STUDIES ON SUNFLOWER RUST: III. OCCURRENCE, DISTRIBUTION, AND SIGNIFICANCE OF RACES OF PUCCINIA HELIANTHI SCHW.

Discovery of rust-resistant sunflower seedlings in 1952 made it possible to distinguish races of Puccinia helianthi Schw. occurring on cultivated sunflowers (Helianthus annuus L.). The races isolated in 1954 helped to identify two discrete factors for resistance derived from wild annual sunflowers from the Renner, Texas, area. The four races which theoretically could be distinguished with two resistance factors have all been found in Manitoba. Collections from other provinces, countries, and continents, with few exceptions were classified on the Canadian differentials as race 1. Differences among isolates of race 1 were observed on resistant derivatives of H. argophyllus T. and G. Use of the four rust races described made it possible to identify different factors for resistance in sister selections of sunflowers with identical rust reactions in the field, and to demonstrate that the original rust resistant accessions 953-88 and 953-102 each contained at least two distinct resistance factors. Use of the races has also made it possible to recognize the existence in H. argophyllus of factors for rust resistance which appear to differ from those in the Canadian differential varieties.