การใช้ Bacillus spp. ร่วมกับแมกนีเซียม เพื่อควบคุมโรครากขาว (Rigidoporus microporus) ของยางพารา

Abstract

Currently, exchangeable magnesium status in rubber (Hevea brasiliensis) planting area in the southern of Thailand is mostly below the optimum range at 0.3 cmolc kg-1. Insufficient magnesium affects the growth of rubber trees and makes them sensitive to white root rot disease caused by the fungus Rigidoporus microporus. However, using Bacillus spp. as a bio-control agent was the effective method. This research aimed to study the effect of magnesium with bacteria B. subtilis strain SM1 on R. microporus strain NK6 inhibition. The experimental result showed that B. subtilis strain SM1 can grow on Potato Dextrose Agar (PDA) supplemented with magnesium in the form of Kieserite added in 4 levels at 0, 0.10, 0.30, and 0.50 cmolcL-1. It was noticedthat B. subtilis strain SM1 was able to grow in PDA medium with magnesium added in every level while R. microporus strain NK6 was also able to grow in PDA medium with magnesium added. However, R. microporus fungus in PDA medium with magnesium added at 0.3 and 0.5 cmolc L-1, it caused abnormal morphological characteristics of the hyphae compared to the control without magnesium supplementation. Testing three strains of antagonistic bacteria, including B. subtilis strains SM1 and B. subtilis strain LPDD3-2, as well as B. amyloliquefaciens strain PT7, in conjunction with the supplementation of magnesium in the growth medium at concentrations ranging from 0.1 to 0.5 cmolc L-1, using the dual culture method. The experimental results showed that B. subtilis strain SM1, when grown on PDA medium with magnesium added at 0.5 cmolc L-1 was able to inhibit the growth of R. microporus strain NK6 by 75.29%, which was higher compared to other strains of beneficial bacteria. The volatile extracellular metabolite test revealed that B. subtilis strain SM1 with magnesium added at 0.50 cmolc L-1, was able to produce volatile compounds that inhibit the growth of R. microporus strain NK6 by 68.90%. This inhibition was higher than that observed at different concentrations of magnesium. Moreover, B. subtilis strain SM1 also forms heat-tolerant and non-heat-tolerant antagonistic substances that inhibited the growth of R. microporus strain NK6 by more than 50% when combined with magnesium at a concentration of 0.5 cmolc L-1 using the agar well diffusion method and the poisoned food cell method. Moreover, the non-sterile and sterile antagonist’s substance of B. subtilis strain SM1, was able to inhibit the growth of R. microporus strain NK6 by more than 84.44%. As a result, the hyphae of R. microporus strain NK6 appeared abnormal, twisted, and had swollen hyphal tips. When tested in soil, the combination of B. subtilis strain SM1 with magnesium at 0.50 cmolcL-1 was found to effectively inhibit the growth of R. microporus strain NK6 as well. Antagonistic bacteria was able to grow at 2.24 cm from the soil’s height of 10 cm. However, B. subtilis strain SM1 with magnesium added in every level could not resist R. microporus strain NK6 when directly inoculated to the rubber limb branch. The fungus was able to grow and cover the rubber limb and was found to destroy the rubber rod cells in all treatments. The use of B. subtilis strain SM1 in combination with the application of magnesium at a concentration of 0.5 cmolc kg-1has shown an impact on the growth of tap roots and lateral roots of rubber tree at 14 and 28 days after planting. Especially, the treatment of B. subtilis strain SM1 with magnesium added at 0.5 cmolc kg-1, which also reduced the occurrence of white root rot disease caused by R. microporus strain NK6 fungus. The hydroponics experiment result showed that using bio-agent, B. subtilis strain SM1 with magnesium added at 60 mg L-1, has been found to reduce theoccurrence of white root rot disease caused by R. microporus strain NK6 fungus in the stem and roots by 67.67–70.33%. Furthermore, The use of B. subtilis strain SM1 in combination with the application of magnesium at a concentration of 60 mg L-1 have resulted in improved growth of rubber trees, including increased plant height, roots development and increased nutrient accumulation in rubber leaves such as nitrogen, phosphorus, and magnesium. Planting rubber trees in potting soil and application a fertilizer grade 15-15-15 along with using magnesium at a concentration of 0.5 cmolc kg-1 combined with the use of B. subtilis strain SM1 bacteria, can help reduce the occurrence of white root rot disease caused by R. microporus strain NK6 fungus in the above-ground and root parts by 44-95%. The application of 0.50 cmolc kg-1 of magnesium, combined with bacteria B. subtilis strain SM1 also increased growth of the height above the ground, roots development and nutrient accumulation in rubber trees. It’s evident that B. subtilis strain SM1 is an effective bacterium in controlling white root rot disease in rubber trees, both in laboratory and experimental greenhouse conditions. Therefore, B. subtilis strain SM1 should be studied and developed into a bio-agent for convenient application.