Fungal Diseases of Hop and their Control

Dr Gary G. Grove, Plant Pathologist; Mr Mark E. Nelson, Research Technological Supervisor, Department of Plant Pathology, Washington State University, Prosser, Washington

The overall objective of the Washington State University hop fungal pathology program is to identify, investigate and ameliorate losses caused by fungal pathogens of hop with current emphasis on powdery mildew and downy mildew. Short-term objectives are to make safe, effective fungicides and appropriate decision making tools available to hop growers with longer-term objectives focused on development of better, more cost effective disease management strategies.

An understanding of any fungal disease epidemic is essential to identify when protection is actually required, better time control measures, and reduce the total number of fungicidal applications. To address this need research is underway to determine when hop powdery mildew spores are actually present in hop yards and when infection periods are likely. Sensitive molecular detection techniques are under development to accurately differentiate powdery mildew spores from other fungal spores collected in spore traps. Detection and quantification of spores collected in spore traps is correlated with environmental factors to refine a recently developed model that predicts when control measures are necessary. Several disease management programs/strategies based on these results are under evaluation.

An internet-based hop decision assistance system is under development and now online (http://hops.wsu.edu). This site contains fungicide characteristics, labels, efficacy information, research results, disease forecasting information, and links to other hop-related web sites. The powdery mildew forecasting model is expected to be available to the public beginning April 2007. The fungicide database developed for Palm handheld computing devices and smartphones currently is in beta testing. The Windows mobile version is expected to be available in late 2007.

Powdery mildew was first reported in commercial production in Washington in 1997 and rapidly developed into an extremely serious problem. Researchers at WSU responded to the crisis by screening synthetic fungicides, bio-control agents, and other “soft” products to identify and support registration of those found to be efficacious in management of the disease. Downy mildew has been a serious pathogen of hop in the United States for about 100 years and so has been a long-term priority. Work of WSU researchers has significantly contributed to the recent registration of a number of safe and effective products for management of these two diseases. Additional research has focused on resistance of pathogens to fungicides. Availability of effective products with diverse modes of action is necessary for growers to implement effective resistance management strategies for these diseases.