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.