The potato industry in North America is currently experiencing an invasion of sorts by an old pest that has recently undergone some important and sinister changes. The pest is Potato virus Y or PVY and the threat is new strains of the virus that can cause both internal and external tuber symptoms. To combat this problem, a team of PVY researchers from across the U.S. received a Specialty Crops Research Initiative (SCRI) grant to study the new strains of PVY and to educate the potato industry about what to do to manage this new threat. The grant is entitled "Development of comprehensive strategies to manage Potato Virus Y in potato and eradicate the tuber necrotic variants recently introduced into the United States."
In the good old days we only had to deal with the "ordinary" strain of PVY, which bears the designation PVYO. PVYO generally causes a recognizable mosaic symptom in the foliage of most commonly produced varieties, and because of this growers, with the help of state seed certification programs, were able to manage the disease fairly well by the simple, time-honored practice of roguing (removing) visibly infected plants out of seed fields. This situation became much more difficult with the increase in production of a number of varieties that don't show PVY symptoms nearly as well as many of the older varieties. Russet Norkotah, Shepody, Silverton Russet, and Gem Russet, among others, are some of these "latent PVY carrier" varieties. As if this development weren't enough of a problem, the new strains of PVY usually don't show symptoms as strongly as PVYO even in the older varieties. As a result, PVY of all strains has been increasing in the North American seed potato system over the last 10 years or so.
The first of the new PVY invaders to be encountered were designated "PVYN." In this case, the "N" stands for "necrotic" but this designation is a little misleading because this strain of the virus causes a necrotic reaction in the leaves of tobacco, not potato. PVYN is actually responsible for a relatively mild symptom in potato, a feature that makes this strain more difficult to manage because it is more difficult to see and remove. Oddly enough, since the discovery of PVYN in North America back in the early 1990's, this particular strain of the virus is only rarely encountered. Much more commonly found these days are the so-called "recombinant" strains of PVY that contain genetic material from both the PVYO and PVYN strains. One of these recombinants, often labeled PVYN:O or PVYNWI is becoming very prevalent across the U.S. A more important new strain is one designated as PVYNTN which stands for "N type, Tuber Necrotic." This strain often causes mild foliar symptoms, but it causes devastating surface necrotic rings on tuber surfaces that render the tubers unmarketable. Yellow flesh varieties seem to be the most susceptible, but a few white, red and russet varieties have also been found to be susceptible to PVY tuber necrosis.
Our industry has been fortunate so far as there have only been a handful of cases wherein PVYNTN has been responsible for actual losses to commercial growers. If PVYNTN is allowed to increase in the North American seed potato system, this situation could get a great deal worse in the future. To underline the urgency of this prediction we need look no further than Europe where the NTN strains of PVY now make up more than 80% of the PVY population. We believe that we can learn from the European experience and take action immediately, before the situation gets out of hand over here.
A website for the SCRI grant has been created and can be accessed at http://www.potatovirus.com On th.e site can be found a wealth of information about PVY in general, as well as the purpose and methods that will be employed to gather more information and, hopefully eradicate these new strains of PVY. Also available on the website is information about the researchers involved and how to contact them. The group will be studying all aspects of PVY in the North American industry, including surveying the strain makeup of the North American PVY population, studying the symptom expression of new strains in a full range of North American Varieties in both foliage and tubers, developing methods of PVY management, investigating the role of weed hosts as reservoirs for the virus, and the true economic impact of PVY on yield and quality in commercial potato production.
The success of an ambitious project like this one will require the efforts of the entire potato industry on the entire continent. Changes may need to be considered by seed certification agencies to reduce PVY in seed potatoes, particularly seed, which will be planted back into a seed program for another year of increase. PVY of all strains spreads very easily and the most important source is usually contained in the seed that is being planted. Greater education of the entire potato industry to create a greater awareness of the potential for economic damage due to the new strains will also be necessary. Growers must realize that the most effective management for PVY is to avoid planting infected seed in the first place.
—By Phil Nolte, University of Idaho