Soybean host status to Meloidogyne incognita and nematode biodiversity in local soybean cropping systems

Abstract

Root-knot nematodes (Meloidogyne spp.) are the major nematode pests of local soybean crops, resulting in up to 100 % yield losses. The host response of locally adapted soybean genotypes to the predominant Meloidogyne incognita was determined, while nematode (plant-parasitic and non-parasitic) abundance, diversity and occurrence in local soybean cropping systems and especially in glyphosate-treated versus non-treated (conventional soybean and natural vegetation) soils were also assessed. The host status of 36 soybean genotypes was determined for M. incognita in glasshouse experiments under two temperature regimes. Substantial variation existed amongst the cultivars for all nematode parameters. Only line PRF-GCI7 and the resistant standard cultivar LS 5995 had reproduction factors (Rfs) < 1, while DM 6.2i RR had an Rf = 1 and percentage resistance (%R) ≤ 2 %, indicating resistance. These genotypes retained their resistance even at the higher temperature regime. From the surveys conducted, M. incognita, Meloidogyne javanica, Pratylenchus brachyurus and Pratylenchus zeae were the predominant endoparasites. Ectoparasitic nematode genera that dominated in soil samples were Helicotylenchus, Scutellonema, Criconema, Criconemoides, Tylenchorhynchus and Nanidorus. Seven species, viz. Pratylenchus flakkensis, Pratylenchus scribneri, Pratylenchus vulnus, Rotylenchulus brevicaudatus, Telotylenchus avaricus, Tylenchorhynchus brevicaudatus and Quinisulcius capitatus are first reports for soybean in South Africa. The highest plant-parasitic nematode diversity was associated with conventional soybean cultivars followed by natural vegetation and the glyphosate-tolerant soybean cultivars. A total of 32 non-parasitic nematode genera were also listed for the three ecosystems, viz. 21 for glyphosate-tolerant soybean, 23 for conventional soybean and 28 for natural vegetation. Bacterivore genera (Acrobeles, Acrobeloides, Eucephalobus and Panagrolaimus) generally dominated in soils of all three ecosystems, followed by fungivore genera (Aphelenchus and Aphelenchoides). Low abundance, diversity and occurrence were recorded for predators and omnivores. No correlations were apparent for non-parasitic nematode genera and the three ecosystems. According to soil food web analyses, soils from all three ecosystems were disturbed and degraded. A field experiment was also conducted to determine the response of nematode communities to glyphosate. This was done over two consecutive growing seasons for a soybean/maize cropping cycle. The abundance of six plant-parasitic nematode genera, Criconema, Helicotylenchus, Meloidogyne, Nanidorus, Pratylenchus and Tylenchorhynchus did not differ between glyphosate-treated and non-treated plots. Of the 14 non-parasitic nematode genera identified (Acrobeles, Acrobeloides, Aphelenchoides, Aphelenchus, Aporcelaimellus, Cephalobus, Discolaimium, Ditylenchus, Eucephalobus, Teratocephalus, Leptonchus, Panagrolaimus, Tylencholaimus and Tylenchus), only a few differed significantly in abundance between the glyphosate-treated and non-treated plots. Faunal analyses showed that soils from glyphosate-treated plots were degraded, less enriched and fungal-mediated. Conversely, soils from non-treated plots were disturbed and enriched, and bacterial-mediated. This study re-emphasised the challenges posed by plant-parasitic nematodes, in particular Meloidogyne and Pratylenchus, to local conventional and genetically-modified soybean crops. It also gave an insight regarding the importance of non-parasitic nematodes as bio-indicators of soil quality in soybean cropping agro-ecosystems. Ultimately, it showed that nematode assemblages generally did not differ among glyphosate-treated and non-treated plots. More research over longer study periods should, however, be conducted to determine whether glyphosate has an effect on both plant-parasitic and non-parasitic nematodes that prevail in local soybean-based cropping systems.