There are two different aspects to this topic. One relates to the use of high nitrogen fertilizers resulting in larger softer plants that are more susceptible to aphid attack, with subsequent transfer of virus. The other relates to activation of latent virus or the appearance of symptoms in otherwise asymptomatic infected plants. Plants are riddled with all sorts of viruses that don't cause symptoms when the plants are grown optimally, and the viral load can be large without any indication of infection. Plants become symptomatic when stressed. I brought robust totally asymptomatic petiolarid primulas from Scotland to the SE of England and within days of experiencing heat stress they became distorted and streaked. It doesn't matter whether the plants are greenhouse grown or field-gown, the principles are the same. I doubt there is such a thing as an unvirused plant, it is just a question whether the infection is sufficiently active or the plant sufficiently stressed for the symptoms to appear. J.