This article appears as the cover story in the May 2017 issue of Potato Grower.
Supplying disease-free seed for the potato industry has always been a challenge, but new hurdles have been placed in front of us with the emergence and spread of three viruses that cause necrosis in tubers: the necrotic strains of potato virus Y (PVY), potato mop-top virus and tobacco rattle virus. A multidisciplinary team of 31 researchers from nine universities and the USDA Agricultural Research Service with expertise in potato breeding, entomology, plant pathology, agronomy and economics are working together with seed certification programs and the potato industry to understand the biology of these viruses and provide practical management options.
One objective of the program is to develop and distribute accurate and reliable sampling and testing procedures that will allow seed certification and other testing labs to determine virus incidence in dormant tuber samples. This could provide an alternative to the post-harvest winter grow-outs currently done in Hawaii, Florida or greenhouses.
Other diagnostic tests are being developed to quantify from soil the stubby root nematode vector of tobacco rattle, and the causal agent of powdery scab, Spongospora, which is also the vector of mop-top. This would allow growers to determine whether the tobacco rattle and mop-top vectors (and ultimately the viruses) are present in their fields and pose a threat prior to planting the crop. Other research under this objective includes the development of remote sensing diagnostics that use changes in the light reflectance properties of virus-infected potato plants to distinguish among healthy and diseased plants.
Another major objective is to link pathology and potato breeding to characterize the susceptibility of popular varieties to foliar and tuber disease caused by the three viruses, and to develop the tools and germplasm that will ultimately generate virus-resistant potatoes acceptable to processors and consumers. Resistance to the viruses is limited to a few newly released and not widely planted varieties. However, the more common varieties vary widely in their symptom expression. Foliar symptoms are useful for seed certification and rogueing operations, while a lack of tuber symptoms is helpful in generating a consumer-acceptable crop. Until virus-resistant varieties can be developed and widely planted, varieties that do not express tuber symptoms are a useful stopgap measure.
To this end, researchers are conducting experiments to define foliar and tuber symptom expression in varieties infected with PVY, mop-top and tobacco rattle. Additionally, various research groups are working to develop molecular markers for virus resistance traits that can be used to speed up the process of developing virus-resistant varieties. This has been extremely useful in the development and release of several PVY-resistant varieties in recent years. There have also been advances in identifying new sources of resistance to mop-top and tobacco rattle.
The third major objective is to develop sustainable practices that can be implemented on the farm to reduce the impact of these viruses on production in the field and on the quality of tubers in storage. Efforts are under way to define the seasonal occurrence of important aphid vectors of PVY. This information helps define the high-risk periods of PVY spread into and within the crop, and guides the timing of applications of oils and insecticides to minimize aphid feeding. Coordinating aphid, weather and crop landscape information to develop predictive models can help growers better understand the risk of virus spread and take action to reduce their exposure.
While most of the effort to date has focused on management of PVY, future efforts will move toward mop-top and tobacco rattle. Both these viruses are soil-borne due to their dependence on soil-inhabiting vectors for spread and long-term survival. Initial efforts will focus on determining incidence in the seed potato crop and detecting the vectors and virus in soil. Researchers are also looking at the use of host resistance, vector control and crop rotation as potential best management practices to reduce disease incidence and risk of exposure. Since the soil-borne vectors and viruses are easily moved in contaminated soil, it is imperative that growers identify infested fields and take steps to minimize movement of soil from those areas, such as using only thoroughly cleaned equipment.
Recent evidence, including anecdotal observations by packers and processing plants, indicates that these tuber necrotic viruses can impact the fresh market and processing quality of tubers. The impact often increases with increasing time in storage. Storage time and conditions also affect the ability of diagnostic assays to detect virus in tubers and thereby provide an accurate assessment of virus incidence in the harvested crop. Researchers are investigating how storage parameters can impact the effects of virus on the stored crop and how these parameters can be modified to mitigate the impact of virus infection.
All the research is conducted and evaluated with an eye toward economics and communicating the information to the relevant sectors of the potato industry. While investigations into the fundamental mechanisms of potato/virus/vector interactions have led to industry-changing discoveries, it is important to always recognize that recommendations from the project must be compliant with the business of growing, storing and selling potatoes. Changes in production or storage may be warranted, but only if they are economically sound. Outreach and information transfer is accomplished through the project website (currently under development), presentations and discussions at grower meetings, and field days and demonstration plots.
In 2016, a demonstration plot of 42 potato varieties grown from seed infected with three different PVY strains was established in Othello, Wash. The field day event was attended by over 150 people and served to educate growers, seed inspectors and others about some of the challenges the emerging viruses are posing to the industry. Similar events are planned in Washington, Wisconsin and Maine in 2018.
The potato virus research community has come together from across the U.S. to partner with seed certification and other segments of the potato industry to understand emerging virus disease challenges and to collectively develop ways to manage and minimize impacts on the crop and farm income. This is an ongoing and dynamic project that attempts to keep current with ever-changing disease pressures and industry needs. Continued critical evaluation and counsel from all segments of the potato community is welcome and required for the continued success of the project and the industry.