As this column goes to press, Montana will have just released the results of the post-harvest grow-out and testing for our 2015 crop. Tuber samples from every G1 through G4 field were collected at harvest, trucked to Belgrade, Mont., in late October, and then on to Seattle, where they were trans-loaded onto a cargo ship bound for Oahu, Hawaii.
The Montana crew arrived in Hawaii in November to plant the potatoes and the crop was evaluated from late December through mid-January. Montana picks leaves from 100 percent of the post-harvest plots and sends them back to the lab in Montana for virus testing. This is in addition to the visual assessment for viruses, other diseases, variety mix, herbicide injury and general vigor problems. (Idaho also lab tests 100 percent of its seed lots, while many other seed production states test varieties that don’t express visual symptoms.)
The post-harvest test became a requirement for certification of seed potatoes in the U.S. in the 1970s. With the increasing prevalence of potato virus Y (PVY) and its changing strains, the importance of the test is amplified. Mosaic readings from summer inspection often increase tenfold in winter readings, and lots with no detectable PVY in summer may show virus infection in the post-harvest test. Tubers from states representing 69 percent of the U.S. seed acreage are being tested on Oahu, Hawaii, 24 percent are tested at Homestead, Fla., 6 percent are being grown out in greenhouses, and 1 percent of acreage receives no post-harvest test. These figures represent seed that will be planted for re-certification the following year. Some states currently do not perform field grow-outs on seed destined for commercial plantings. The recently introduced National Harmonization Standards, implemented by most states to facilitate international and domestic trade, dictate seed must be certified and subjected to a post-harvest test to be transported across state and international borders.
The post-harvest grow-out provides U.S. seed growers with the crop health status of seed potatoes before the majority start shipping for commercial planting and well before planting for certified seed production. European countries do not have the luxury of tropical growing areas within their borders and rely solely on laboratory testing for determining if seed lots are within disease tolerances for certification. With laboratory testing, they have the ability to start testing immediately after harvest and to obtain results as quickly as they can run the samples through the lab. The Canadians have also moved toward laboratory testing, but they send seed lots that will be exported to the U.S. for recertification to Hawaii and Florida for grow-out.
One question that continually comes up is, Should the U.S. seed potato industry move toward laboratory-based post-harvest testing? Investigations on the feasibility of direct tuber testing are one objective of the Specialty Crop Research Initiative on potato necrotic viruses, of which Montana is a participant. We are evaluating alternative methods for direct testing of tubers for viruses in the lab versus a field grow-out.
The methodology for high-sensitivity testing for viruses is now available through the use of PCR-based assays; these methods are employed extensively in Europe. In the U.S. they are routinely used in potato virus research, but our certification labs do not currently have the ability to perform the laboratory assays on a high-throughput basis.
To begin feasibility studies, Montana is testing tuber tissue cores from 18 seed lot samples (400 tubers per sample) using the PCR-based assay to determine PVY incidence. These same tubers were sent to Hawaii to be planted in the field. Lab and field results will be compared. I anticipate that we will get comparable results from lab and field testing, but the lab results will only provide data on PVY incidence and cannot provide all the observational data that is collected in the field. What we hope to achieve is the development of straightforward methodology that can be readily transferred to other certification labs.
Direct tuber testing does not completely replace a field grow-out, but it can be advantageous if very early results are needed on specific viruses, or if there is a natural disaster causing extensive losses of the post-harvest plots. The U.S. potato industry has the unique advantage of comprehensive field grow-outs in tropical locations but will benefit by increasing its options for post-harvest testing.