Control of A Tomato Plant Root-Knot Nematode By Induced Resistance Of Oxalic Acid Derived From Aspergillus Niger

Abstract Aspergillus niger F22 producing oxalic acid (OA) as a nematicidal component is currently used as a microbial nematicide. OA is known to induce systemic resistance in plant diseases caused by fungi, bacteria, and viruses, but the induced resistance of OA has not yet been elucidated in plant diseases caused by root-knot nematodes (RKNs). In this study, we investigated the functional mechanism of induced resistance of A. niger F22 formulation (Nemafree, 20% SC) and OA in tomato plant RKN disease caused by Meloidogyne incognita and analyzed their effectiveness against the disease. Foliar spray and soil drench treatments of Nemafree and OA were effective in the management of M. incognita in tomato plant in-pot experiments. When Nemafree and OA were applied 4 days before inoculation of M. incognita eggs, the treatments of Nemafree (4,000-fold dilution) and OA (0.22 mM) reduced root gall formation by more than 50%. The soil drench treatment also effectively suppressed RKN disease in field experiments. Moreover, the treatments of Nemafree and OA enhanced the transcriptional expression of pathogenesis-related 1 gene, plant proteinase inhibitor-II, and polyphenol oxidase genes and improved the production of total phenols, flavonoids, and lignin in the tomato plants infected with M. incognita. These results demonstrate that RKN diseases can be effectively controlled by induced resistance even at low concentrations of Nemafree or OA. Accordingly, our study provides evidence for more economical and efficient application strategies of microbial nematicides that control RKNs under field conditions.