Hop Disease Management

Dr Cynthia M. Ocamb, Associate Professor, Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon

Hop powdery mildew (Sphaerotheca macularis) was found in Washington hop yards during 1997. In 1998, Idaho and Oregon hop yards were found to be infected with hop powdery mildew. The incidence and severity of the disease is variable, depending on variety and weather, but the threat of economic yield losses are great. Subsequently, growers need to practice control of hop powdery mildew and the introduction of this fungus has resulted in increased pesticide applications as well as buyers’ rejections of hop cones.

There are seven different genes known to confer resistance to hop powdery mildew in specific hop varieties, and these “differential” hop varieties (seven in all) are used for screening powdery mildew to determine the virulence genes present in this pathogenic fungus. If a strain of powdery mildew is able to infect and sporulate on a differential hop variety, then that fungal strain is thought to contain the virulence gene(s) able to overcome that specific plant resistance gene(s). Characterization of hop powdery mildew virulence genes in the Pacific Northwest population will assist in development of new hop germplasm with resistance to powdery mildew. If regular collections of powdery mildew are made during each growing season, then the virulence and aggressiveness of the pathogen can be monitored as the fungus adapts in the Pacific Northwest hop production. Hop powdery mildew is an obligate parasite and is maintained as actively growing colonies on detached hop leaves or intact plants. We currently keep a collection of 36 isolates on detached leaves and on intact plants contained in individual chambers.

Powdery mildew collections made during 2001-05 from hop fields in the Pacific Northwest show the ability of hop powdery mildew to routinely overcome three of the seven resistance genes in laboratory evaluations. A few isolates collected during 2001, 2002, and 2004 were able to overcome another three resistance genes. Isolates collected during the latter part of the 2003 growing season from both Washington and Oregon were able to overcome the remaining resistance gene present in the cultivar ‘Nugget’ in laboratory evaluations, while ‘Nugget' infections have not been confirmed in growers’ fields. ‘Nugget’-infecting strains were detected at increasingly higher frequencies during 2004-05 from commercial yards in Oregon and Washington, and from Idaho during 2004. The detection of powdery mildew strains with wider virulence, over-coming plant resistance genes, is of concern for hop production in the Pacific Northwest. Loss of resistance in varieties such as ‘Nugget’ appears to be imminent and will require growers to manage disease more closely, probably requiring a program of protective fungicides when this dominant resistance gene breaks.

Wild hop and commercial breeding material is currently undergoing screening in my laboratory with characterized powdery mildew isolates. New germplasm has been found that is not infected by powdery mildew isolates that represent the broad range of virulence genes that I have detected in my powdery mildew collections.