This article appears in the February 2017 issue of Potato Grower.
Seed certification is a quality control program that consists of a number of components intended to ensure that specified quality standards are met. One of the more important of these components is post-harvest testing. Post-harvest testing may consist of an off-season grow-out in the field or greenhouse, laboratory testing, or some combination of these.
The vast majority of Idaho seed lots are post-harvest tested in a winter grow-out conducted in Waialua, Hawaii. This grow-out consists of a visual assessment of grower-submitted samples for potato leaf roll virus and a laboratory test of harvested leaves for potato virus Y (PVY). Smaller, early-generation seed lots are usually retained for lab testing by the Idaho Crop Improvement Association’s (ICIA) seed certification laboratory. The results of the post-harvest test are reported to the seed grower as a percentage based on the number of plants observed or tested.
While the process of post-harvest testing and the reporting of results is relatively straightforward, we do occasionally receive questions about why reported post-harvest test results differ from what is observed in the field the following season. This has been a particular issue with PVY levels observed in some seed lots. Why does this occur?
It is important to bear in mind that the only way to determine the actual amount of PVY in a seed lot is to test every plant or tuber. Since this practice would be neither practical nor economically viable, the amount of virus in a seed lot is estimated based on the submitted sample. Unfortunately, there is an expected and unavoidable margin of error associated with sampling. As a result, an individual sample may under- or over-estimate the amount of virus in the seed lot.
The precision of the estimate provided by the post-harvest sample is dependent on a number of factors, including sample size. Most important is the collection of a sample that is representative of the seed lot. Seed growers typically select a representative sample by systematically collecting tubers while placing the crop in storage. However, this goal can be challenging if the seed lot is very large or is not homogeneous. In these cases, we recommend that growers submit extra samples and/or subdivide lots so that better estimates can be obtained.
Finally, the type of plant tissue tested (leaf vs. sprout vs. dormant tuber) can influence the estimate provided by the post-harvest test. It’s important to note that some pathogens are not uniformly distributed within the plant tissue, so even if a sample is representative and sufficiently large, it can still result in a test result that does not match field observations.
To summarize, post-harvest test results are estimates only. When purchasing a seed lot, buyers should request a North American health certificate. This document not only provides post-harvest test results, it also gives additional details, such as the type of testing and sample size, that can be useful in evaluating those results.