The effects of micronutrient-enriched media on the efficacy of Bacillus subtilis as biological control agent against Meloidogyne incognita

Abstract

The root-knot nematodes Meloidogyne incognita poses a serious threat to horticultural food and plantation crops. One of the effective and efficient measures to overcome this nematode is using bacteria as a biological control agent. This study aimed to test the effectiveness of Bacillus subtilis bacteria grown on media with additional micronutrients as a biological control agent for M. incognita. The test was carried out by growing bacteria in 100 mL of nutrient broth (NB) added with FeCl₂, MnCl₂, and CuCl₂. Each type of micronutrient was administered at concentrations of 35 ppm, 40 ppm, 45 ppm, 50 ppm, and 55 ppm. As a control, NB media was used without the addition of micronutrients. The test results showed that the best performances on bacterial growth were 55 ppm MnCl₂, 45 ppm MnCl₂, 40 ppm MnCl₂, 40 ppm CuCl₂, 50 ppm MnCl₂, and 35 ppm MnCl₂. These six treatments were then used in an antagonism test. A total of 150 J2 M. incognita in 4 mL of suspension was put into a petri dish with a diameter of 5 cm. After that, 1 mL of each bacterial suspension was put into a petri dish following the treatment. The mortality rate of M. incognita in each treatment was 77.5% in 55 ppm MnCl₂, 74.5% in 45 ppm MnCl₂, 77.25% in 40 ppm MnCl₂, 73.25% in 40 ppm CuCl₂, 78.75% in 50 ppm MnCl₂, 79% in 35 ppm MnCl2, and 53.25% in control. The same six treatments were also used to measure the chemotaxis index of M. incognita on tomato roots. Tomato seeds of the Tantyna variety were soaked for 60 minutes in a bacterial suspension grown on micronutrient-enriched media. After seven days, the tomato seed roots were removed and placed in 5 mL 20% Pluronic F-127 Gel which had been infested with 100 J2 M. incognita. After 6 hours, the number of nematodes approaching the roots was observed. The chemotaxis index in the treatments ranged from 16% to 24%, while that in the control was 33%. This study corroborates that the addition of micronutrients MnCl₂ and CuCl₂ to the growth medium of B. subtilis can increase antagonistic activity against M. incognita and suppress its chemotaxis response in tomato roots soaked in the bacterial suspension.