Resistance to Puccinia sorghi Schwein. based on the Rp1-D gene has been used successfully in North America for the past 15 years to control common rust on sweet corn (Zea mays L.). The objective of this preliminary research was to examine rust reactions of Rp-hybrids grown for processing in the midwestern United States against biotypes of P. sorghi virulent against Rp1-D. In Sept. 1999, isolates of P. sorghi virulent on corn with the Rp1-D gene were collected throughout the midwestern United States. Rust reactions of 41 Rp-resistant, processing sweet corn hybrids and nine non-Rp hybrids were evaluated during the 1999-2000 season in Argentina, Hawaii, Mexico, and South Africa, where populations of P. sorghi are virulent against Rp1-D. Sporulating uredinia were observed on all hybrids in all locations. Although rust reactions varied among locations, mean standardized scores of nine non-Rp hybrids that were included in the trial as controls ranked nearly the same as in previous trials. Thirteen hybrids with standardized scores above 0.25 were more susceptible than the hybrid with the lowest mean rust rating, `Green Giant Code 27'. Thirty-two hybrids were intermediate in reaction to P. sorghi virulent against Rp1-D. Reactions were moderately resistant for nine hybrids with mean standardized scores below -0.50, including two moderately resistant, non-Rp hybrids (`GG Code 27' and `GG Code 6') that were included as controls. Additional trials are necessary to confirm reactions of these hybrids. If the Rp-hybrids that were moderately susceptible or susceptible in this trial are infected by P. sorghi virulent against Rp1-D, secondary inoculum will be abundant and infection will be severe if the weather is wet.
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