Inheritance of resistance to bacterial disease in Zea mays (L.)

RUIZ; ROSSI; BALZARINI; BONAMICO

Cba

Simposio; FIRST PLANT BREEDING SYMPOSIUM; 2021

INTA

In the last decade bacterial disease (BD) has expanded its range and prevalence in maize crops. In the worldwide, main BD reported in maize are Goss?s wilt and leaf blight, Stewart?s bacterial wilt, Maize white spot and Bacterial leaf streak. The continuous application of copper-based products to control BD has favored the spread of copperresistance genes among bacteria including the pathogenic strains. A preferable means of reducing losses to maize BD is the use of genetically resistant materials. However, few knowledge exist on inheritance of resistance to diseases caused by foliar pathogenic bacteria strains affecting Argentinean maize crops. The objective of this study was to explore inheritance of resistance to BD from a multi-environment phenotypical evaluation of a diverse panel of maize lines. An exotic germplasm of near 200 maize lines from the International maize and wheat improvement center (CIMMYT) was evaluated for resistance to BD in a multienvironment trial (MET) conducted during 2019/2020 and 2020/2021 crop seasons defining four environments (Río Cuarto 2020, Río Cuarto 2021, Sampacho 2021, and La Cautiva 2021) of South Córdoba, Argentina. A partially replicated (p-rep) design was used, with 25% of the genotypes with three replications and the remaining genotypes with one replication. All plants in each plot were evaluated and scored for BD by observing symptoms at flowering stage. Lesions observed in maize plants were compatible with those of Xanthomonas vasicola pv. vasculorum and bacterial isolates are being studied to pathogen identification. At each environment, each plant was classified by severity (SEV) in a 5-grade scale (1 = no symptoms; 2 = scattered lesions; 3 = up to 50% of the lower leaves presenting lesions, with severe injuries in 25% of the lower leaves; 4 = up to 75% of the leaves presenting lesions, with severe injuries in 50% of the lower leaves; 5 = 100% of the leaves with lesions, with severe injuries in 75% of the lower leaves). Alternative mixed lineal models were evaluated to model plot average SEV for each maize line at each environment. REML estimates of genotypic (G), environmental (E), and GxE variances were obtained to calculate broad sense heritability (H2) of BD resistance in each environment and across environments, as follows: In each environment: 𝐻2 =𝜎𝑔2𝜎𝑔2+ (𝜎𝑒2𝑝⁄ ) Across environments 𝐻2 =𝜎𝑔2𝜎𝑔2+ (𝜎𝑔𝑒2𝑒⁄ )+ (𝜎𝑒2𝑝⁄ ) where 𝜎𝑔2is the genotypic variance, 𝜎𝑔𝑒2is the variance of G×E interaction, 𝜎𝑒2is the error variance, 𝑒 is the number of environments, and 𝑝 is a weighted mean of the number of replications per genotype.We found that the exotic maize germplasm showed great phenotypic variability for resistance to BD across local environments. Río Cuarto 2020 had the lowest average SEV of 2.52, while La Cautiva 2021 had the highest one (SEV= 3.46). The mixed linear model with best fit included heterogeneous and unstructured variance for GxEinteraction. G and GXE variance components explained 55% and 26% of total G+E+GE phenotypic variance, respectively. BD heritability ranged 0.43 in La Cautiva 2021 to 0.73 in Río Cuarto 2021; while heritability across environments was 0.53 (Table 1).Table 1. Mean plot severity of bacterial disease severity and broad sense heritability (H2) of resistance in an exotic maize germplasm assessed in South Córdoba, Argentina.Environment mean ± E.E. H2RC20 2.52 ± 0.058 0.65RC21 2.93 ± 0.064 0.73SA21 3.10 ± 0.054 0.59LC21 3.46 ± 0.064 0.43Across environments 2.97 ± 0.032 0.53RC20: Río Cuarto 2020, RC21: Río Cuarto 2021, SA21: Sampacho 2021, LC21: La Cautiva 2021.The results suggest high potential for developing broad genetically controlled resistance of bacterial disease in maize.