Nematode assemblages in grain crop fields in South Africa

Abstract

Grain crop yields and quality are adversely affected by infection and parasitism of plant-parasitic nematodes; especially root-knot (Meloidogyne) and lesion nematodes (Pratylenchus) that dominate in local production areas. Local grain producers experience poor crop performance and yield decline despite optimal management practices being implemented, e.g., sufficient fertilizer application, proper disease and pest control, and others. Hence they requested this study of which the aim was to determine the abundance and diversity of plant-parasitic and beneficial nematodes in 18 fields identified. Root and rhizosphere samples of maize, soybean, sunflower, and wheat were obtained from the 18 fields during two subsequent summer growing seasons 2020 for Survey 1 and 2021 for Survey 2) were grown on fields investigated. Motile life stages and eggs and second-stage juveniles (J2) of the sessile endoparasitic genera Meloidogyne and Rotylenchulus were extracted from 5 g and 50 g roots, respectively, using standardized procedures. Similarly, specialised and accredited techniques were used to extract motile individuals of both plant-parasitic and beneficial nematodes from soil samples. Prominence values (PV) were calculated for all nematode genera and/or families, while univariate analyses (Repeated Measures ANOVA) were done to elucidate the population dynamics of predominant plant-parasitic nematode genera/families. Multivariate data analyses (Principal Component Analyses) examined whether associations existed between nematodes and selected abiotic factors. In 50 g roots Meloidogyne dominated, followed by Rotylenchulus (PV of 8915 and 978 eggs and J2, respectively). Morphological and molecular species identification for Meloidogyne evidenced M. incognita as the predominant species, followed by M. javanica and M. arenaria; M. enterolobii were not identified. These results are crucial for proper planning of grain crop production in fields where mostly mixed communities of the genus occur. Meloidogyne and Rotylenchulus also dominated in 5 g roots, followed by migratory lesion nematodes. In soil, Pratylenchus was predominant followed by the ectoparasitic families Dolichodoridae and Criconematidae. This shift in predominant plant-parasitic nematode genera occurring in soils of grain fields investigated in this study is interesting and novel; it accentuates producers to implement proactive measures to mitigate damage that can be caused by the predominant ectoparasites. Densities of predominant plant-parasitic nematodes in grain crop rhizospheres differed significantly for the two seasons, with multivariate analyses not portraying that any of the selected abiotic parameters included may have contributed to this phenomenon. iii Exceptionally low levels of beneficial nematodes were present in soils from 17 of the 18 fields sampled, with bacterivores, fungivores, omnivores and predators in descending order of domination. Using nematodes as bioindicators of soil health hence suggests the poor level of soil health in the production areas surveyed in this study. Conclusively, results suggest that practicing monoculture as well as including highly susceptible summer crops such as soybean and sunflower in maize-based rotations are favourable to high population density build-ups of the predominant endo- and ectoparasitic nematodes identified. The findings of this study will assist nematologists and producers in making informed decisions to combat nematode problems in maize-based cropping systems.