Selection and application of antagonistic microorganisms to control root and stem rot of durian caused by Phytophthora palmivora (Butl.) Butl.

Antagonistic microorganisms isolated from rhizosphere soil and roots of durian in Chanthaburi province could inhibit Phytophthora palmivora, a causal agent of root and stem rot of durian on potato dextrose agar medium. These antagonists comprised with 132 fungal and 68 bacterial isolates. Trichoderma spp. (7 isolates) and Penicillium sp. (1 isolate) were selected and prepared as powder formulation, by using diatomite powder as carrier. Efficacy of powder formulation of each fungal isolate for the control of P. palmivora was evaluated under laboratory condition. Fungal mixture was prepared by amending antagonist powder with diatomite, rice bran and organic compost at the ratio 1:8:5:16 by weight. Each fungal mixture was used solely or combined with low dosages (0.25 fold of normal rate) of fungicide, 1.25 g/l metalaxyl or 0.63 g/l fosetyl Al for the control of P. palmivora. All isolates of Trichoderma spp. and Penicillium sp. in the fungal mixture, when incorporated with Phytophthora-infested soil, significantly reduced population of P. palmivora. Efficacy of fungal mixtures were superior to metalaxyl or fosetyl Al. Integration of metalaxyl or fosetyl Al (low dosage) with a fungal mixture could not enhance the reduction of P. palmivora population as compared to the solely use of a fungal mixture. However, combination of a fungal mixture with metalaxyl provided higher efficacy than with fosetyl Al. Under lathhouse condition, application of a fungal mixture in soil for 4-5 days prior to inoculation of P. palmivora around the root zone of durian significantly reduced disease incidence as compared to a non-treated control. T.harzianum isolates CHAN-03-13 and M4 were the two most promising isolates that provided complete protection of all test plants. Integration of a fungal mixture isolate M4 with metelaxyl reduced disease incidence better than the combining use of fosetyl Al or the solely use of each fungicide. Efficacy of antagonistic bacteria, prepared as a mixture of bacterial powder, talcum, rice bran and organic compost at the ratio 1:8:5:16 by weight for the control of P.palmivora was tested in laboratory. Only one isolate of gram-negative bacterium could reduce population of P. palmivora in soil. However, all bacterial mixtures failed to control root and stem rot of durian seedling under lathhouse condition.