The Use of Trichoderma hamatum and Fluorescent Pseudomonad BJ10-86 in Controlling Phytophthora capsici on Chili (Capsicum

Abstract

Phytophthora capsici is an important plant infecting oomycete with a worldwide/cosmopolitan distribution. The pathogen has a wide host range, especially capsicums and cucurbits, and attacks nearly all plant parts and at all growth stages. Chemical control is proofing no longer effective due to reported pathogen resistance development, not to mention the related environmental concerns and the consumer demands for low, if not zero, use of fungicides. Biological control is a sustainable option. Several biocontol agents have been reported effective against this pathogen. In this research, the biocontrol activities of Trichoderma hamatum and fluorescent pseudomonad BJ10-86 against P. capsici were studied in order to propose an optimization of their application. The objectives of the biological control application were to improve chili pepper seed germination, reduce damping off disease incidence, and improve the growth of the seedlings. The studies were done in vitro as well as seed treatment assays and polybag application experiments. Fluorescent pseudomonad isolate BJ10-86 was subjected to molecular identification by amplifying the 16S rRNA region and sequencing the nucleotides before searching for similar sequences in the BLAST search engine. The isolate was identified as Pseudomonas aeruginosa and deposited in the GenBank under the accession number KM102771 as P. aeruginosa BJ10-86. The in vitro experiments involved plating the micro-organisms in dual and triple culture assays on agar media. The results showed that both biocontrol agents were compatible and increased the inhibition of P. capsici. Chili pepper seeds treated with the biocontrol agents, individually or in combination, were sown in commercial nursery media that had been pre-inoculated with P. capsici zoospores. Results of data collected over a period of 35 days indicated that seed treatment significantly (P=0.008) reduced damping off disease incidence. Combined application of T. hamatum and fluorescent pseudomonad BJ10-86 was the best biocontrol treatment with an area under disease progress curve (AUDPC) of only 36.61 units compared to 92.87 units in the control treatment. This combination was seen to increase biocontrol efficacy to 60.58%. Both organisms evidently had a synergistic interaction. Seed treatment had no effect on seed germination and seedling growth. In further experiments, the objective was to utilize synergism between T. hamatum and fluorescent pseudomonad BJ10-86 in order to reduce root and collar rot caused by P. capsici and promote chili pepper plant growth under field conditions. The treatments involved individual and combined polybag application of the biocontrol agents as conidial and bacterial cell suspension as soil drench. These were either administered 14 days before transplanting or during the transplanting day. The results indicated that fluorescent pseudomonad BJ10-86 increased the chili pepper plant height and number of leaves. Combined T. hamatum and fluorescent pseudomonad BJ10-86 had synergistic effects against root and collar rot disease incidence through delaying the disease onset. For better effects, these combined application of biocontrol agents should be at least 14 days before transplanting. Based on these findings, it is proposed that chili pepper seeds should be treated with a combination of both organisms. These organisms are effective at concentrations of 2×108 CFU ml-1 and 9.4×105 spores ml-1 for P. aeruginosa BJ10-86 and T. hamatum respectively. It is also important to administer both biocontrol agents as field treatment by drenching the planting holes several days prior to transplanting of the seedlings. This is meant to suppress root and collar rot disease by reducing the pathogen inoculum in soil and delaying the disease onset.