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Biological Control of Root Rot of Soybean in Thailand with Antagonistic Bacteria
Dissertation Abstract:
One hundred bacterial isolates from soil samples collected from the rhizosphere of soybean seedlings grown in Chiang Mai Province, Thailand, were tested for antagonism to seven root rot pathogens [(Sclerotium rolfsii Sacc., Pythium aphanidermatum, (Edson) Fitzp., Phytophthora megasperma f. sp. glycinea Kuan and Erwin., Rhizoctonia solani Kuhn, Fusarium solani (App. & Wollenw.) Snyd. & Hans and F. oxysporum (Schlecnt.) Snyd. & Hans]. Among the bacterial isolates, 48 proved antagonistic to one or more pathogens when grown on dual culture tests in the laboratory. This was based either on growth competition or antibiotic activity to the pathogen. Forty-two of the antagonistic isolates showed high potential as biological control agents against one or more pathogens during either blotter or tissue tests. These isolates were used further to control root rot disease of soybean in the greenhouse. Separately coating soybean seeds (SJ 4) with cell suspension of different bacterial isolates and sowing them in sterilized soil amended separately with each of seven pathogens resulted in the control of the disease which often caused rotting of seeds and roots. The bacterial antagonist-coated seeds also had improved stand, which resuI ted in greater fresh weight of plants and lower disease incidence.
When the seeds were coated with the two most effective bacterial isolates and sown in field plots amended separately with inoculum of the seven pathogens, improved plant stand and greater seed weight and yield resulted. Statistically, significant yield increase of up to 25 percent was noted in P. aphanidermatum-infested soil.
The promising bacterial isolates were tentatively classified to be under genus Pseudomonas, family Pseudomonas. These results suggest that once certain isolates of bacteria are established in the soil, soybean seedlings are partially protected from root rot pathogens in the field.