Synergism between urea and urease-positive bacteria in controlling root-knot nematodes

Abstract

Synergistic effects between urea and urease-positive bacteria against root-knot nematodes were assayed in laboratory and greenhouse trials. We isolated 85 urease-positive bacterial strains from soil, compost, sludge, and animal faeces; they belonged to four clades and sixteen genera of Eubacteria based on 16S rRNA BLAST. Twenty-five representative strains were assessed for nematicidal and urease activities. The urease-positive bacteria were nematicidal. A cubic relationship with a critical threshold was found between a strain’s nematicidal and urease activities. Sixteen strains with urease activity above this threshold had a 100 % nematicidal activity level. These 16 strains and urea alone were assessed for urease activity in pot experiments with tomato, which found that high-dose urea was nematicidal to root-knot nematodes (400–800 mg per kg soil) and phytotoxic (1,600 mg), while low-dose urea (≤200 mg) was non-nematicidal. However, nematodes could be controlled at 200 mg urea per kg soil in combination with urease-positive bacteria from seven different strains, and the nematode control efficacy of urea was always non-linearly positively correlated with soil urease activity. To test bacterial persistence, soils were recycled for another experiment, in which nine strains were significantly nematicidal, with a stronger relationship between nematode control efficacy and soil urease activity. These results suggest ways to manipulate soil bacteria to improve soil urease activity for the management of plant-parasitic nematodes. While urease-positive bacteria may have practical applications in controlling nematodes, their use will require careful field investigation and appropriate environmental safety evaluation.