This article appears in the January 2021 issue of Potato Grower.
The first tenet of producing a quality, high-yield potato crop is “Plant Clean Seed.” Quality assurance for healthy seed is accomplished through seed potato certification programs throughout the U.S. In Montana, as in all programs, we perform visual inspections for virus diseases and other disorders during the summer and post-harvest test. In addition, we perform comprehensive laboratory testing for potato virus Y (PVY) in the summer for early-generation seed lots, and for all seed lots in the post-harvest test. Montana is the only state to collect comprehensive summer leaf testing data, which provides a unique opportunity to assess virus spread using historical data from both summer and post-harvest tests. Virus spread can be calculated from the amount of virus planted (summer leaf test results) followed by post-harvest test results of the same lots.
Kate Fuller and Chris McIntosh are agricultural economists at Montana State University and the University of Idaho, respectively. Fuller had previously studied the benefits of virus screening in grape, another vegetatively propagated crop. McIntosh has studied the impact of PVY on yield and revenue derived from potatoes in Idaho. They asked two questions:
-
How much increase in virus can be predicted if a lot with a known amount of PVY is planted?
-
How does screening for PVY in summer leaf samples and the post-harvest test reduce the cost of PVY and ultimately enhance the revenue to the commercial potato industry in Idaho?
Seed potato certification programs across the country have established tolerances for mosaic caused by PVY. If left unchecked, this disease reduces yield and quality, and, depending on the strain, can cause necrotic symptoms in the tubers. This disease is challenging to manage because it is spread by aphids. Summer test data is the most important predictor of post-harvest test results.
Analyzing the summer and winter data, the spread during the season was calculated as 5.8 to 8.3 times. If a lot starts out with 2 percent PVY, it will spread during the growing season, potentially resulting in PVY levels of at least 15 percent. This far surpasses tolerances for PVY for recertification for a seed potato grower. If that seed were replanted for commercial production, it would be at a level where yield and quality could be significantly impacted. By employing rigorous testing measures, levels of PVY are continually monitored through generations of the seed lots during the summer and post-harvest, allowing the Montana seed grower to make well-informed decisions on retention of seed lots for recertification, and employing best management practices based on how much virus is detected in a seed lot to be replanted for recertification.
These management tactics include isolation of lots with virus from lots that have tested clean, as well as enhanced protection measures including mineral oil sprays and insecticides. The employment of these methods result in tangible benefits to the commercial producer by providing him or her with seed with no or very low levels of virus. These benefits were reported in an article in the scientific journal Plant Disease under the title “Valuing Disease Prevention in a Vegetatively Propagated Annual Crop: Benefits from the Montana Seed Potato Certification Program.” Based on data from Idaho, where yield loss due to PVY was calculated for seed lots with various levels of PVY, Fuller and McIntosh were able to estimate that the annual benefit from Montana’s program to Idaho commercial potato producers averaged $205 per acre for a total benefit of $22 million to the state.
These data show that the investment the Montana seed potato producers put into raising a clean crop are not only a benefit to their own farms, but also to the commercial growers who purchase their seed. During the summer, the MSU potato lab employs 35 to 40 students to pick leaves from farm fields and test them in the lab. In the winter, leaves are picked from every post-harvest test plot in Hawaii, shipped back to our lab in Montana, and tested for PVY, PVX and PVA. These testing costs are borne 100 percent by the seed grower. The data from these tests add value to the seed they will sell the next growing season. The conclusions from this paper show that the costs borne by Montana seed potato growers to produce clean seed pay dividends to Idaho potato farmers, and to all customers who purchase Montana seed potatoes.