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SUMMARY Strain D of tobacco necrosis virus (TNV) was transmitted by zoospores of 3 different isolates of Olpidium brassicae (Wor.) Dang. to roots of Mung bean and lettuce grown in modified Hoagland's solution diluted 1/20. On Mung bean roots necrotic local lesions formed one day after exposure to virus and zoospores. Virus in lettuce was assayed by inoculation to leaves of French bean. Virus transmission was favoured by decreasing salt concentration and increasing the pH value of the nutrient solution and depended also on the concentrations of virus and zoospores. With 105 zoospores/ml. transmission to lettuce was obtained with as little virus as 0.05 ,ug./l. When the virus concentration was 5 ,ug./l., 50-100 zoospores/ ml. were effective. Fungus infection as measured by the number of zoo- sporangia in the root was not strictly correlated with virus infection. Exposure of roots to virus + zoospore mixture for 1 min. sufficed to infect them with virus. More transmission occurred when virus was added before or together with zoospores, than after. Roots, exposed to zoo- spores for 10 min., then washed, were more readily infected by TNV when virus was introduced during the first hour or two after zoospore attach- ment to the root cells than later; there was some transmission even when virus was withheld till 4 hr after washing. Immersing roots, inoculated with fungus and virus, in hot water (60') killed the fungus but not the virus, and varying the interval between inoculation and heating showed that virus became established after 2-3 hr. Isolates of Olpidium brassicae naturally contaminated by strain D or A of TNV were freed from contamination by inoculating lettuce roots with dilute zoospore suspensions. Zoospores mixed or naturally contaminated with TNV were partially separated from it by centrifugation. Virus trans- mission was prevented by adding concentrated homologous antiserum to zoospores that had already been exposed to virus, or by adding very dilute antiserum to virus before mixing it with zoospores. The extent to which transmission was prevented by antisera to other strains of TNV depended on the degree of their serological relationship to strain D. The present evi- dence does not support the suggestion that TNV is carried inside the fungus.

Transmission of Tobacco Necrosis Virus by Zoospores of Olpidium brassicae