Root-knot nematode abundance and diversity in grain fields of New-Era producers in South Africa

Abstract

Production of maize, the third most important grain crop for human consumption after rice and wheat, is hampered by Meloidogyne which is the economically most important plant-parasitic nematode (PPN) genus infecting and damaging local grain crops. The research aim of this study was to determine the abundance and diversity of PPN in local grain fields of New-Era producers. Root and rhizosphere samples were obtained during the 2018/19 and 2019/20 growing seasons from 15 fields on which maize (2019 & 2020) and sunflower (2020) were grown. Eggs of sessile endoparasitic genera, e.g., Meloidogyne and Rotylenchulus, were extracted from 50 g roots using an adapted NaOCl method, while motile individuals of these two and other PPN genera were extracted from 5 g roots using the sugar-flotation method. From soil samples, both PPN and beneficial nematodes were extracted using the decanting and sieving method, followed by the sugar-flotation method. Prominence values (PVs) were calculated for all nematode genera and/or families, while univariate and multivariate data analyses were done to elucidate the population dynamics of predominant PPN genera over four sampling intervals and investigate whether associations existed between nematodes and selected abiotic factors. Only Meloidogyne and Rotylenchulus occurred in 50 g root samples, with Meloidogyne being predominant. These two genera also dominated in 5 g roots, with Helicotylenchus and Pratylenchus also being present but to a lesser extent. In soil, Rotylenchulus was predominant, followed by Helicotylenchus, Meloidogyne and Pratylenchus while other genera and families occurred in low densities. Beneficial trophic groups in the soil, in order of domination, were bacterivores, fungivores, omnivores and predators. Meloidogyne and Rotylenchulus abundance in 50 g roots, and that of fungivores in soil were the highest at Dwaalkraal and Hoopstad for the 2nd sampling intervals of both surveys. For the 2019 survey, localities with soils containing high sand (87 - 93%), K (293 - 240 mg/kg) and N-NH4 (1.05 - 1.75 mg/kg) strongly correlated with high Meloidogyne and Rotylenchulus abundance in 50 g roots, and that of bacterivores in soil. For the 2020 survey, high Meloidogyne abundance in 50 g roots and high fungivore abundance in soil correlated with localities containing soil with high Sand (84 - 94%) and pH values (5.25 - 6.6). Morphological and molecular species identification for Meloidogyne evidenced M. incognita as the predominant species, followed by M. javanica, M. arenaria and M. enterolobii. These results revealed information that is novel and crucial for proper planning of grain crop production in fields where mostly communities containing mixed Meloidogyne spp. occur. The population densities of Meloidogyne and

Rotylenchulus were exceptionally high in root samples, while the latter genus had high densities in soil samples too. This suggests that practicing monoculture as well as rotation of maize and sunflower are favorable to high population density build-ups of these two genera. Despite the known adverse effect of Meloidogyne on maize and sunflower, the impact of Rotylenchulus on both crops is unknown. Findings of this study will assist nematologists and producers to make informed decisions to combat nematode problems in maize-based cropping systems.