy tissue causing new infections.

White flag shoot emerging in spring, infected with powdery mildew. Photo by David Gent, USDA Agricultural Research Service, Bugwood.org

Another means of overwintering is by sexual spores produced in a survival structured (termed a chasmothecium). However, chasmothecia have not been associated with the outbreaks this season. Ascospores are discharged and land on newly emerged shoots where they germinate, infect the plant and eventually produce a new spore mass of asexual spores (conidia). Conidia are produced in large numbers over multiple cycles and are dispersed via wind, rain splash, insects, tractors, equipment and humans.

The life cycle of Podosphaera macularis, the causal agent of hop powdery mildew. Prepared by V. Brewster, Field Guide for Integrated Pest Management in Hops, Third Edition.

Fungicide applications alone are not sufficient to manage powdery mildew. Complete chemical or mechanical removal of the first flush of growth in spring is an important management strategy. Under high disease pressure, mid-season removal of diseased basal foliage delays disease development on leaves and cones. Desiccant herbicides should not be applied until bines have grown far enough up the string so the growing tip will not be damaged and bark has developed on stem.

In trials in Washington, removing basal foliage three times with a desiccant herbicide provided more control of powdery mildew than removing it once or twice, particularly on cones. Established yards can tolerate some removal of basal foliage without reducing yield. This practice is not advisable in baby yards (less than 2 years), and may need to be considered cautiously in some situations with sensitive varieties such as Willamette.

Bud infection leading to flag shoots in the following year occurs sometime between mid-July to late September/early October. The earlier disease occurs during this period, the more likely it is that flag shoots will be produced in the following year. In Washington, post-harvest applications of fungicide have not been shown to be effective in reducing disease in the subsequent season.

However, there may be some value in post-harvest fungicide applications in hopyards where the disease attacked late in the season (i.e., September rather than July) and mechanical pruning is not possible in spring. In these situations, post-harvest application of products such as horticultural oils and sulfur may reduce the risk of flag shoot development and the number of chasmothecia that may overwinter.

Remove all unharvested plants promptly after harvest to minimize the number of chasmothecia in affected yards. Cultivation after harvest will bury infested plant debris bearing chasmothecia that can serve as a source of the pathogen in the following year. Most importantly, implement an appropriate powdery mildew program in spring that starts with the mechanical or chemical removal of the first flush of growth in spring, manage plant growth to limit superfluous growth, and apply well-timed fungicide applications. Weak, stressed or baby plants should not be pruned.

Though frequently cited in popular literature, optimal fertilization, soil health and water management alone are not adequate for disease control, although these practices may moderate disease levels. Likewise, biorational compounds, biological controls, manure teas and various botanicals and natural products have shown minimal to no efficacy against powdery mildew under moderate to severe disease pressure. Organic producers should consider a combination of sulfur and oil-based fungicides and, most importantly, select resistant cultivars.

For more information on the life cycle, appearance and management of powdery mildew of hop, refer to the Michigan State University Extension article, “Powdery mildew arrives in Michigan hopyards.”

Controlling downy mildew

Downy mildew was not a major issue for most Michigan hop growers this season. As harvest wraps up, we have received inquiries around the availability and efficacy of post-harvest methods to combat this damaging disease. To properly answer this question, we need to fully understand the disease cycle of Pseudoperonospora humuli, the causal agent of downy mildew of hop (see image below) and how growing conditions in Michigan can affect the efficacy of late season treatments. 

The disease cycle of Pseudoperonospora humuli, the causal agent of downy mildew in hop. Photo by V. Brewster, Compendium of Hop Diseases and Pests. View larger image.

Mycelium, the vegetative part of the downy mildew pathogen, overwinters in infected buds and crowns. As shoots emerge in spring, they may already be infected with this overwintering mycelium. As the hop bine begins to grow, the mycelium produces microscopic spore-bearing structures on the underside of leaves giving the underside a gray, fuzzy appearance. These structures give rise to spores called sporangia.

Sporangia move via wind and rain and act as the major cause of disease spread during the season, infecting new leaves, shoots and eventually even cones. The reproductive cycle that produces sporangia may repeat multiple times over the season, depending on temperature and moisture availability. Alternately, mycelium may also yield a resting spore (termed an oospore) through sexual recombination. It is unclear what role oospores play in the disease cycle.

When considering a post-harvest treatment, remember that the downy mildew fungus overwinters in the plant itself and is protected by the plant epidermis. While there is limited data from other production regions to suggest post-harvest treatments are beneficial in terms of a reduction in disease next season, there is a general correlation between disease presence and severity from season to season that warrants further research.

