This article appears in the April 2018 issue of Potato Grower.
Potato virus Y (PVY) continues to be a chronic issue in seed potato production across the U.S. Despite most seed certification agencies implementing more comprehensive post-harvest laboratory-based testing, the number of seed lots with detectable levels of PVY has not declined over the past decade. This “steady state” of PVY incidence suggests that the problem will require an industry-wide approach to shift this trend downward.
Prior to 2006, the ordinary strain of PVY, PVYO, was prevalent and induced recognizable foliar symptoms in most potato cultivars. This enabled reasonable estimates of PVY during the growing season and post-harvest grow-outs, and it allowed seed growers to rogue infected plants early in the season. In the past decade, several factors have combined to drive the emergence of several new strains of PVY, which tend to induce mild or transient symptoms in many cultivars. Recent surveys indicate that PVYO now accounts for less than 10 percent of the total PVY in the U.S. The mild symptoms induced by the new PVY strains may present a challenge to visual inspection of the crop and to rogueing operations, which may contribute to more PVY in the seed crop. Furthermore, these mild strains often have much less of an impact on plant growth or tuber production relative to PVYO. They may be passed from the mother plant to daughter tubers or transmitted by vectors more efficiently infecting a greater number of marketable seed tubers.
One of the primary challenges of PVY is a lack of effective management options available to growers. There are steps, however, that, when used in combination, may mitigate PVY incidence in the harvested crop. An important first step for all growers, whether commercial or seed, is selecting seed with the lowest amount possible of PVY. Choosing seed with low PVY incidence based on post-harvest test results will reduce the amount of initial inoculum in the planted crop and subsequent virus spread within that crop and nearby fields. The importance of the initial inoculum load has long been recognized in seed potato production. To this end, beginning in 2018, Idaho seed potato growers will no longer be allowed to use seed stocks with more than 1 percent PVY for seed production.
However, the efforts of certified seed growers may not by themselves be sufficient. While many states have adopted laws requiring commercial growers to plant certified seed, these laws often include so-called “year-out” clauses that permit the planting of saved seed. Commercial growers should contemplate the wisdom of planting untested seed of their own production, as this seed may have high PVY levels and can serve as a source of inoculum that will contaminate fields planted with clean seed.
Volunteer potatoes serve as another important source of PVY. Because of the mild 2017-18 winter that parts of Idaho experienced, it is likely that volunteer potatoes will be a serious problem in the 2018 growing season. Many of the emerging volunteers will be infected with PVY and will become infested with aphids. Effective management of volunteers by both commercial and seed growers is a key component to an overall PVY management plan.
Aphid management can also help to mitigate in-season spread of PVY. PVY is transmitted from infected to healthy plants by winged aphids as they probe the plants in search of a new food source. While colonizing aphids are easily controlled by standard at-planting insecticide applications and subsequent sprays as the crop matures, it is transient, non-colonizing aphid species that are responsible for most of the PVY spread in both seed and commercial crops. These transient aphids develop on other crops or wild plants and move through potato fields in search of preferred host plants. Acquisition and transmission of PVY by these aphids occurs within 15 to 30 seconds, which is much faster than contact insecticides are able to kill the aphids. Thus, contact insecticides are of limited effectiveness in PVY management. However, use of insecticides to control aphids in nearby crops or natural areas can help reduce PVY spread by reducing the number of transient aphids moving through the potato crop.
Use of crop oils also has potential to reduce in-season movement of PVY, but effectiveness will likely be dependent on when applications are initiated, frequency of applications, method of application (air vs. ground), and type of irrigation (pivot vs. other). Crop oils must cover all plant surfaces, be re-applied to new growth and after rain or irrigation events, and be applied continuously from emergence to vine desiccation in most seed production areas of the U.S.
Use of crop borders is yet another tool that has been explored for decades. Research has shown that for small plots, crop borders can effectively reduce in-season spread of PVY. Aphids that alight on the crop border before entering the potato crop will probe the border plants and cleanse their mouthparts of virus before moving into the potato crop. The utility of borders decreases as field size increases.
None of these management options by themselves offers a silver bullet to the problem, and they may not be practical for all situations. Outcomes from ongoing research will help to refine these tools and identify additional strategies that can help combat PVY. However, even the most effective management strategies can be overwhelmed. A grower who plants seed, whether purchased or saved, with high PVY levels places all other growers in the area at risk. This last issue is why PVY management really is a community effort and needs a wide net to capture all possible advantages of the different management strategies.
Kasia Duellman is an Extension Seed Potato Specialist with the University of Idaho. Stewart Gray is a research plant pathologist with USDA-Agricultural Research Service based in Ithaca, N.Y. Alan Westra is the southeastern regional manager with the Idaho Crop Improvement Association.