Plant pathogens (this includes hop viruses and viroids) naturally accumulate in plants as they age and in cultivated hop yards the maximum crop yield potential naturally falls as time passes over a period seasons. Many people simply rationalize this as the plants are “getting old”. Plant viruses and viroids number in the thousands and coexist naturally in our surrounding environment. These viruses transfer from plant to plant, or even between plant species by a number of different vectors (insects, mechanical, root contact, etc.) . Plants that carry viruses and viroids often do not express symptoms unless the plants are stressed or weakened.
In nature, these pathogens are controlled and mitigated by plant vigor (healthy plants can carry a virus load with little external effects); annual cold and heat cycles that eliminates or sets back virus loads, or elimination of virus by new plant regeneration via seed. Viruses and viroids are considered obligate pathogens because they coexist with and rarely kill their host. Some viruses are actually beneficial.
Virus accumulation is an important consideration in long-lived perennial crops like hops. Single favored female hop cultivars are repeatedly cloned for years (sometimes decades) from the original mother lineage. This monoculturing totally bypasses the way viruses and viroids are are naturally controlled. Hop planting stock that is derived from older cultivars and hopyards often carry higher virus loads than planting stock that is derived from newer hop stock and cultivars that have been treated for virus reduction. The USDA is aware of these facts and has a congressional mandate to create clean seed and planting stock for ALL commercially important crops in order to remain competitive with the rest of the world. A single small USDA lab in the Pacific Northwest is the sole source for clean hop stock that has been heat-treated and cultured to remove viruses that are considered harmful. (see http://nationalcleanplantnetwork.org/HOPS_CPN/ for details.) Only a limited number of commercial hop cultivars are treated due to budgetary constraints. Many less popular hop cultivars do not ever qualify to go through this program for virus treatment. The hop cultivars that do get treated are available in only very limited quantities; sometimes as only few dozen plants or unrooted cuttings for growers and propagators nationwide. This is the only current government-funded source in the USA that creates cleaned hop stock. There are currently no other certified sources or certifying agencies for hops in the USA.
Great Lakes Hops (GLH) actively obtains, maintains, and propagates the cleanest newest nuclear hop mother stock available (emphasis on AVAILABLE) to us from this government program.
However, that being said - read carefully and understand the following.
2. Even the most popular individual hop cultivars are only retreated for virus removal every 5 to 8 years after it has been determined that the hop cultivar is still commercially important and the viruses have re-accumulated to a level that warrants re-treatment. (See the USDA National Clean Plant Program for their detailed program specifications and methods.) GLH's role as a propagator of hop stock is merely to multiply and provide larger numbers of these hop cultivars to hopyards using the cleanest culture methods available to us. It takes GLH two years minimum to propagate and make commercial quantities of the newly acquired clean hop cultivars available to hopyards. It is a very difficult time-consuming process, and is little appreciated unless you are a propagator who has created thousands of replicate plants from a single mother plant.
3. Any "virus free" claims made by any hop propagators and nursery suppliers are misleading in our opinion and the term is only directly applicable to the original mother stock obtained directly from the USDA program. ( Due to how easily hop re-acquire viruses, even the USDA program does not warranty or certify the individual nuclear mother hop plants provided remain virus free once they leave their lab facilities.) GLH produces cloned hop transplants that originated from these limited available mother plants and these resulting transplants may be tagged to show this origination. This identification is in no way, shape, or form indicative that these cloned transplants are certified or re-tested individually to be "virus-free".
4. All plants accumulate viruses and other pathogens – that is the reality of nature. Viruses accumulate naturally in all plants over time; thus the effort to monitor and replace heavily virus infected plants in commercial hop cultivation is a continually ongoing effort. Yes; even "clean" plants naturally re-acquire viruses and that is why they have to be continually re-cleaned as years pass. Older less-utilized hop varieties commonly have a higher virus load of several viruses and viroids. It is known that rhizome stock obtained from old production hopyards tend to carry higher levels of virus. Hop viroids are extremely difficult to remove from motherstock using standard thermotherapy treatments and can re-appear in treated hops after a latency period..
5. Hopyard plant populations rarely show uniform virus expression. In large commercial hop yards; virus expression is quite sporadic and often will often not be visible unless plants are stressed by other factors. Large commercial hopyards routinely rouge out individual weak plants that express heavy virus symptoms and replace them with splits or rhizomes from other plants in the same field. This is a normal grower practice and is similar to how virus levels are managed in many other perennial crops.
6. Worries over viruses in hopyards have been overblown, in our opinion. Over the past decade, GLH has repeatedly tested hop plants and rhizomes sourced from numerous huge PNW hopyards. We have yet to find one sample that tested negative for viruses from these sources. These hopyards are aware of the numerous viruses and viroids present and yet are very successful at providing the majority of the hops used worldwide.
It is far more common to find during a full professional consult and review that it is a case of improper cultural practices. Lack of water and fertilizer at critical times, incorrect soil pH, soil compaction, and poor disease and weed controls are evident as the main contributing factors to poor hop vigor and yields. Hop growers should re-examine every aspect of their cultural programs before dismissing the obvious in favor of the virus explanation.
GLH continually monitors hop vigor levels in all our produced hop transplants and replaces propagation stock with cleaner plants as often as they become available. The USDA is aware that the increased interest in growing more hop varieties for craft brewers has put more pressure on them to ramp up the budgets and programs that produce more treated mother hops that hop propagators use to create hopyard transplants. Commercial hop growers should check with their plant suppliers to see if they are sourcing the cleanest mother stock available for creating the hop cultivars they want to plant.
GLH is a MDARD licensed and inspected plant nursery.
Detection and Elimination of Viruses in USDA Hop (Humulus lupulus) Germplasm Collection
Viruses and Viroids Infecting Hop: Significance, Epidemiology, and Management