The influence of common leaf rust (Puccinia sorghi Schw.) on 2 sweet corn (Zea mays L.) hybrids was compared in rusted and nonrusted plots for several maturity and ear quality characters. Differences were found for time of silking, ear length, ear diameter, and percentage of moisture between rusted and nonrusted plots. The percentage of Brix ranged from 4% to 25%, with the rusted plots always having reduced mean values. Correlations (P = 0.01) were found between ear diameter and percentage of moisture, percentage of Brix and percentage of moisture, and between ear length and ear diameter.
Evaluation of the progeny populations from crosses between a resistant sweet corn inbred (Zea mays L.) and 3 susceptible inbreds indicated that the variation for partial resistance to corn leaf rust (Puccinia sorghi Schw.) depended on the parents used. Heritability estimates were high with both additive and dominance gene effects important in character expression. Epistasis was shown to influence rust reaction in at least one cross. The ability to improve partial resistance in 2 sweet corn populations was demonstrated by 3 methods of selection.
Partial resistance effectively reduces common leafrust (Puccinia sorghi Schw.) epidemics in sweet corn (Zea mays L.). One measurable field component of partial resistance is delayed first pustule appearance (FPA), influenced by latent period and infection efficiency. This is the first report of delayed FPA against common rust in sweet corn. Our objectives were to determine 1) if differences in delayed FPA could be assessed in a field environment, 2) the relationship between delayed FPA and disease severity, and 3) the inheritance of delayed FPA. FPA was screened in a field environment during 1992 and 1993 using two susceptible U.S. station inbreds and 32 breeding lines selected for partial resistance to common rust. The range in mean delay in FPA on a genotype basis was 2.4 to 5.0 days in 1992 and 1.5 to 7.4 days in 1993. Although the rank correlation of genotypes between years was small (r = 0.27), several breeding lines had longer delayed FPA in both years while others produced shorter FPA delay in both years. Seven commercial sweet corn hybrids with a wide range of partial resistance did not differ (P ≤ 0.05) for delayed FPA. There was no correlation between disease severity and delayed FPA (r2 = 0.00 to 0.21) for breeding lines or commercial hybrids, indicating that selection for delayed FPA may be conducted independently of disease severity. In a generation mean analysis conducted in 1993 and 1994 for three high x low delayed FPA crosses, the genetic control of delayed FPA was primarily additive. Dominant genetic effects were found in two crosses and epistasis was not significant in any cross.
Interplot interference, or the influence of one host genotype on another when grown in adjacent plots, can be a problem with the evaluation of partial resistance to airborne pathogens. Moreover, interplot interference may affect selection in the plant breeding nursery. To estimate the degree of interference in a typical sweet corn (Zea mays L.) breeding nursery, the partial resistance of 3 hybrids grown in several field plot treatments was examined. Treatments consisted of differing spatial arrangements of the 3 hybrids. In a 2-year study, mean rust level differed significantly for 5 field plot treatments. As the potential for leaf rust increased, the ability to distinguish between hybrid disease reaction diminished. In addition, the variability in disease reaction was reduced as rust potential increased for hybrid and field plot treatment, indicating that when high levels of leaf rust existed, disease gradients tended to flatten.
A sugary-1 breeding population of maize (Zea mays L.), AS11, has hypersensitive resistance (Rp1d gene) to common leaf rust (CLR) (Puccinia sorghi Schw.) and is resistant to maize dwarf mosaic virus (MDMV) strains A and B. It is released for its potential value in sweet corn improvement.