At their annual meeting in October of 2007, the seed potato producers of the Idaho Crop Improvement Association made a momentous decision: They voted unanimously to test the entire winter seed grow out for PVY mosaic using a laboratory (ELISA) test. This dramatic change was done in an effort to reduce the amount of PVY in the seed system and ultimately in the State of Idaho in general. This new rule has been in effect for two seasons; the seed crop currently in storage will be the third since the change.
The winter grow-out for the Idaho seed potato industry is currently conducted in the Imperial Valley of California, near the city of Brawley. Samples from each of the around 600 seed lots produced in the state during the last season are collected during October, treated to break dormancy and planted during the first week or so of November. During the winter each sample will be subjected to inspections and also tested for potato virus Y (PVY).
As many of you know, seed potatoes have to undergo an entire battery of inspections before the standards for certification have been met. Some of the most important of these inspections are the summer inspections (two of them) and the winter grow-out. The summer inspections provide information for the seed producer on how much virus was in the seed that he planted. The winter inspection provides information on how much virus is in the seed that the seed producer intends to sell, either for commercial production or, in some cases, for recertification.
So, how successful has this PVY testing program been? The results over the last two seasons of seed production are looking very encouraging. We have seen reductions in the amount of PVY in Idaho seed. One of the measures that we have to base this judgment on is how many of the lots in the seed system have low-enough levels of mosaic virus that they could still be entered into the seed program and be re-certified.
Over the last three years, since the winter testing program was instituted, the number of lots that are eligible for recertification has increased by 9.5 percent. This is a clear indication that the amount of PVY in Idaho seed has gone down considerably. As this program continues to have its positive effect, the next few years should see even greater reductions in PVY.
Another part of PVY reduction has to do with the use of effective management procedures by the seed producers. In order to facilitate this process, the University of Idaho has just published a new Extension bulletin entitled, "Potato Virus Y Management for the Seed Potato Producer." The bulletin is authored by Phillip Nolte, Juan M. Alvarez and Jonathan L. Whitworth. For seed producers, or anyone else who might be interested, this publication is available online at: http://info.ag.uidaho.edu/catalog/detail.
As I have written before, research here at UI indicates that relatively small percentages of PVY in seed (10 percent or less) are likely to have no discernable effect on yield. Studies performed on Russet Burbank, Russet Norkotah and Shepody indicated that 1 percent PVY in the seed lot would result in a yield reduction of about 1.5 cwt/acre in all three of these varieties. If we take these numbers out to higher percentages of virus, 10 percent virus can be expected to result in a yield reduction of 15 cwt/acre. We have had several opportunities to test these predictions, and these numbers have, so far, held up pretty well.
The decision by the seed potato producers of ICIA for ELISA testing of the winter grow-out appears to be having the desired effect. PVY levels have dropped for the last two years in a row. We'll keep you informed.