The genetics of blackarm resistance IV. Gossypium punctatum (Sch & Thon.) crosses.

Tim blaekarm ress factors, B1 and B2, discovered by Knight & Clouston (1939), have now been successfully transferred from American Upland (G. hirsutum) cotton to two varieties of Sakel (G. barbadense) without affecting the spinning quality and, in the Sakel genotype, they give a very marked resistance. With a view, however, to the possibility of increasing this resistance, and even of building up complete immunity, the resistance of many types of cotton has been examined genetically. In the synthesis of blackarm-resistant American strains, factor B1 is dangerous, since certain American varieties possess a gene complementary to it which causes pronounced dwarfing. On the other hand, B2, though of marked value, does not, alone give entirely adequate protection against the disease. As part of a systematic search for additional resistance factors, a genetic examination of the three punclaium types, BAR 3, Gambia Native and 'Hindi Weed' cotton, was undertaken. The results of this research are described in this present paper.