Hop Viruses

Introduction

Hop (Humulus lupulus) is a perennial, dioecious plant in the Cannabaceae family. Female hop plants are grown commercially for their cones, which provide the bitter acids and aromatic compounds important to brewing.

Several viruses and viroids can infect hop. These pathogens are especially challenging because hops are perennial and commonly propagated vegetatively. An infected plant can therefore remain in a hop yard for many years, and infected rhizomes, cuttings, or plantlets can introduce a pathogen into a new yard.

Some infections cause conspicuous leaf symptoms or plant stunting. Others are essentially symptomless but can still affect plant vigor, cone yield, bitter acids, or essential-oil composition. Prevention through pathogen-tested planting material is the foundation of virus and viroid management in hops.

Important Viruses and Viroids of Hop

Pathogen	Typical symptoms	Primary means of spread	Potential impact
Apple mosaic virus (ApMV)	Chlorotic rings, arcs, bands, oak-leaf patterns; sometimes symptomless	Infected planting material; mechanical transmission and plant-to-plant/root contact	Reduced vigor, cone yield and α-acids; strongly cultivar- and environment-dependent
Hop mosaic virus (HpMV)	Often symptomless; chlorotic mosaic or mottling in sensitive cultivars	Infected planting material; aphids, especially damson-hop aphid	Reduced vigor, yield and quality in susceptible cultivars
Hop latent virus (HpLV)	Usually symptomless	Infected planting material; aphids	Generally mild, but effects depend on cultivar and mixed infections
American hop latent virus (AHLV)	Usually symptomless	Infected planting material; aphids	Usually limited visible effects
Hop latent viroid (HLVd)	Usually no obvious foliar symptoms	Infected planting material; mechanically through contaminated sap	Can reduce yield and α-acids and alter essential-oil composition
Hop stunt viroid (HSVd)	Yellow-green foliage, shortened internodes and sidearms, progressive stunting	Infected planting material; mechanically through contaminated sap	Potentially severe reductions in yield and α- and β-acids

Apple mosaic virus (ApMV)

Apple mosaic virus is among the most economically important viruses of hop. Symptoms include chlorotic ringspots, arcs, bands, or characteristic oak-leaf line patterns on leaves.  Symptom expression can vary considerably among cultivars, seasons, and environments, and infected plants may sometimes appear healthy. Infection can reduce plant vigor, cone yield, and α-acid production.

Hop Carlaviruses

Three related viruses in the genus Carlavirus occur in hop: Hop mosaic virus (HpMV), Hop latent virus (HpLV), and American hop latent virus (AHLV). Infections are often mild or symptomless. However, HpMV can cause chlorotic mosaic, mottling, and reduced vigor in susceptible cultivars. Symptoms may be sharply bounded by major leaf veins.

The damson-hop aphid (Phorodon humuli) is an important vector of hop carlaviruses. Aphid transmission is nonpersistent: an aphid can acquire and transmit virus during relatively brief feeding probes. For this reason, insecticides used to manage aphids should not be expected to completely prevent virus transmission.

Hop viroids

Don’t Overlook the Viroids. Viroids are very small infectious RNA molecules. Unlike viruses, they do not encode proteins, but they can nevertheless disrupt plant growth and metabolism.

Hop latent viroid (HLVd) causes few or no obvious symptoms in most hop cultivars. Its name, however, should not be interpreted to mean that infection is harmless. Research has demonstrated cultivar-dependent reductions in cone yield and α-acid content associated with HLVd. Infection can also alter essential-oil and other secondary-metabolite profiles important to brewing quality. This is an important change from older descriptions of HLVd that characterized it primarily as a symptomless infection.

Hop stunt viroid (HSVd) is potentially much more damaging. Early symptoms may include pale or yellow-green basal foliage and yellow speckling along major veins. Over time, infected plants may develop shortened internodes and sidearms, reduced vigor, and obvious stunting. Cone production and α- and β-acid yield may be substantially reduced. Effects vary among cultivars, and symptoms may take several years to become obvious. The long period between infection and obvious symptoms makes HSVd particularly challenging: apparently healthy plants can be infected and serve as sources of inoculum.

How Do Viruses and Viroids Get Into a Hop Yard?

