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Horizontal Resistance of Red Rustproof Oats to Crown Rust. H. H. Luke, Research Plant Pathologist, Plant Science Research Division, ARS, USDA, Plant Pathology Department, University of Florida, Gainesville 32601; W. H. Chapman(2), and R. D. Barnett(3). (2)(3)Chairman and Assistant Agronomist, respectively, Agricultural Research and Education Center, Quincy, Florida 32351. Phytopathology 62:414-417. Accepted for publication 1 November 1971. DOI: 10.1094/Phyto-62-414.

Some cultivars of Red Rustproof (RRP) oats (Avena byzantina) exhibit a specific (necrotic) reaction to a few races of crown rust (Puccinia coronata f. sp. avenae), but their principal defense against this pathogen is through horizontal resistance (late-rusting and slow-rusting). A late-rusting cultivar is one that exhibits crown rust symptoms 10-14 days later than susceptible cultivars. When symptoms develop, late-rusting cultivars exhibit a susceptible reaction. Late-rusting has been observed only under natural field conditions, and is not associated with any stage of plant growth (age of the host). Late-rusting seems to be dependent upon environment or upon spore concentration. A slow-rusting cultivar is one on which the percentage of crown rust infection increases more slowly than it does on a susceptible cultivar. Slow-rusting cultivars usually exhibit a lower percentage of infection than susceptible cultivars throughout the growing season. Slow-rusting can be reproduced under artificial conditions. Apparently, a different mechanism is involved from that responsible for late-rusting. Both, however, seem related to the maturity dates of specific cultivars, because late-maturing cultivars exhibit pronounced slow-rusting and late-rusting, whereas early-maturing RRP cultivars do not.