Description and life history
Fungi causing powdery mildews are obligate parasites that are able to grow and reproduce only on living host-tissues. Powdery mildews affect a large number of plant species; however, each strain of powdery mildew fungus attacks only one or a few plant species. The fungus causing hop powdery mildew is known as Podosphaera macularis and attacks only hop.

Powdery mildews are characterized by the appearance of white to grayish ‘powdery’ spots or patches on affected plant tissues. Powdery mildew covered hop shoots, called flag shoots, are characteristic of early season infection. Cones and flowers are also affected.

Powdery mildew on hop leaf
Powdery mildew spike
Powdery mildew on hop cones

The powdery substance is actually a mat of fungal growth called mycelium that contains the reproductive structures of the fungus. Severe infestations may result in the entire plant surface being covered with powdery mildew growth.

Two types of reproductive structures may be present. The first are called conidia or conidial spores, and are the asexual reproductive structures of the fungus. Chains of conidia are produced on outgrowths of the mycelium called conidiophores. When mature, these conidia are released and carried by wind currents to new host tissue where they can begin a new infection. The conidia are able to survive for only a few hours in the absence of a host.

When conditions become unfavorable for asexual reproduction, sexual spores called ascospores may be produced. The ascospores are enclosed inside spherical structures called cleistothecia which appear as pinhead size brown to black spots on infected tissue. The cleistothecia are much more persistent than the sexual spores and may survive for many months. Under favorable conditions, the ascospores are released explosively from the cleistothecia to begin a new infection cycle.

Powdery mildew may over winter as mycelium in aerial buds or as cleistothecia in hop yard debris. In early spring, infections resulting from mycelium overwintering in aerial buds produces mildew covered flag shoots. Spores from the flag shoots disperse causing secondary infections which if not controlled can "move up the plant" eventually infecting hop flowers and cones. Cleistothecia would typically produce infection later in the spring. To date however, cleistothecia have only been reported from a few cones grown in Washington state and appear to be rare in the Pacific Northwest hop growing regions. High humidity and temperatures between 70° and 75° Fahrenheit favor conidia germination and growth; however, hop powdery mildew is able to grow and produce conidia quite well in dry climates at temperatures from 55° to 85° Fahrenheit. (back to top)

Damage to Hop
Powdery mildew infections on leaves and stems do not generally result in serious economic damage, but do serve as a source of inoculum for flower and cone infestations that can cause serious economic damage. Infections on flowers result in the death of the infected flower so that no cone is produced. Heavy infestations can completely destroy the crop. Infection of young cones inhibits cone growth and results in cones developing into small hard knobs. Infection of older cones results in the development of distorted or red cones with a mushroom-like odor, but apparently has no effect on the lupulin glands. (back to top)

Powdery Mildew Management
A successful program for managing powdery mildew, or for any pest, consists of several important components: Pest identification, monitoring for pest or pest damage, control guidelines (e.g. action or economic thresholds), and one or more effective management options.

Identification. Proper identification of the pest is essential for determining if control is necessary and for selecting the most effective control option. Use the pictures and descriptions provided in this web site or OSU's "On Line Guide to Plant Disease Control" web site to aide in identifying powdery mildew. If you need assistance, contact the a University specialist using the information provided at the bottom of this page.
Scouting. It is important to check fields for powdery mildew on a regular basis. The latent period of the disease is 5 to 7 days., Therefore, fields containing susceptible hop varieties should be scouted at least once per week beginning as soon as vines emerge in the spring. Scouting should continue until after harvest. Due to the possible presence of powdery mildew-susceptible off-types, even fields planted to resistant varieties should be scouted. The number of plants checked in each field will depend on a number of factors, including, the number of fields that need to be checked each week, the size of the fields, the size of the hop plants (larger plants will require more time to check), and the availability of scouts.

A suggested method would be to travel a ‘W’-shaped path that covers the entire field checking a minimum of 5 to 10 plants for each of the four legs of the ‘W’. In the early part of the season this can be done fairly quickly. Later in the season, when the plants are larger, and cones and flowers are present, more time will be needed to effectively check the same number of plants. Visually examine stems, leaves, flowers, and cones of selected plants for signs of the disease. A ladder or a pruning pole will be needed to inspect plant parts beyond hand-reach. It is important that flowers, cones and foliage at the trellis top be examined. (back to top)

Control guidelines.
Powdery mildew can be extremely prolific. The spores are wind-blown and the disease can spread rapidly within and between hop fields resulting in complete crop loss. There is essentially a zero tolerance for powdery mildew on hop. If powdery mildew is found, the field will need to be treated. Because of the potentially devastating economic losses resulting from infection of hop flowers (burrs) and cones, the objective of powdery mildew control measures should be to prevent these infections.

Control options.
All practical methods must be used to reduce or eliminate primary infection, the initial infections resulting from overwintering forms of the fungus: conidia from overwintering mycelium on aerial buds and ascospores from overwintering cleistothecia. Reducing or eliminating primary infections will proportionally reduce the levels of secondary infection: infections resulting from the in-season production and spread of conidia occurring on a 5 to 7 day cycle. (back to top)

Cultural control measures:
Good field sanitation can be an extremely effective means of reducing or eliminating sources of primary infection. Stripping the leaves, chemically or manually, from the bottom three to four feet of the plant will remove some primary inoculum and remove the leaves most susceptible to secondary infection. It will also help to increase airflow and reduce humidity in the hop field, creating a less favorable environment for powdery mildew growth and reproduction.

