The abundance, identity and population dynamics of Meloidogyne spp. associated with maize in South Africa

Abstract

Maize, a staple food in sub-Saharan countries, is also an important livestock feed source in many parts of the world. Plant-parasitic nematodes cause estimated yield losses of 12 % upwards in local maize fields. Although Pratylenchus spp. were initially regarded as the economically most important plant-parasitic nematodes of maize. However, improving the efficacy of a NaOCl-method for extraction of Meloidogyne spp. from maize roots during 1995 proved otherwise. Meloidogyne incognita and M. javanica were from then on listed as the predominant species that adversely affected local maize production, followed by Pratylenchus zeae. The objectives of this study were to i) conduct a follow-up audit of nematode pests of maize in South Africa and ii) determine the nature of relationships between initial (Pi) and final population densities (Pf) of M. incognita and M. javanica, respectively, in microplot experiments as well as the effect of a seed treatment (active substance: abamectin) on Pi levels of a mixed M. incognita and M. javanica (70:30 ratio) population. Root and soil samples were obtained from 78 commercial maize fields (irrigation and rain-fed) in local maize-production areas during the 2014/15 and 2015/16 growing seasons. Plant-parasitic nematodes were extracted from the samples using standard protocols, counted and identified. Molecular species identification was done, for Meloidogyne only, using the sequence-characterised amplified region (SCAR) - polymerase chain reaction (PCR) and NaDH5-gene sequencing. Meloidogyne incognita, followed by Meloidogyne javanica, Meloidogyne arenaria and Meloidogyne enterolobii were the predominant root-knot nematodes identified. Meloidogyne enterolobii is a first report for local maize, listed as a non- or poor host crop. Crops locally used in rotation with maize are, however, highly susceptible to this species and will allow high build-ups that will be difficult to manage. Use of the NADH5 technique was not able to discriminate among the four species, as was obtained by SCAR-PCR, but grouped them in one clade with numerous thermophilic Meloidogyne spp. (sequences selected from NCBI Genbank). The NADH5 could also not indicate the presence of mixed species (31 % of the populations identified) as was obtained with SCAR-PCR. Substantial variation existed among the Pf levels of M. incognita and M. javanica for different Pi levels used. The Pf of M. incognita at the highest Pf level (10 000 eggs and J2 / root system) was substantially higher compared to that of M. javanica. A sharp decline was recorded for the production potential of M. javanica compared to that of M. incognita at Pi levels 7 500 and 10 000. Furthermore, substantial reductions of 2.2 (micro plot study) and 2.5 times (glasshouse study) were recorded in Meloidogyne spp. Pf densities for the abamectin compared to control treatment. Lower Pfs illustrated the benefits this environmental sustainable and cost-effective treatment may offer to local producers in reducing Meloidogyne spp. numbers. Identification of a more aggressive, threat species, M. enterolobii for which maize has been known to be a poor host, was found in a maize field in Mpumalanga. This study generated new information that will be useful for producers, related crop as well as seed and chemical industries. It accentuates the need to re-assess nematode assemblages in crop fields and exploitation of alternative and sustainable management tools to combat nematode pests