California Grow-Outs

Winter a busy time for seed certification officials

Published in the April 2012 Issue Published online: Apr 09, 2012 Phillip Nolte, UI seed potato pathologist

The winter meeting season is starting to wind down as we prepare this month's column. One thing that's becoming more and more evident at this point is that segments of the North American seed potato industry have seen some significant changes in the amount of potato virus Y (PVY) this season. Many growers will be faced with the prospect of using seed with higher-than-usual levels of PVY. What does this mean for the coming season? What will be the consequences of numerous exemptions?

The commercial producer can do very little to manage PVY except to purchase and utilize seed with the lowest level of virus obtainable. Studies performed at the University of Idaho indicate that 1 percent seed-borne PVY can result in an average yield reduction of 1.5 cwt/acre. In turn, a 10 percent PVY incidence in the seed lot will result in a proportional yield loss of 15 cwt/acre. And as you would expect, the yield losses continue to mount as virus levels continue to increase.

We do not recommend using seed with PVY levels greater than 10 percent, but producers of some varieties, such as Russet Norkotah, may have difficulty finding seed containing levels lower than 10 percent PVY. This doesn't mean that planting seed with 20-25 percent PVY will be a complete disaster, but it does mean that a proportional yield loss can be expected. More importantly, these fields can serve as a source of infection for other nearby seed potato fields. So, if you are planting virus-infected seed, make every effort to isolate those fields and plant downwind of other (seed) potato fields to minimize virus spread and continued, future challenges.

PVY can be spread by contact between infected potato plants, but aphid vectors are responsible for the vast majority of the spread that occurs. One of the problems that occurred last summer was the industry saw a dramatic increase in the number of aphids in many areas of the continent. The industry's old nemesis-the green peach aphid (Myzus persicae)-has been pretty will kept in check, but this species is probably not responsible for the increases in PVY observed during the 2011 season.

It is now generally acknowledged that many non-colonizing `transient' aphids, coming from ripening grain, other cereals and other crops such as soybean, are very likely responsible for the majority of the PVY dissemination in North America. In the West we believe that the most important species is the bird-cherry oat aphid (Rhopalosiphum padi), resulting from flights off of senescing grain crops. In the Midwest, however, the likely culprit implicated in last year's outbreak is thought to be a relative newcomer, the soybean aphid (Aphis glycines). This species builds populations on soybean during the growing season and later disperses, as a result of crowding and plant development stage, in pursuit of new soybeans to colonize.

Aphid population increases tend to be cyclic in nature with an occasional year where the numbers explode to levels beyond that considered to be "normal." When this occurs, the spread of PVY and other virus problems in potato can be quite extensive. In many cases, the majority of the spread is later in the season and late season infections usually do not manifest as foliar symptoms, yet many of the developing tubers will become infected.

Interestingly, the aphid populations throughout much of the West, Midwest and Northern production regions in 2011 were considered to be normal, or even below average in many places. But it is important to remember that considerable PVY spread can occur in years when aphid populations are reportedly at, or below normal. In fact, current season (2011) reports of below-average aphid numbers may have contributed to a level of complacency with respect to the need to remain vigilant in PVY control.

For the seed producer, management of PVY begins with planting seed free of PVY or is only infected at very low levels and infected plants are rogued out early in the season. Aphid feeding studies performed at the University of Idaho (J. Alvarez) indicate that aphids are attracted to PVY-infected plants and will preferentially choose them over healthy plants.

In other words, the aphids can find the PVY-infected plants in the seed fields even if we can't, even at low population densities. After probing an infected plant, these non-colonizing aphids then probe or taste adjacent plants in the field and eventually move on. The seed producer will generally never even know that these transient, non-colonizing aphids were present. This means that late-season spread of PVY in seed fields can be quite extensive while going completely unrecognized by the seed producer.

Since the PVY levels in the seed you plant are so important, the winter inspection results are probably the best information you can get regarding PVY. The summer seed inspections provide information on how much PVY was in the seed when it was planted. The winter grow-out provides information on how much PVY is in the seed that was harvested and will be used to plant the next crop. We recommend using a laboratory test, such as ELISA, for determining PVY levels rather than relying on strictly visual methods which are not as accurate.

The PVY the industry is dealing with today is not your father's PVY. The virus is changing and the new strains are, for the most part, inducing mild foliar symptoms, making certification inspections and roguing operations more difficult. The emerging tuber necrotic strain is especially worrisome because it can cause necrotic rings on the surface of the tubers of susceptible varieties reducing quality and salability.

The tuber necrotic strain is present in most seed production areas, but fortunately it has only been found in a limited number of seed lots. Seed lots harboring the tuber necrotic strain should be flushed out of the certification system, regardless of the level of virus in the seed lot. This is the only way the industry can prevent PVY from becoming the next tuber quality disease affecting the entire potato industry.

The authors of this article are all part of a Specialty Crops Research Initiative Grant entitled "Development of comprehensive strategies to manage Potato virus Y in potato and eradicate the tuber necrotic variants recently introduced into the United States." The project website, www.potatovirus.com, contains a wealth of information about PVY, including photographs and links to publications that discuss management of PVY for seed producers. PG

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