Plants or plant debris left in fields after harvest may harbor large numbers of overwintering cleistothecia or conidia that can infect aerial buds. These plants and plant debris should be removed from the field and/or destroyed by composting. Fields should be cultivated to bury all non-composted debris.

Re-growth of hop plants after harvest can provide a source of conidia and cleistothecia that increases the amount of overwintering powdery mildew. If post-harvest re-growth cannot be prevented, fields should be scouted as previously described. If powdery mildew is found, the field should be treated to reduce the amount of inoculum overwintering.

Do not apply excessive nitrogen. Hop powdery mildew grows and reproduces better on young, tender plant tissue than on older, more established plant tissues. Excessive nitrogen fertilization can produce a flush of tender foliage providing an environment that favors establishment and growth of primary and secondary fungal infections.

Pruning back hop crowns two to four inches before plant growth occurs during the early spring can greatly reduce the amount of overwintering powdery mildew in aerial buds. For this practice to be effective crowns must be hilled during the previous season to promote crown growth so that deep pruning does not weaken hop crowns.

Scratching hop fields using a light harrow may also be an effective means of reducing primary infections due to powdery mildew on aerial buds. (back to top)

Legislative control measures:
A quarantine has been established in order to prevent the importation of powdery mildew infested plant material into Idaho . All persons growing hops, commercially or privately, should be aware of and comply with the provisions of the quarantine. Even in the absence of the disease, growers will need to inspect their hop fields for hop powder mildew on a regular basis.

Biological control measures. Naturally occurring biological control agents do not normally exist at levels that provide economic control of powdery mildew. A commercial formulation of a biofungicide is available (see table below). The active ingredient consists of the spores of a fungus, Apelomyces quisqualis, that is a parasite of fungi causing powdery mildews. The spores of the parasitic fungus germinate and invade cells of the powdery mildew fungus destroying the powdery mildew. A. quisqualis must be applied when powdery mildew is present but before it is at high levels. A. quisqualis requires cool, humid conditions for germination and is less effective in hot dry weather. (back to top)

Resistant hop varieties.
Different hop varieties carry genes that code for resistance to powdery mildew. Seven powdery mildew resistance genes, labeled Rb, and R1-R6, have been identified from different hop varieties. There are also races of hop powdery mildew able to overcome resistance coded for by these genes. These powdery mildew races have genes, called virulence genes, for overcoming resistance coded for by these resistance genes. There is a one to one relationship between genes coding for resistance in hop varieties and genes coding for overcoming resistance in powdery mildew races. To successfully infect a hop variety containing a particular resistance gene, a powdery mildew race must have a virulence gene corresponding to that resistance gene. For example, the variety Nugget contains the R6 resistance gene. The hop powdery mildew race introduced into the Pacific Northwest cannot successfully infect Nugget. Therefore, this hop powdery mildew race does not contain the virulence gene, V6, for overcoming resistance coded for by the R6 gene. Cascade has the R5 resistance gene. It is known that the hop powdery mildew race introduced into the Pacific Northwest is able to overcome resistance coded for by the Rb gene, so it must have the corresponding Vb gene for virulence. (back to top)

Several hop varieties are resistant to the race of hop powdery mildew introduced into the Pacific Northwest.

Nugget, Cascade, and Mt. Hood appear to be resistant. Horizon, a variety originally though to be resistant, was found to be infected with powdery mildew in Washington state.

Fuggle, Perle, Tettnang, and Hallertau are moderately resistant. Columbus, Tomahawk, and Zeus, were originally categorized as moderately resistant as well, but have suffered high levels of infection in Washington state.

Cluster, Chinook, Willamette, Liberty, Chelan, Eroica, Symphony, and Galena are susceptible.

Chemical control measures:
Because of the recent introduction of hop powdery mildew into the region, the list of chemicals are available for use in hop is constantly changing. A number of the “softer’ compounds, AQ10, and various potassium bicarbonate and crop oil formulations have full EPA labels for hop. Several fungicides are available through Section 18 Emergency Exemption labels. A list of currently available chemicals currently available, along with use information is provided OSU's On Line Guide to Plant Disease Control web site. Whatever chemicals are used, thorough coverage is a must. Therefore spray volumes (30-250 gallons per acre) and slow sprayer speeds (1-2 mph) must be adjusted according to plant size to assure complete coverage. Check the Pest Management News page on this site for availability of compounds through Section 18 and 24c registrations.

Due to constantly changing labels, laws, and regulations, the University of Idaho can assume no liability for the suggested use of chemicals contained in this document. The pesticide label is a legal document. The user is required by law to use the pesticide in a manner consistent with label directions. If for any reason directions given in this publication are not consistent with the label, you are still legally bound by label restrictions. (back to top)

Growers with question about identification and management of hop powdery mildew can view the on-line version of OSU's On Line Guide to Plant Disease Control or contact the University of Idaho hop pest management specialist:

Dr. Jim Barbour
Parma Research and Extension Center, 29603 U of I lane
Parma, ID 83660

Phone: (208) 722-6701
Fax: (208) 722-6708
Email: jbarbour@uidaho.edu