General Resistance in Maize to Southern Rust (Puccinia polysora Underw.)

Southern corn rust (Puccinia polysora Underw.) is a major tropical disease that can lead to severe yield losses in the tropics. Occasionally it becomes of concern in temperate regions, as in 2010. We summarize 45 yr of research on the occurrence, genetic basis, and breeding of general resistance. Trials were conducted in Colombia, Hawaii, Nigeria, the Philippines, Texas, and Thailand. Temperate maize proved uniformly susceptible. No racially specific monogenes were effective. Resistance varied continuously among tropical inbreds and many of these showed stable intermediate tolerance. Two diallel populations revealed high correlations of inbred values with hybrid array means and slight heterosis for resistance. Narrow-sense heritabilities were 51% and 66%, and mean square ratios for general to specific combining ability (GCA:SCA) were 7.8 and 10.7 to 1. Generation mean analyses were performed on six sets of families. Mean and additive effects were significant across families, while significant nonadditive effects were rare. Heritability values ranged from 19% to 49%. A set of recombinant lines segregated a primary susceptibility locus on chromosome 6. Studies of 137 near-isogenic inbred lines (NILs) of resistant tropical inbred Hi27 revealed a dominant susceptibility quantitative trait locus (QTL) in the NIL for dwarf-1 locus, but linkage could not be confirmed. Premature senescence was associated with rust infections in many NILs, emphasizing the ubiquity of modifying genes. No correlation existed between resistance to southern and common rusts. Resistant tropical inbreds and populations were bred to serve as sources for future improvement.