Clayberg, C. D.
Further attempts here to break the Tm\2-nv linkage have been unsuccesful and suggest a very close linkage of these genes. Fortunately, the linkage was recently broken by Dr. Paul G. Smith at Davis, and I have tested and confirmed this in material he kindly sent. My additional data are being presented to encourage others interested in working with this source of resistance to use the Tm\2-nv^+ stocks now available from Dr. Smith or myself, rather than to attempt rebreaking the linkage in their own backcross generations. Several sibling BC\2 families culled to a total of 427 resistant segregants were grown here in the field in 1959. These plants were progeny tested, using open-pollinated set seed, in winter 1959-60 to identify those also carrying nv. Of these, 420 plants had nv segregants in their progenies. Inoculation of the remaining seven families with TMV yielded all susceptible plants. Thus the seven plants no doubt represented field escapes from the virus. The 420 plants, combined with the 75 previously tested (see TGC 9:28), indicate a linkage between Tm\2 and nv of the order of 0.2 units or closer, at least in the predominating varietal background tested in this instance where Fireball (Harris Seed Co.) was the recurrent parent used in the backcrossing.