Infected planting material. For most hop viruses and viroids, the most important pathway into a new hop yard is infected vegetative propagation material. Rhizomes, rooted cuttings, and plantlets produced from infected mother plants can carry a pathogen even when the plants appear healthy. For this reason, know the health status of your planting material. Do not use an established hop yard as a source of propagation material unless mother plants have been appropriately tested.

Pathogen-tested foundation hop material is available through clean-plant programs, including the USDA-supported National Clean Plant Network for Hops and Clean Plant Center Northwest at Washington State University.

Aphids. HpMV, HpLV, and AHLV can be transmitted by aphids. The damson-hop aphid is the principal aphid vector associated with natural spread in hop yards. Because transmission occurs rapidly during aphid probing, aphid management alone cannot guarantee prevention of virus spread. Start with virus-tested plants and integrate aphid monitoring and management into the overall disease-management program.

Tools, equipment, and plant-to-plant contact. ApMV and the hop viroids can be spread mechanically when infected plant sap is moved from one plant to another. Activities that cut, wound, or otherwise damage plants—including pruning, training, basal growth removal, mowing, and harvesting—can potentially contribute to pathogen movement. Hop roots can also persist extensively underground. This makes complete removal of infected perennial plants difficult and may facilitate persistence within affected portions of a yard.

Management: Keep Them Out and Limit Their Spread

There are no curative field treatments for hop plants infected with viruses or viroids. Management therefore depends on exclusion, sanitation, early detection, and appropriate removal of infected plants.

1. Start clean. Plant pathogen-tested material from a reputable clean-plant source whenever possible. This is the single most effective strategy for preventing viruses and viroids from becoming established in a hop yard. This is particularly important when:

• establishing a new hop yard;
• introducing a new cultivar;
• obtaining plants from another farm;
• establishing mother plants for propagation; or
• replacing plants in an existing yard.

2. Scout for mosaic or mottling, ringspots or oak-leaf patterns, unusual yellowing, shortened internodes, weak or shortened sidearms, reduced plant vigor, poor cone development, and patches of progressively stunted plants. Because several important pathogens cause latent or highly variable symptoms, laboratory testing is necessary for confirmation.

3. Practice good sanitation. To reduce mechanical spread:

• Work in healthy or pathogen-free yards before working in infected or suspect yards.
• Remove plant debris and sap from pruning, cutting, and harvesting equipment.
• Appropriately disinfect cutting tools between suspect plants or production blocks.
• Clean equipment before moving between hop yards.
• Avoid moving infected roots, crowns, or plant debris between yards.
• Consider non-mechanical approaches to basal growth management where appropriate if viroids are present.

Cleaning is important before disinfection. Plant sap and organic debris can reduce the effectiveness of disinfectants.

4. Manage aphids as part of an integrated program. Monitor for damson-hop aphid and follow current Wisconsin recommendations when management is warranted. Remember that insecticides can reduce aphid populations but may not prevent transmission of non-persistently transmitted viruses. Clean planting material remains the first line of defense.

5. Take Hop stunt viroid seriously. If HSVd is confirmed, remove infected plants, including as much crown and root tissue as practical. Plants adjacent to confirmed infections should be considered potentially exposed. Depending on the situation, testing and/or removal may be appropriate. Monitor the affected area for regrowth from infected roots and prevent volunteer hop plants from establishing. If infection is widespread, removal of the entire affected planting followed by re-establishment with pathogen-tested material may ultimately be more effective.

Can an Infected Hop Plant Be “Cleaned”? Specialized clean-plant laboratories can sometimes recover pathogen-free material from valuable infected cultivars using meristem or microshoot-tip culture, sometimes in combination with temperature or other treatments. However, viroids can be particularly difficult to eliminate. Research with HLVd and HSVd demonstrates that successful elimination varies considerably among cultivars and treatments. Regenerated plants must therefore undergo repeated molecular testing before they can be considered pathogen-free. For commercial growers, purchasing tested material from an established clean-plant program is preferable to attempting to propagate from known infected plants.

For assistance with diagnosis and appropriate sample collection, contact the UW–Madison Plant Disease Diagnostics Clinic or UW–Madison Vegetable Pathology.

Additional Resources

• UW–Madison Plant Disease Diagnostics Clinic – Plant disease diagnostic services and information on submitting samples
• National Clean Plant Network for Hops / Clean Plant Center Northwest, Washington State University –  Source of pathogen-tested hop foundation material and information on hop clean-plant programs