Genetics and Breeding of Resistance to Southern Rust in Corn.

Southern corn rust (Puccinia polysora Underw.) is a major disease of maize in the tropics that leads to severe yield losses. Our multi-national trials since 1967 revealed no resistance in temperate corn. Multiple races of the pathogen occurred in all trials and monogenic (Rpp) immunity alleles were valueless. Resistance varied widely among tropical inbreds. Genetic studies include diallel populations in Colombia and Hawaii that showed high correlations of inbred values with hybrid array means and slight heterosis for resistance. Narrow-sense heritabilities were 51% and 66% and GCA:SCA mean square ratios were 7.8 and 10.7. Generation mean analyses were performed on three sets of families based on crosses of susceptible and resistant inbreds. Mean and additive effects were significant across families and non-additive effects were significant in some. Heritability values ranged from 19% to 49%. Recombinant inbred lines based on susceptible (B68) and resistant (Ki14) parents segregated in Philippines and Hawaii as if under control of two primary susceptibility QTLs. Molecular markers identified a major QTL on Chromosome 6, with significant additional correlates. Linkage drag studies revealed major QTLs affecting rust susceptibility in a series of ~150 near-isogenic lines (NIL) of resistant tropical inbred Hi27. A NIL of exceptional susceptibility was dwarf-1 (3S-33) that was concluded to carry a closely linked dominant susceptibility gene from temperate maize. Premature senescence was associated with rust on NILs that otherwise affected plant chlorophyll, plant colors and vigor, suggesting that many genes act as loci modifying yield response. No correlation existed between resistance to southern rust and resistance to common rust. Breeding to increase general resistance to southern rust was effective in many countries when backcrosses were followed by F2 progenies. Tropical synthetic HIS3 was created from 20 multinational inbreds and showed superior general resistance.