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What would beer be without hops? These plants have been an integral part of brewing for well over a thousand years. Today, these plants are used worldwide to add zest, finesse and fruitiness to any quaff.

But these plants have a pathogen problem. It’s a small piece of genetic material known as the hop stump viroid (HSVd). It’s not a living entity per se but it does have a drastic effect on the plant. Once this circular piece of RNA gets into a cell it interferes with the normal biochemical process of turning DNA into proteins. Specifically, it attaches to RNA, silencing the transcribed signal so no protein can be produced.

In the hop plant, this silencing can have drastic effects. When HSVd infects the plant, various proteins are silenced. Most are involved in the formation of flavonoids and polyphenols. For us, these molecules add taste. But to the plant, they are essential to defending against pathogens. When these genes are silenced, the plant becomes more susceptible to a variety of diseases such as powdery mildew, aphid infestation, and root rot.

Because HSVd is not an actual living organism, the hop plant has no natural resistance to infection. There are no antigens to develop an immune response and anti-silencing mechanisms are simply not a part of the normal living process. This means any interaction with a contaminated carrier, such as damaged leaves, tubers, and insects could lead to infection.

In a natural environment, the spread of the virus is relatively limited. But in a horticultural setting, where plants are continually being manipulated, damaged, and harvested, the spread can become epidemic. The simple use of a machete or a root mass harvester can spread the virus throughout a field. Making the situation worse is the presence of a natural reservoir for the viroid, grapes.

The only way to combat the disease is to get rid of the viroid. Considering our advancements in genetic engineering, the answer may be found in the lab. All that would be needed is to find a silencer of the viroid, place it into the plant, and grow it. Yet, this route is arduous and could become relatively expensive. Also, there appears to be little to no genetic option currently available.

There is, however, another path to clearance. It’s efficient, cost-effective, and best of all, completely natural. The trick is to do the same to hops as we all do to beer: make them cold.

The process is known as cold treatment and meristem tip culture. The technique involves culturing the hops in a cold environment, usually around forty to fifty degrees Fahrenheit. We use the roots and the underground stem only. As the plant grows, it forms shoots, the meristem. Here, the cells are rapidly dividing and are the perfect home for the viroid. In warmer climates, the entire plant is growing allowing the viroid to circulate but when it’s cold, growth is limited to these areas. After one to a few months, the viroid accumulates in this area of the plant and can be removed simply by clipping. The result is a viroid-free plant that can then be cultivated.

To learn more about the importance of this technique to the brewing industry, I reached out to Peter Wolfe. He’s a brewing scientist at Anheuser-Busch and a hops aficionado. Anheuser-Busch is a member of the Hop Research Council, which funds and directs hop research to benefit the U.S. hop industry. As Peter puts it, “Quality ingredients are absolutely crucial. When hop growers succeed, we succeed.”

Considering part of Wolfe’s job is to ensure supply to the largest brewing company in the world, keeping HSVd at bay in a cost-effective and accessible manner is a top priority. “Once you have the facility, which costs about fifty thousand dollars, you can make a plant viroid-free for about three thousand. This may appear a bit costly at first but consider you can take that plant and turn it into a crop over time. It’s also an open access system. Once the plant has been made, it will be grown and conserved. Anyone who wants to grow their own can get the viroid-free hop plant for a nominal fee of fifty dollars.” This price point makes is accessible not only to large companies, but also to those homebrewers and backyard hop growers.

Of course, there is one small caveat to the technique. Even though the plant is viroid-free, it is still susceptible in the wild. This means over time, the crop may once again become infected. But as Wolfe points out, this is a short term problem. “As the number of cleared hop species grows, anyone who suffers can quickly and easily replace their crops.” But, Wolfe also suggests the best way to keep a clean crop is to have viroid-free environment. “Make sure the field is clean and virus free before you plant the new crop. And unless you have no choice, ensure your neighbor isn’t cultivating grapes.”

The cold therapy technique is now fully available through a program called the National Clean Plant Network. The hop program is located at Washington State University and welcomes any and all hop questions.

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