On the diversity of nematode antagonists in an agricultural soil, and their steerability by root-knot nematode density and cover crops

Plant-parasitic nematodes are harmful pathogens for many agricultural crops. Within this category, root-knot nematodes (RKN, Meloidogyne spp.) are worldwide regarded as the most impactful because of their wide geographical distribution and their polyphagous nature. Host plant resistances against RKN have been successfully introduced in a few crops only. As the use of nematicides is becoming increasingly restricted because of environmental and human health concerns, there is a need for alternative strategies to control RKN. One such approach is the stimulation of local nematode antagonists. We investigated this in an experimental field setting with two main variables: density of the Columbia root-knot nematode Meloidogyne chitwoodi, and the type of cover of crop. For each of the three M. chitwoodi densities, the effects of ten cover crop treatments were tested on both the resident (DNA) and the active (RNA) fractions of the bacterial and fungal communities. In our analyses, we focussed on changes in the abundance of plant-parasitic nematode antagonists. From the eight bacterial and 26 fungal genera known from global literature to harbour potential antagonists of plant-parasitic nematodes, we detected respectively five and 14 genera in our agricultural field. Among the bacterial genera, four included bacterial species for which nematode antagonism has been documented. The fungal genera included facultative nematode parasites (e.g., Arthrobotrys spp.), endophytes strengthening host defences (e.g., Acremonium spp.), as well as multiple obligatory nematophagous species. This study revealed that conventionally managed arable fields may harbour an unexpectedly high diversity of nematode antagonists. Multiple antagonists were stimulated by cover crops in a cover crop-specific manner, and, to a lesser extent, by increased RKN densities. The richness in putative nematode antagonists did not translate into M. chitwoodi suppression probably because most antagonists have a facultative nematophagous lifestyle and will only predate nematodes under poorer nutritional conditions.