TRANSMISSION EXPERIMENTS WITH AN ISOLATE OF BEAN YELLOW MOSAIC VIRUS FROM GLADIOLUS NANUS - ACTIVATION OF THE INFECTIVITY FOR VICIA FABA BY A PREVIOUS PASSAGE ON THIS HOST

An isolate of bean yellow mosaic virus from Gladiolus nanus showed some peculiarities in its mechanical transmissibility from one host to another which are summarized in Fig. 1. The virus was easily transmitted from Nicotiana clevelandii to Nicotiana clevelandii, but only with difficulties from N. clevelandii to Vicia faba. From V. faba the virus was easily transmitted to V. faba and to N. clevelandii. A greatly increased infectivity for V. faba was retained after one passage on N. clevelandii following a passage on V. faba, it was lost, however, after several passages on N. clevelandii. Since the "adaptation" to V. faba was thus reversible and since single lesion isolates showed the same behavior, a selection of preexisting strains from the original inoculum cannot be the explanation for the observed phenomenon. The most likely explanation is that in V. faba variants are produced (induced or spontaneously?) and that those variants which are especially well adapted to this host are propagated preferentially. After transfer to N. clevelandii other new variants may develop which gradually dilute out those with a high infectivity for V. faba.