Race-specific resistance in oat does not provide adequate protection against crown rust because new virulent races of the pathogen often arise within a few years after cultivars with race-specific resistance are released. This study assessed levels of partial resistance in a series of experimental oat lines that showed reduced crown rust severity over 30 years of exposure to a diverse crown rust population. Oat breeding lines with putative adult plant slow-rusting resistance to Puccinia coronata were evaluated in small isolated field plots for crown rust severity in natural epidemics and in controlled inoculations in the greenhouse for infection efficiency with a single virulent P. coronata isolate. Lines were also tested as seedlings against isolates from a local P. coronata population to detect race-specific resistance. In field tests, areas under the disease progress curves for all 14 experimental lines tested were significantly less than for cv. Starter, the susceptible check, and values for nine of the lines were significantly less than for Portage, a standard slow-rusting cultivar. Four lines with good field resistance were susceptible as seedlings to >85% of the local P. coronata population. One of the lines, MN841801, remained relatively free of rust in field plots over 7 years of tests, even in the 3 years when Starter was destroyed by crown rust. In greenhouse inoculations, adult plants of MN841801 developed only 7% as many uredinia as Starter. MN841801 can be an excellent source of effective, durable adult plant resistance to crown rust.