Azelaic Acid Induces Resistance in Coffea arabica Against Infection by Hemileia vastatrix, the Causal Agent of Coffee Leaf Rust

Abstract

Coffee leaf rust (CLR), caused by the fungus Hemileia vastatrix, is a very devastating disease affecting coffee production in many countries worldwide and causing yield losses that range from 15% to 50%. High CLR intensity on coffee trees impairs photosynthesis and causes intense defoliation resulting in fewer and smaller fruit berries on trees. New control methods for CLR that will reduce the use of fungicides and production costs need to be investigated and used in an integrated disease management program. In this study, it was hypothesised that azelaic acid (AzA), a C9 dicarboxylic acid (oxylipin) known to be involved in systemic acquired resistance, could boost defence reactions on the leaves of coffee (Coffea arabica, cultivar Catuaí Vermelho—IAC44) against infection by H. vastatrix. In the in vitro assay, urediniospores germination was significantly reduced (81%–86%) by AzA with concentrations from 1 to 20 mM. Fungal sporulation was much more intense on the leaves from water-sprayed plants compared to leaves from AzA-sprayed plants at both 22 and 30 days after inoculation (dai). The area under CLR progress curve and the intensity of fungal sporulation were significantly lower by 82% and 83%, respectively, and the incubation period was higher by 31% for AzA-sprayed plants compared to water-sprayed ones. The AzA-sprayed plants infected by H. vastatrix displayed less photosynthetic impairments considering the greater values for rate of net CO₂ assimilation, internal CO₂ concentration, transpiration rate, and maximum photochemical efficiency of photosystem II compared to water-sprayed and infected ones. On top of that, these plants displayed higher concentrations of chlorophyll a + b and carotenoids and a more robust antioxidative metabolism (increased ascorbate peroxidase, catalase, and glutathione reductase activities at 12 dai). Interestingly, activities of chitinase, β-1,3-glucanase, phenylalanine ammonia-lyase, peroxidase, and polyphenoloxidase were lower for AzA-sprayed plants as a result of reduced colonisation and sporulation of H. vastatrix in contrast to water-sprayed and infected ones. Higher concentration of superoxide anion radical for AzA-sprayed plants and infected by H. vastatrix at 12, 22, and 30 dai may have helped to reduce the colonisation of coffee leaf tissues by H. vastatrix besides having a citotoxic fungistatic effect against the fungus. These results strongly support the potential of AzA to negatively affect the germination of urediniospores from H. vastatrix as well as to hamper the infection process of H. vastatrix on coffee leaves.