Screening of antagonistic bacterial strains against Meloidogyne incognita using protease activity

Twenty-six bacterial strains that had demonstrated antagonism to some fungal and bacterial pathogens were evaluated for their ability to inhibit Meloidogyne incognita Kofoid & White. The inhibition rates of egg-hatching and second-stage juveniles (J2) mortality of M. incognita by these strains ranged from -16.5 to 87.4% and from 1.3 to 77.8%, respectively. The 12 strains causing J2 mortality over 40% were chosen for greenhouse experiments in which their biocontrol efficacy reached 33.3-65.6%. On the other hand, among the 26 strains, 20 demonstrated in vitro protease activity and 14 revealed chitinase activity. Significantly, strains Bacillus sp. AR156 and GJ24 in greenhouse tests showed the strongest protease activities. The analyses of the relationships of the efficacy of the 12 strains with their protease and chitinase activities, respectively, indicated that biocontrol efficacy was highly correlated with protease activity (r = 0.92, P<0.001) but barely correlated with chitinase activity. The strong positive correlation between protease activity and efficacy suggests that in vitro protease activity could be used as a parameter for selecting biological control agents (BCAs) against root-knot nematodes. Consistently, the biocontrol efficacy of AR156, GJ24, abamectin reached 74.3, 73.4, and 40.9% in the field in Huai-an, Jiangsu; and 71, 69.9, and 37% in Zao-zhuang, Shandong, respectively. The fact that the strains with high protease activities also had significantly higher biocontrol efficacy than abamectin in the field implies that in vitro protease activity may be adopted as a reliable new parameter for speeding up the process of screening the biological control agents (BCAs).