Potato Grower

28 POTATO GROWER | APRIL 2018 Diggin' In Diggin' In Diggin' In Diggin' In Diggin' In Diggin' In VIRUS CONTROL | Kasia Duellman, Stewart Gray & Alan Westra Minimizing in-season spread of PVY Casting a Wide Net 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, PVY O , 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 PVY O 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 PVY O . 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 Varietal reaction to seed-borne infection by three strains of PVY. Plants on the left were grown from healthy seed. Notice that plants grown from PVYO-infected seed are compromised in growth, easily identifi ed by foliar symptoms and are not likely to produce viable tubers. Plants infected with PVYN:O and PVYNTN have more mild or transient symptoms that could be missed during visual inspections.

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