Southern corn rust (Puccinia polysora Underw.) is a major tropical disease that can iead 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 Thaiiand. Temperate maize proved uniformly susceptible. No racially specific monogenes were effective. Resistance varied continuously among tropical inbreds and many of these showed stable intermediate toierance.Two diallel populations revealed high correlations of inbred values with hybrid array means and slight heterosis for resistance. Narrowsense heritabilities were 51% and 66%, and mean square ratios for gênerai to specific combining ability (GGA: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 HÍ27 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.

Genetic studies were made of general resistance in maize (Zea mays L.) to common rust, Puccinia sorghi Schw., by means of diallel and generation mean analyses of crosses among 11 inbreds. Most populations were grown under severe natural epiphytotics on Oahu, Hawaii with rust spore burdens from the time of emergence. Susceptible checks averaged 6.8 on the 1 to 7 rust scale, where 7 represented infections covering more than 40% of mature leaf surface. Few inbreds, including Oh545 and CM105 (Cuban Flint origin), displayed effective general resistance under these conditions. The nine parents of a diallel showed high general combining ability (gca) for resistance, and a high coefficient of linear determination (r2 = 0.91) between parental averages and their gca effects. Heterosis was not evident for resistance, and specific combining ability effects were small although statistically significant. Generation mean analysis was made of 20,000 plants from 25 sets of biparental crosses and their F2 and backcross progenies. Heritability estimates were 83.4% (broad‐sense) and 47.0% (narrow‐sense) for the summarized data, and substantially higher for the highly resistant sources Oh545 and CM105. From crosses with the highly susceptible inbreds — B37, CM104 (India) and AA8 (Hawaii) — Oh545 was inferred to carry two major, partially dominant resistance genes.

Studies were made to determine the extent and genetic control of husk number variations in maize (Zea mays L.), and to test their association with ear insect resistance and response to environmental influences. Husk numbers were highly uniform within inbreds and single‐cross hybrids, ranging from 6 to 19. Twelve inbreds were chosen to represent this range for diallel and generation‐mean analyses. Analyses were made in Cali, Colombia, and in Waimanalo, Hawaii, of husk numbers from 10‐ and nine‐parent diallels. General combining ability (g.c.a.) mean squares were highly significant at both locations. Specific combining ability mean squares were also significant, but were exceeded by the g.c.a, values by ratios of 5.3 to 1 (Colombia) and 19.9 to 1 (Hawaii). Hybrids showed a significant positive heterotic effect of 11%, and array means correlated well with parental values (r = 0.93). Means and array means for the hybrids grown at both locations were also highly correlated (r = 0.94 and r = 0.86, respectively). F2 and backcross progenies of 10 crosses involving high ✕ low husk numbers were analyzed. Backcross progenies commonly reverted sharply toward parental means, and F2 and F1 means were quite similar. Environmental variability was high and narrow‐sense heritability estimates were low (average 24%). The Cuban Flint‐derived inbred CM111 (India) provided the best genetic source for increasing husk numbers, which is proposed as a practical and efficient alternative to selection for low insect injury for improved ear pest resistance.Damage by the larvae of ear insects was evaluated for earwrm (Heliothis zea Boddie) and fall armyworm [Spodoptera frugiperda (J. E. Smith)] in relation husk numbers in Hawaii and Colombia. Resistance was notably higher in tropical lowland types of maize with high husk numbers. Ear damage in Hawaii (largely earworm) correlated significantly (r = −0.40) with husk numbers that ranged from 6 to 14 in a broad‐based composite. Ear insect damage in Colombia (largely fall armyworm) also correlated significantly (r = −0.68) with husk numbers that ranged from 8.0 to 14.9 in a 45‐entry diallel. Husk numbers were highest for lowland tropical races of maize and lowest for temperate and montane races, correlating significantly (r = −0.71) with elevation at which the race was indigenous. It was concluded that ear pests provided strong selection pressure for higher husk numbers during evolution of maize. Environmental effects on husk numbers appeared solely to relate to temperature during growth, and a reduction of one husk was observed for a corresponding increase of 4 C in growing temperature during early growth.