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. (more…)