Disease causing pathogens naturally accumulate in plants and the maximum crop yield potential falls as time passes over a period years. Many people rationalize this as the plants are “getting old”. Virus accumulation is an important consideration in long-lived crops like hops. The normal productive years of a commercial hopyard variety that starts with few or no viruses present is about 10 to 12 years before they are typically replaced with newer fresh stock. Initial planting stock that is obtained from old hopyards that are loaded with high levels of viruses will carry this same load and yields are less and plants have to be replaced in fewer seasons. The USDA is aware of this and has a congressional mandate to provide clean seed and planting stock for all commercially important crops. Hops are included in this list and reputable professional propagators source and use this clean nuclear mother stock whenever possible.
Great Lakes Hops (GLH) actively searches for, obtains, maintains, and propagates the cleanest nuclear mother stock available (emphasis on AVAILABLE) to us in our specialized MDOA licensed nursery facility. Most of our nuclear mother stock is sourced directly from the USDA National Clean Plant Program / Facility located in the western USA. This is the only current recognized source of hop propagation stock that has been treated and tested for virus removal and is certified With a VF number at the time of its release. We do not propagate or sell hops or rhizomes as a sideline or as uninspected planting stock. All of our hop transplants are grown specifically for new hop acreage.
However, that being said - read carefully and understand the following.
2. Even popular individual hop varieties are only retreated for virus removal every 5 to 10 years after it has been determined that the hop cultivar is still commercially important and the viruses have accumulated to a level that warrants re-treatment. See the USDA National Clean Plant Program for their detailed program specifications and methods.
3. Any virus free claims made by all downstream propagators and suppliers are only applicable to the mother stock obtained directly from the USDA program. ( Even the USDA program does not warranty the individual nuclear mother hop plants provided remain virus free once they leave their facilities.) Plants propagated or cloned from VT mother stock cannot claim all the plants are still virus free unless they are all retested individually. GLH produces transplants that originated from such original virus-free mother plants are accompanied with the VT number to show the plants origination. This provides assurance of the plant being true to type and history of its origins.
4. All plants accumulate viruses and other pathogens – that is the reality of nature. Viruses accumulate slowly and 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 to protect maximum potential yields in all commercially important crops. Older less-utilized hop varieties commonly have a higher virus load of several possible viruses and viroids. It is known that rhizome stock obtained from old production hopyards tend to carry higher levels of virus.
5. Hopyard plant populations rarely show uniform virus expression and transmission plant-to-plant is slow. In large commercial hop yards; virus expression is quite sporadic and often will not be visible unless plants are severely stressed by other factors. Large commercial hopyards routinely rouge out individual plants that express heavy virus symptoms and replace them with splits or rhizomes from other plants in the same field. This is a normal practice and is similar to how a virus is managed in many other crops.
6. If a grower is experiencing problems in the hopyard and the majority of plants are not reaching the trellis wire, forming full length sidearms, or yielding poorly; then other cultural factors such a pH, water, fertility, soil compaction are the most common causes and need to be reexamined thoroughly FIRST.
Indeed, upon a full professional consult and review of hop yard practices, it is almost always found to be a case of wrong cultural practices, lack of water and fertilizer at critical times, incorrect soil pH, and soil compaction that are the main contributing factors to poor vigor and yields.
In summary, GLH continually monitors hop virus levels in all our hop varieties and replaces nuclear mother stock with cleaner plants 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 treated virus free mother hops that professional propagators use to create new planting stock. Commercial hop growers are advised to check with their plant suppliers to see if they are sourcing the cleanest mother stock available and if the plants cloned are regularly inspected by the Dept. of Agriculture.
Detection and Elimination of Viruses in USDA Hop (Humulus lupulus) Germplasm Collection
Viruses and Viroids Infecting Hop: Significance, Epidemiology, and Management