Cell wall contents and estimation of genetic parameters for polysora rust resistance in Tropical Maize (Zea mays L.)

Polysora rust (Puccinia polysora Underwood) of corn (Zea mays L.), known as southern rust, has enormous destructive potential on susceptible hosts. It occurs in the tropics and in warm temperate regions, and is especially severe in Hawaii's wet winter season. No temperate sweet corn hybrids can be grown commercially without fungicides in these months, but tropically adapted hybrids have an intermediate level of resistance that minimizes yield losses to this disease. Monogenic sources of resistance are ineffective in Hawaii, as they are throughout the tropics. Genetic parameters for general or horizontal rust resistance were estimated by evaluating six generations from crosses of the tropical sweet corn inbred, Hi38-71 (resistant), and a susceptible inbred, G24. Empirical scores ranging from 1 (highly resistant) to 9 (highly susceptible) were applied under uniform field epiphytotics of the rust. A simple three-parameter model (m, [a] & [d]) did not fully explain those data. However, a digenic model that the incorporated epistasis adequately explained the variation in resistance. Epistatic interactions [aa] and [dd] were highly significant, while the [ad] interaction was not. Heritability values were 68% for broad-sense and 53% for narrowsense, and were accompanied by estimates of about two controlling loci. The high significance of non-additive and epistatic effects makes clear that the breeding for rust resistance should be based on selection of both parents. Additional data were taken on Set M of recombinant inbreds (RILs), based on the cross of Hi38cl and Ia453sh2 (susceptible). Previously reported, the parents averaged 3.1 (Hi38cl) and 5.3 (Ia453sh2), while the 55 RILs averaged 4.5 with skew showing high resistance. Under a second more severe epiphytotic, only 8 of the 52 tested RILs were comparable in resistance to the Hawaii parent. Data from the GMA (Generation Mean Analysis) and RIL studies, both of which were based on the moderately resistant Hi38, are similar in confirming that two to three loci are required to provide adequate field resistance for sweet corn growers. Crude protein of Hi 27 was higher as 10.43% than that of Hi38-71 but acid detergent fiber and neutral detergent fiber of Hi27 were lower as 34.70% and 65% than those of Hi38-71 in the cell wall contents. In parallel studies in Hawaii it is clear that tropical field corn hybrids must have much higher levels of resistance, and these resistance alleles appear to be available only among tropical inbreds.