Potato virus disease is really a big problem affecting potato production

I looked some potato production fields yesterday, It seems that potato virus diseases indeed is a big problem. In some field, 90% of potato plant display virus-like symptom, most of them are mosaic. In some field , which were grown in green house, and removed the cover recently, the potato grows much better. Discussed with the farmers, the yield difference is up to 40%. A big difference!