Systemic fungicides are typically described as locally systemic, acropetal systemic (moving upward) or basipetal systemic (moving downward). Locally systemic materials may not be useful for treating downy mildew at this time because they do not move far from the site of application and don’t reach the sites where the pathogen overwinters. Truly systemic fungicides are taken up by the xylem or phloem tissue of the plant and moved to new tissues. Many of the systemic materials today are only translocated outward via the xylem or water-conducting tissues. Distribution of a systemic fungicide in the phloem or carbohydrate-conducting tissues (basipetal translocation or downward movement) would include translocation into the crown and roots where downy mildew overwinters.

Fungicides labeled for hops that move systemically downward include Aliette and phosphonate fungicides. Given the overwintering location of the fungus, a systemic fungicide with downward movement would be the best option. That being said, with little remaining leaf area and bines shutting down from shorter day length, there may be limited value to a fungicide application at this time. Based on the lack of supporting data, post-harvest treatments for downy mildew are not recommended. Growers with high levels of downy in their hop yards should instead focus on developing an early and aggressive protectant treatment program for next spring. Refer to the MSU Extension article, “Battling downy mildew as hop harvest approaches,” for more information.

Post-harvest insect pests

Let’s consider some of the problematic insect pests still lingering after harvest. Potato leafhoppers, damson hop aphids and twospotted spider mites were all reported in hopyards this season, and growers are considering what treatment strategies are available post-harvest. 

Potato leafhoppers were a real issue for some growers, but a treatment now would not affect populations next season because the potato leafhoppers currently in hopyards will not survive the winter. Potato leafhoppers move north on spring storms and reinfest each year. Once potato leafhoppers arrive, they reproduce in the hopyards and can cause significant damage. However, their inability to survive the winter wipes out the entire population in Michigan each year.

Potato leafhopper nymph (left/top) and hop leaf covered in nymphs and showing signs of feeding damage with necrotic leaf margins (right/bottom). Photos by Erin Lizotte, MSU Extension

Damson hop aphids were observed in higher numbers as harvest approached and some growers had problem infestations at harvest. Again, we must look at the life cycle of the pest to determine if a post-harvest treatment could help keep numbers down next season.

Hop aphids overwinter as eggs on Prunus species (genus of trees and shrubs that includes plums, cherries, peaches, nectarines, apricots and almonds). In early spring, eggs hatch into stem mothers that give birth to wingless females that feed on the Prunus host. In May, winged females are produced and travel to hop plants where additional generations of wingless females are produced. As cold weather approaches, winged females and males are produced, move back onto a Prunus host, mate and lay eggs for before winter.

We expect this migration away from hops and onto plants in the Prunus genus occurs sometime in September, making post-harvest treatments ineffective during most seasons.

A wingless damson hop aphid on hop. Photo by Erin Lizotte, MSU Extension

Twospotted spider mites were an issue for some growers this season. Again, if we examine the life cycle of the pest, we can make better decisions about the potential impact of post-harvest management.

Hops are a unique situation when it comes to mite management. Many horticultural crops use post-harvest treatments in infested sites to reduce overwintering populations, but hop growers remove the plant itself quite early in the season, likely removing a large portion of the mites with it. The twospotted spider mites that remain overwinter as mated females on plant debris and trellis structures in the hopyard. Mites remaining in the hopyard are susceptible to miticide applications, but likely account for a relatively small number in fall when they are beginning to migrate to overwintering sites.

Unless the infestations were at an economically significant level, avoid miticide applications if possible. Refer to the MSU Extension article “Twospotted spider mite numbers up in hopyards” for more information on managing twospotted spider mites.

Twospotted spider mite, eggs and cast skins. Photo by Erin Lizotte, MSU Extension

Sanitize and fertilize in fall

Consider the importance of sanitation at this time. Remove all bines and leaves from the hopyard after the first hard freeze. Plant tissues can harbor insects and disease and should be removed, fully composted, buried or burned. Growers who did not harvest this year (as in first year hops) should remove aerial plant parts after a hard frost to prevent increased pest and disease pressure next season.

Growers planning to utilize compost fertilizer can apply it this fall. Also, consider sub-soiling between rows in areas in need of better drainage, applying herbicides for perennial weed control and removing unproductive or diseased crowns.

Trickle and drip lines and tape are designed to be self-draining, but manifolds and supply systems need attention to make sure no water pockets remain to freeze. Winter rodent damage can turn usable drip tape and trickle line into junk rapidly. Lines that are to be moved for next year are best stored in the barn. Lines overwintering in the field stand less rodent damage if not covered by plastic, plant material or mulch. To read more about preparing irrigation systems for winter, refer to the MSU Extension article “Protecting irrigation equipment from winter damage.”

Take time to evaluate the season

Lastly, it is well worth your time to set aside a moment to reflect on the season. Take note of trouble areas in the hopyard and consider planning how to address pest or nutrient issues in the following season. Review your spray records to ensure they are complete and begin investigating a food safety plan for next season. For more information on record keeping, visit the MSU Pesticide Safety and Education Resources webpage.

This material is based upon work supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2017-700006-27175. 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.