Finger millet host plant resistance and associated changes in plant metabolites as a response to blast disease infection

Abstract

Finger millet blast, caused by fungus Magnaporthe grisea, is a highly devastating disease affecting all growth stages and causing a severe economic impact on the crop. Host plant resistance remains the most cost-effective and reliable management strategy. This study aimed at (i) determining new stable sources of blast resistance in selected finger millet genotypes in Uganda, and (ii) determining finger millet metabolites produced in response to infection by Magnaporthe grisea on susceptible and resistant genotypes. For the first objective, ninety-seven diverse finger millet genotypes were evaluated for resistance to leaf, neck, and finger blast across two locations (NaSARRI and Ikulwe) during the 2023A and 2023B seasons, using an alpha-lattice design with three replications in 2-meter row plots. For the second objective, a screen-house experiment was conducted using three finger millet varieties (resistant, moderately resistant, and susceptible) selected from field evaluations and arranged in a completely randomized design (CRD) with three replications. Controlled pathogen inoculation was done, and blast-infected tissues (leaf, neck, and finger) were sampled at seedling, booting, flowering, and maturity stages for metabolite analysis using spectrophotometry and HPLC. Disease severity was assessed periodically using a 1-9 scale for leaf blast, 1-5 for neck, and visual percentage of blast florets for finger blast that was adjusted to a 1-5 scale. These were later converted to Area Under Disease Progress Curve (AUDPC) before analysis. A significant variation was observed across genotypes, and genotype by environment (GxE) interaction, for leaf, neck, and finger blast severity. The stability analysis revealed twenty-two finger millet genotypes with stable resistance to at least leaf, neck, and/or finger blast. The selected finger millet genotypes demonstrated a significant difference in metabolite concentrations across the different sampling times with progress in blast infection. The study further identified saponins, salicylic acid (SA), and phenolics such as tannins, among others, as key metabolites associated with low blast severity in finger millet. These accumulated at higher levels in resistant and moderately resistant genotypes relative to the susceptible one. The study therefore, identifies new stable blast resistance and key biochemical indicators that may guide breeding efforts for disease resistance in finger millet.