It’s July in northeast Wisconsin. The air is fresh, the birds are chirping, and the potato fields on the isolated seed potato farm in Langlade County are a healthy green. This is a highly-specialized farm where precious disease-free tubers from the state’s elite seed farm are carefully nursed to supply the diverse requirements of commercial growers. Everything on the seed farm seems to be going well this year, and even the eagle-eyed inspectors have not found any evidence of disease. The farmer looks up at the bluebird sky and smiles at the promise of a warm summer day, unaware that a silent enemy may be approaching. In swirls and currents of air coming from the south, a scattering of feather-light insects is alighting unseen on the edges of his fields. They are anxious to insert their needle-sharp mouthparts into the leaves and begin feeding. These are aphids, which act as unwitting flying hypodermics, quietly moving from one plant to the next, sampling the sap they need to live. These harmless creatures are from southern growing areas where a myriad of crops exist that can potentially infect them with any number of virus diseases that they can carry to potatoes. Unlike the July sky, the enemy the aphids carry promises trouble.
Hijackers, tricksters and deceitful agents, plant viruses are some of the most devious and threatening enemies of seed potato growers in North America. They are the number one cause of seed lots failing to meet the standards necessary to wear the Wisconsin Certified Seed label. How do they do it, you ask? Scientists have studied potato viruses for decades. They are very small micro-organisms that can enter plant cells with the aid of vectors, such as the aphids, but are incapable of multiplying without the help of the potato host’s resources. Once inside a cell, virus particles use different strategies to hijack the cell’s resources to make more copies of themselves. Plants, like humans and other organisms, have defense systems that will recognize the presence of an enemy and try to stop its attack. But some viruses, like Potato virus Y, counterattack by turning off, or ‘silencing’ the plant’s defenses, allowing them to keep using the plant’s resources to multiply.
Potato virus Y (PVY), a tiny rod-shaped particle, has quickly made its way up the list of important enemies of seed potato growers in North America in the last decade. The virus has mutated successfully over time, producing strains that vary in their severity, and with the aid of its aphid vectors, PVY has successfully spread around the world, infecting many different plants (potatoes, peppers, tomato, etc.). In a disturbing recent development, PVY strains have evolved to now infect potatoes without being seen. Even the finely-tuned eyes of an inspector—specialized in spotting the green mosaic patterns and distortions on potato leaves typically caused by PVY—are unable to diagnose infected plants. These new strains pose a threat to the effectiveness of the seed potato improvement association’s primary control tactic, which relies on detecting and removing infected plants before the virus can spread. Since there are no tools to combat the virus directly and aphids can inoculate plants faster than they can be killed with pesticides, seed potato growers face significant new challenges in continuing to produce virus-free seed.
However, nature has interesting ways of dealing with quarrels, and in the wild (where there is no farmer to take care of potato plants and propagate them from new disease-free tubers) alternative approaches are needed to allow plants to overcome virus infection. Wild potatoes have developed strong defenses that allow them to resist viruses. Potato researchers have identified several PVY resistance genes in wild relatives of potato, and now we are working hard to understand how the resistance works. Using new genetic tools and technologies, we are working to incorporate resistance into Wisconsin varieties.
Conceivably in the future, PVY will become old news. Langlade County seed growers will be able to combine virus resistance with the proven, integrated approaches that begin with a solid disease-free foundation from the elite seed farm and employ rigorous inspection and rogueing. This will allow the farmers to relax and enjoy those bluebird days of July!
Learn more about Potato virus Y and its importance to the potato industry at www.potatovirus.com.