Wheat spindle streak mosaic virus was transmitted via water from soil-free roots of wheat [Triticum aestivum] plants infected by growing in infective soil, but not from plants grown in sterile soil and infected with the virus by sap inoculation of the leaves. The zoosporic fungi Lagena radicicola, Olpidium brassicae, P. graminis, Rhizophydium graminis and Pythium spp. usually were present in the roots of plants infected from soil, and in the roots of most plants in 10 sets infected successively by root association in sterilized sand. When isolates of each of the zoosporic fungi were cultured on wheat infected with the virus by sap inoculation of leaves, the virus was transmitted by root association in sand only from plants infected with P. graminis. The optimum temperature for transmission of the virus in soil, 15.degree. C is above the optimum for the development of the virus, 10.degree. C, but within the lower range of temperatures favorable for the development of P. graminis.