A "Pan American" approach to durable crown rust resistance in oats : evaluation of a seedling resistance covariance method.

Abstract

Crown rust, caused by Puccinia coronata, is the most important disease of oats worldwide. Methods of control have usually been based on single major genes that confer complete resistance, but with the emergence of new races this resistance is easily overcome. Thus, partial resistance is believed to be more effective in controlling the disease because it promotes coexistence of host and pathogen and thus slows the evolution of pathogen virulence. Therefore, creating durable resistant lines depends on several aspects and choosing parental lines is one of the most important. However, methods to predict the best parental combinations and their progeny performance are not reliable. Since the University of Minnesota oat breeding program began in 1889, many cultivars have been released. On average 150 new lines are selected every year for multiple location yield trials. With the number of lines available and the necessity for reducing the number of crosses to save labor, time, money and space, we attempted to identify a method to predict the superior test cross combinations involving parental lines with genetic diversity between resistance genes. We used a covariance analysis based on the results from the seedling rust tests of parental lines, where the original data based on rust reaction types (HR – highly resistant; R – resistant; MR – moderately resistant; MS – moderately susceptible and S – susceptible) were transformed to numbers (5, 3, 1, -1 and -3), respectively. Every possible pairing combination was done with all parental candidates. A large negative covariance indicated that the parents had substantial genetic diversity between them, which was preferred because then a resistant gene in one parent could match the deficiency in another parent. Eleven populations from different crosses iii were evaluated for crown rust resistance. The covariance between parents for these crosses varied from -0.5864 to +0.5304, which indicated more and less genetic diversity among parents. The most similar parental pair was used as a control to test our hypothesis. In our study covariance analysis was not able to predict the best parental combination for maximum potential crown rust resistance in progenies, but crossing parental lines with higher resistance levels produced lines with consistently lower disease infection and a higher proportion of resistant individuals.