Virus-Like Agents of Hop Plants

Dr Ken Eastwell, Associate Plant Pathologist, Department of Plant Pathology, Washington State University, Prosser, Washington

The current program at Washington State University continues its long involvement in virus research of hops. Unlike other broad categories of plant pathogens, viruses and virus-like agents are unique in that once they infect hop plants, there are no practical means by which the pathogen can be eliminated. The only opportunity for control occurs before plants become infected. Hop plants are particularly prone to diseases caused by viruses and viroids because the plants are exposed to possible infection over many growing seasons and because the plants are vegetatively propagated. The latter provides a crucial route for inadvertent propagation and rapid distribution of infected plants. Progress in hop breeding continues to increase the yield and quality of hops. However, the relatively recent introduction of new genetic material in the development of commercially important cultivars has created the potential for unanticipated interactions between virus-like agents and hop production.

Hop plants are infected by a limited number of viruses and viroids that are currently controlled by roguing infected plants and renewing hop yards with virus-free roots. Three carlaviruses are frequently encountered in hop production. The wide distribution of these viruses and efficient vectoring by aphids has made exclusion of these viruses impractical. Their greatest impact on production is likely during the establishment of new hop yards. Ilarviruses, on the other hand, are economically important viruses of hop production and it is important that we can detect all of the virus strains that may be encountered. Moreover, it is currently not known where the source of infection of hop plants originates other than through infected propagation material. We are characterizing the ilarviruses of hops at the molecular and serological level and developing reliable diagnostic methods in aid of answering these questions. Specific antibodies for carlaviruses and ilarviruses are being developed through the use of synthetic peptides representing different regions of the virus coat proteins. Three viroids are known to infect hop plants. Of these, Hop stunt viroid is of most concern. The interaction of Hop stunt viroid with the diverse genotypes of commercial hop cultivars currently in production in North America is not known. Determination of the effect of this viroid on production and its relative transmissibility from different hop cultivars is needed to develop meaningful control programs.

The principal objective of this research program is to develop appropriate disease management strategies. This is achieved through characterization of virus-like agents associated with disease. The direct result of this research is the development of reliable diagnostic methods leading to a firm understanding of modes of transmission. Additionally, methods are required to obtain foundation plants that are free of virus-like agents. Currently, meristem culture is used to achieve this goal, but more efficient approaches are being sought.