Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2005
Publication Date: 9/1/2005
Citation: Leonard, K.J., Huerta-Espino, J., Salmeron, J.J. 2005. Virulence of oat crown rust in Mexico. Plant Disease. 89:941-948.
Interpretive Summary: Crown rust is the most damaging disease of oat in North America. Oat production in the southwestern U.S. may be vulnerable to spread of crown rust from adjacent areas of Mexico where oat is grown primarily as a forage crop. Likewise, new virulent races of crown rust that appear on formerly resistant oat varieties in Texas may spread by wind-borne spores into Mexico and endanger oat crops there. We surveyed crown rust races in collections of infected oat plants from four regions of Mexico looking for possible evidence of spread of rust races between the U.S. and Mexico and between regions within Mexico. We found that races in each of the four regions of Mexico were largely distinct, indicating relatively little spread of rust from west to east across Mexico. Many rust races found in Texas were also found in the adjacent Mexican States of Nuevo Leon and Coahuila, but the most common races in those States differed from the most common races in Texas, indicating that epidemics in Texas and Mexico arise locally and do not depend on cross border transmission of rust spores. None of the 27 genes for crown rust resistance in oat that we tested will provide long-term protection against crown rust in Mexico. Therefore, Mexican oat breeding programs should incorporate other types of rust resistance that are non-specific and at least partially effective against all races. Reducing crown rust severity in Mexico will protect oat forage and make dairy farming more profitable.
Technical Abstract: Virulence of isolates of Puccinia coronata collected during 1992-1998 from Sonora, Chihuahua, Nuevo Leon, and five States in Central Mexico were compared on a set of 27 differential oat lines with different genes for race-specific resistance. Frequencies of virulence and the presence of specific pathogenic races were compared between the four regions of Mexico and between Mexico and the adjoining States of California and Texas in the U.S. The P. coronata populations in Mexico were highly diverse even though the sexual stage of the fungus is not known to occur there. Virulence frequencies were more similar between Chihuahua and Nuevo Leon than between other regions of Mexico, although the four races common to both Central Mexico and Chihuahua was more than between any other pair of regions of Mexico. No races found in Sonora were found in other regions of Mexico. More races found in Texas were common to Nuevo Leon than to any other region of Mexico. Mean virulence complexity was lowest in isolates from Central Mexico; greatest in Sonora, California, and Texas; and intermediate in Chihuahua and Nuevo Leon. Significant (P < 0.05) associations of virulence occurred for 24 pairs of differential lines in at least three of the four regions of Mexico. Virulence to 19 of the 24 pairs was also significantly associated in Texas; virulence to 13 was also significantly associated in California.