Characterisation of a biological soil culture and its effects on the biology of root-knot nematodes

Abstract

Root-knot nematodes (Meloidogyne spp.) are the number one economically important and damaging nematode pest of agri- and horticultural crops. Withdrawal of synthetic nematicides from markets necessitated the exploitation of biological products with anti-nematodal characteristics. Therefore, crude extracts of a novel, soil-derived biological product (SoilBioMuti; SBM) that contain an array of micro-organisms was studied during this project in terms of i) its bacterial composition and ii) effects on the biology of the predominant root-knot nematode species Meloidogyne incognita infecting local maize crops. Identification of the bacterial genera contained in SoilBioMuti was done using plating as well as molecular (Next Generation Sequencing; NGS) techniques. Bacterial counts on two agar media (nutrient and MRS) were significantly higher in the fresh SBM compared to those in the ‘cooled’ SBM (exposed to 5 ˚C for 24 h before plating). The freshly-prepared SBM product contained approximately 99 % more bacteria than did the ‘cooled’ sample. According to NGS results, 45 bacterial genera were identified from the two freshly prepared stock SBM samples. Non-pathogenic genera consituted 49 % and beneficial bacterial approximately 50 % of the bacterial community of SBM and represented several genera that are known for their anti-nematodal effects. Novel knowledge on the adverse effect of 2.5 % SBM treatments has also been generated regarding the oxygen consumption of infective M. incognita second-stage juveniles (J2). According to data obtained, 100 J2 were optimal to use for determining the oxygen consumption rates (OCR). The OCR recorded for non-filtered (NF) SBM containing J2 were 42% compared to its counterpart treatment without J2. However, the OCR of J2 suspended in sterile tap water did not differ significantly from that of the NF SBM containing J2. The two filtered (F) SBM treatments had the lowest OCR and differed significantly from that of the NF SBM treatments and the tap-water control containing J2. In vitro evaluations on the effect of different SBM product concentrations (2.5, 3, 4 and 5 %) on J2 motility and in vivo testing of the reproduction abilities of M. incognita also produced novel information. For motility assays, all SBM concentrations (F and NF) significantly reduced J2 motility (below 10 %) throughout the 24 h experimental period. For reproduction assays, greenhouse data showed that all NF SBM product concentrations reduced the egg-laying female indices on roots of a susceptible tomato cultivar (Floradade). Although results from a first tunnel experiment yielded no conclusive data for 2.5 % SBM, a follow-up experiment revealed 83 and 77 % reductions in nematode reproduction in maize roots that grew in soils treated with WonderTM + hay and 5 % SBM + hay, respectively. The SBM treatments, however, had no significant effect on various plant-growth variables measured for both tunnel experiments. Although it can be concluded that the bacterial content of SBM has been determined to a certain extent and its adverse effects on the biology of M. incognita has been proven, knowledge on the specific bacteria (and/or other micro-organisms) that may act as the active substances is still elusive