Serial dilutions of potato virus X in either clarified sap or in a highly purified suspension of the virus were mechanically inoculated to leaves of Datura tatula L. Test plants which were inoculated with high concentrations of the virus developed systemic symptoms within the expected minimum time limits. As the virus concentration was decreased, time required for symptom development increased correspondingly and incubation periods in some cases were as long as 22 days. Delayed symptoms were also obtained when very restricted leaf areas were inoculated. In this respect, the results were similar to those obtained at low virus concentrations. The influence of low virus concentration in increasing the incubation period was least pronounced at high temperatures and was greatest at low temperatures. At the lower virus concentrations, variation in incubation periods of individual plants was considerably greater than when virus inoculum was more concentrated. In two tests where known amounts of virus X were inoculated to leaves of D. tatula and to leaves of Gomphrena globosa L., the former test plant was approximately 100 times more sensitive than the latter.

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