Research & Resources

With support from the Hop Research Council, the Clean Plant Center Northwest conducts research on Hop stunt viroid. Just as the name implies, this disease stunts the growth of hops, leading to smaller yields. The prior Director of the CPCNW, Dr. Ken Eastwell, first identified the Hop stunt viroid in 2004. According to Dr. Eastwell, “By 2005 evidence suggested the disease was being spread when new plants were propagated from hop stunt infected plants.” Data gathered from hop research plots indicates Hop stunt viroid not only reduces yield, but it also reduces the alpha and beta acids found in hop cones. Those acids are used to give the beer its flavor. In some cases the reduction of acids was as much as 45 percent.

Infra-red photo of commercial hop garden. Hop stunt disease infected the plants in rows 8 through 34.
Infra-red photo of commercial hop garden. Hop stunt disease infected the plants in rows 8 through 34.

This aerial photo at left is of a commercial hop yard. The red indicates vegetation while the turquoise color represents soil.Researchers took the picture using a false color infra-red camera about one week before harvest. The numbers of plants in each row are the same. All of the hops were planted at the same time and grown in the same way. Yet the plants in rows 1 through 7 have grown so big and thick that it is difficult to see the soil. The plants in rows 8 through 34 came from a different source. The hop bines grew shorter with fewer leaves and more of the turquoise color (indicating soil) is visible. The photo demonstrates the significant negative impact Hop stunt viroid can have on the hop plants’ growth.

Stopping Hop stunt viroid is economically important in the state of Washington and across the U.S. The U.S. hop industry supplies one-third of the world’s annual production of hops. Over 70 percent of each year’s crop is exported to over 60 countries. Pacific Northwest commercial growers produce an annual farm-gate value exceeding $200 million.

Currently, Jeff Bullock, a plant pathology doctoral student, is researching how Hop stunt viroid interacts with infected hop plants. “We want to know if hop stunt viroid somehow silences the plants natural defense mechanisms,” Jeff says, “or is it simply able to evade plant defenses similar to cancerous tissues?” Jeff’s research will also examine how Hop stunt viroid interacts with other viruses that infect hops, such as Hop mosaic virus, to see if Hop stunt viroid causes the other virus symptoms to worsen or weaken.

Hop powdery mildew and Hop stunt viroid

Also with support from the Hop Research Council, researchers at the CPCNW are also studying the interaction between and Hop powdery mildew and Hop stunt viroid. Improved understanding of how these diseases interact in mixed infections in hop plants will lead to improved management strategies. Ultimately, the goal of CPCNW’s research is to reduce the economic impact of these diseases on hop production and yield. CPCNW researchers are also examining how the interaction of Hop stunt viroid and Hop powdery mildew may differ based on hop cultivar, as some cultivars are more susceptible, with severely decreased yields.