DISEASE LESION MIMICRY CAUSED BY MUTATIONS IN THE RUST RESISTANCE GENE RP1

The rp1 locus of maize controls race-specific resistance to the common rust fungus Puccinia sorghi. Four mutant or recombinant Rp1 alleles ( rp1-NC3, Rp1-D21, Rp1-MD19, and Rp1-Kr1N) were identified. They condit ion necrotic phenotypes in the absence of the rust pathogen. These Rp1 lesion mimics fall into three different phenotypic classes: (1) The r p1-NC3 and Rp1-D21 alleles require rust infection or other biotic stim ulus to initiate necrotic lesions. These alleles react strongly to all maize rust biotypes tested and also to nonhost rusts, (2) The Rp1-MD1 9 allele, which has a similar phenotype, also requires a biotic stimul us to initiate lesions. However, Rp1-MD19 shows the race specificity o f the Rp1-D gene. (3) The Rp1-Kr1N allele specifies a diffuse necrotic phenotype in the absence of any biotic stimulus and a race-specific r eaction when inoculated with maize rust.