Resistance to orange rust associated with the G1 molecular marker in parents of Brazilian RB sugarcane varieties
Orange rust, caused by the fungus Puccinia kuehnii, results in high productivity losses in sugarcane. Selection of resistant genotypes is one of the aims of sugarcane breeding programs. Phenotypic and molecular characterization of the parents of such crosses is essential to obtain superior varieties. We evaluated the reaction to orange rust in the field, the pattern of disease evolution, the molecular marker G1, and the usefulness of this information for the prediction of resistant phenotypes in the main sugarcane parents of the Brazilian Interuniversity Network for Development of the Sugarcane Energy Sector (RIDESA). For this evaluation, 63 sugarcane parents conserved in the RIDESA germplasm bank and that participated most in crosses from 1970 to 2000 were included. The experiment was carried out in a complete block design with three replications, using the clone RB036145 (susceptible to orange rust) as an infective line. Eleven severity assessments were performed using a diagrammatic scale. From the disease severity data, the relative area under the disease progress curve (rAUDPC) was calculated, and the genotypes were classified as resistant, intermediate, and susceptible, compared with the rAUDPC value of the SP79-2233 variety (susceptible to orange rust) and of the variety RB867515 (resistant to orange rust). Of the 63 sugarcane parents, 43 were classified as resistant, of which 27 were positive for the G1 marker; 10 as intermediate, including seven positives for G1; 10 were susceptible, among which four were positive for this marker. The molecular marker G1 showed an accuracy of 71% in predicting the resistant phenotype and could be used as a tool for the characterization of resistant germplasm.