The streak disease of tomatoes in Ontario has been shown to be double-virus in nature, resulting from the synergistic action of tobacco mosaic virus (TMV) and potato virus X (PVX). Expression of the disease was found to be a function of both virus components of the complex, a variation in strain characteristics of either TMV of PVX being reflected in the intensity of their interaction in the tomato.The extreme severity of streak in the greenhouse and field tomato crops in Ontario is associated with what appears to be a geographic strain of TMV. This virus, designated as PLV, is characterized by the induction of primary local lesion in petunia without subsequent systemic invasion of that host.Air temperature profoundly influences the development of streak, symptoms being masked above the critical temperature of 26 °C. This phenomenon is a true masking effect with both viruses becoming latent in the tomato host.