Bacterial antagonists of fungal pathogens also control root-knot nematodes by induced systemic resistance of tomato plants

Abstract

The potential of bacterial antagonists of fungal pathogens to control the root-knot nematode Meloidogyne incognita was investigated under greenhouse conditions. Treatment of tomato seeds with several strains significantly reduced the numbers of galls and egg masses compared with the untreated control. Best performed Bacillus subtilis isolates Sb4-23, Mc5-Re2, and Mc2-Re2, which were further studied for their mode of action with regard to direct effects by bacterial metabolites or repellents, and plant mediated effects. Drenching of soil with culture supernatants significantly reduced the number of egg masses produced by M. incognita on tomato by up to 62% compared to the control without culture supernatant. Repellence of juveniles by the antagonists was shown in a linked twin-pot set-up, where a majority of juveniles penetrated roots on the side without inoculated antagonists. All tested biocontrol strains induced systemic resistance against M. incognita in tomato, as revealed in a split-root system where the bacteria and the nematodes were inoculated at spatially separated roots of the same plant. This reduced the production of egg masses by up to 51%, while inoculation of bacteria and nematodes in the same pot had only a minor additive effect on suppression of M. incognita compared to induced systemic resistance alone. Therefore, the plant mediated effect was the major reason for antagonism rather than direct mechanisms. In conclusion, the bacteria known for their antagonistic potential against fungal pathogens also suppressed M. incognita. Such "multi-purpose" bacteria might provide new options for control strategies, especially with respect to nematode-fungus disease complexes that cause synergistic yield losses.

Figure 1. Effect of bacterial antagonists on repellence of M. incognita juveniles.

Juveniles were attracted by tomato roots and moved from a tube connecting two pots either to the side inoculated with an antagonistic strain or to the opposite side. Controls were inoculated on one side with the not antagonistic strain E. coli JM109, or left uninoculated. Juveniles penetrated into the roots were counted on both sides. Error bars represent standard deviations. Different letters indicate significant differences at P≤0.05 according to Tukey's test (n = 10).

Figure 2. M. incognita reproduction affected by bacterial antagonists through induced systemic resistance of tomato.

Juveniles and bacteria were inoculated in opposite pots of split root systems. Controls were inoculated with the not antagonistic strain E. coli JM109, or left uninoculated. A: Experimental setup of the split root system. B: Mean numbers of galls (white bars) and egg masses (gray bars) counted 50 days after nematode inoculation; error bars represent standard deviations, different letters indicate significant differences at P≤0.05 according to Tukey's test (n = 10).

Figure 3. Comparison of the effects by direct and by plant-mediated antagonism on root penetration by juveniles.

M. incognita juveniles (J2) and bacterial strains were inoculated spatially separated in opposite pots of one split-root system (white bars), or co-inoculated with J2 in one pot of another split root system (gray bars). Controls were inoculated with J2 and the not antagonistic strain E. coli JM109, or only with nematodes. J2 penetrated into tomato roots were counted 10 days after inoculation (A). Numbers of galls (B), egg masses (C), eggs per root (D), and eggs per egg mass (E) were determined 50 days after J2 inoculation. Error bars represent standard deviations. Different letters indicate significant differences at P≤0.05 according to Tukey's test (n = 10).