CONICET | Buscador de Institutos y Recursos Humanos

Argentina is the fourth maize producer with a total production of 50 million tons in 2020. 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 copper-resistance genes among bacteria, including the pathogenic strains. Thus, a most acceptable means of reducing losses to maize BD is the use of genetically resistant materials. However, few knowledges exist on inheritance of resistance to BD caused by foliar pathogenic strains affecting Argentinean maize crops. The objectives of this study were (1) to isolate and identify bacteria from symptomatic maize leaves collected from field samples in the southern region of Cordoba, Argentina (2) to explore inheritance of resistance to BD from a multi-environment phenotypical evaluation of a diverse panel of maize lines.A 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 2020 and 2021 in 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. In a previous year used to multiply the exotic panel, leaves with bacterial symptoms (light-colored necrotic streaks and yellow irregular blotches) were collected from the inbred lines to compose pooled samples which were later analyzed in the laboratory to identify foliar bacteria through the use of MALDI-TOF spectrometry and sequencing of 16S rRNA gene. As a result of bacteria identification from, the pathogenic native strain isolated from the maize field trials was identified as Curtobacterium sp. RC18-1/2. A maximum likelihood phylogenetic tree based on 16S rRNA gene sequences, showed the relationships between this strain and other pathogenic bacteria retrieved from the GenBank database (Fischer et al., 2021).In the 2020 and 2021 MET, 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 new isolates are being studied to pathogen identification. At each environment, each plant was classified by the degree of severity (SEV), where: 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. Different 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: where σ_g^2 is the genotypic variance, σ_ge^2 is the variance of G×E interaction, σ_e^2 is the error variance, e is the number of environments, and p is a weighted mean of the number of replications per genotype.As a result from the phenotypical assessment, we found that CIMMYT maize lines showed great phenotypic variability for resistance to BD. Environment 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 GxE interaction. G and GXE variance components explained 55% and 26% of total G+E+GE phenotypic variance of plot mean SEV. Heritability values ranged 0.43 in La Cautiva 2021 to 0.73 in Río Cuarto 2021; while heritability across environments was 0.53 (Table 1).The results suggest an important genetic variability in the phenotypic variability observed for bacterial disease severity with a relative low GxE in the studied environments.