Hop Downy Mildew Research

Matthew H. Wood, Graduate Student, Dept. Plant & Soil Science.

Photos (jpg):

Closeup of potted plants

First trial

Matt inoculating hops

I am conducting research on downy mildew of hop, caused by the pseudofungus Pseudoperonospora humuli. Downy mildew is one of the most important diseases affecting this crop worldwide and is a major factor when hops are grown in the northeastern US. Once the perennial hop plant has a systemic infection, the disease will return yearly when the weather conditions are right, reducing the yield and quality of the harvested cones. The disease will eventually kill the rootstock of susceptible varieties

My research here at UVM is focussed on exploring some potential biological control methods that may help to suppress downy mildew in field grown hops. Biological controls are important because when used to their full potential, they create an efficient and environmentally safe growing system by creating a demand for composted materials and reducing chemical use. Some researchers have demonstrated that plants grown in soils that have been ammended with semi-mature composts are somewhat protected from various diseases, but most of these are caused by soil-borne pathogens. More work needs to be done exploring how to protect plants from leaf infections like downy mildew by using biological agents.

My greenhouse trials explore the use of semi-mature composts such as composted cow manure/bedding and composted pine bark to suppress downy mildew in potted hop plants. These two composts are being tested by two different application methods, as a soil ammendment and as a foliar applied compost tea. The soil ammendment tests will be done by planting hops into potting mix ammended with either 10% composted cow manure or 40% composted pine bark. The compost teas are produced by incubating 1 part compost to 5 parts water for about 5 days, filtering through cloth, and spraying onto the leaves of hop plants grown in normal potting mix.

I am also testing the effectiveness of plant immunization techniques in protecting hop plants from downy mildew. I will harvest downy mildew spores from infected hop plants, add them to water, and inject this solution into healthy hop plants. In other plant/pathogen situations, this has induced a hypersensitivity within the plant to invading pathogens, protecting them from further infection.

The treatments described above are done in a manner that allows the soil ammendment plants several weeks growth before a challenging inoculum is applied, but the compost tea and plant immunization treatments are done only a few days prior to challenge with inoculum. This challenging inoculum is a concentrated solution of downy mildew spores harvested from infected hop plants and sprayed onto the trial plants to induce disease.

The trial plants represent many different varieties that range in susceptability to downy mildew from susceptable and moderate to resistant. These varieties include Aquila, Tettnang, Saaz, Nugget, Chinook, Fuggle, Willamette, and Perle.

Also, included along side the trial plants are control plants, of the same varieties as the treated plants, but either untreated (negative controls) or treated with a fungicide like copper sulfate (Bordeaux mixture) or metalaxyl (positive controls).

The level of disease attained by each plant will be determined by scanning selected leaves with a computerized camera to determine the % area of the necrotic spots. The values obtained from the treated plants will be compared to those obtained from the control plants to deteremine if any treatments are affording protection to the hop plants from downy mildew.

Any questions concerning this research, e-mail Matthew H. Wood..

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