ABSTRACT
Resistance in sweet corn conferred by the Rp1-D gene has controlled common rust, caused by Puccinia sorghi, in North American corn for nearly 15 years. Eleven isolates of P. sorghi virulent on corn with the Rp1-D gene were collected from Rp-resistant corn in 1999 from Wiscon-sin, Illinois, New York, and Minnesota. Isolates were increased on susceptible sweet corn. Urediniospores of nine isolates were bulked. Reactions of individual Rp genes in the rp1 region and reactions of linked combinations of Rp genes in the rp1 region (i.e., compound rust resistance genes) were evaluated against the bulked population of P. sorghi in several greenhouse trials. Reactions of individual and compound Rp genes also were evaluated against individual isolates of P. sorghi. Each trial contained at least two replicates of several lines with Rp genes and one susceptible check. Five to 10 two-leaved seedlings per line were inoculated at least twice with a suspension of urediniospores. Ten days after inoculation, rust reactions were rated:+ = sporulating uredinia, - = no sporulating uredinia, and I = chlorotic or necrotic tissue surrounding small uredinia. Four single genes, Rp1-E, Rp-G, Rp1-I, and Rp1-K, and eight compound genes, Rp1-JFC, Rp1-JC, Rp-GI, Rp-G5, Rp-GDJ, Rp-G5JD, Rp-G5JC, and Rp-GFJ, conferred resistance. Additional characterization of virulence in North American populations of P. sorghi that are avirulent against Rp1-D is necessary to determine if these genes will be as widely effective as the Rp1-D gene has been. Two subpopulations of P. sorghi were detected from the bulked population after it was sequentially cultured for at least five cycles on seedlings with Rp1-C or with Rp1-J. The subpopulation cultured on Rp1-J was avirulent on lines with Rp1-C/L/N, Rp1-B, and Rp1-M; whereas the subpopulation cultured on Rp1-C was virulent on lines with each of these genes. Both subpopulations were virulent on lines with Rp1-D.
