Evaluation Of Local Trichoderma Isolates For Their Efficiency In Biological Control Of Fusarium Oxysporum F. Sp Phaseoli In Common Bean

Abstract

In Kenya common beans are one of the most important food crops widely grown in the country. The production trend of beans has been on the decline over the years mostly as a result of infectious diseases from fungal pathogens. One such pathogen is Fusarium oxysporum f. sp. phaseoli (FOP) which causes Fusarium wilt of beans. In the country yield losses from FOP have been estimated to be between 10% and 100%. The major control method of Fusarium wilt has been with chemical fungicides. However this has been reported to be ineffective under conditions that are conducive for the disease development. This therefore calls for development of alternative methods to control the pathogen. The strategy of integrated disease management (lDM) that incorporates use or biocontrol agents which are safe, easy to adopt and environmentally friendly is one such system that can contribute greatly towards management of this disease. Antagonistic fungus Trichoderma is one of the biocontrol "agents that can be developed for this purpose. The overall objective of the study was to evaluate local Trichoderma isolates for their virulence potential against Fusarium oxysporum I. sp. phaseoli. The focus of this study was to isolate Trichoderma from soil samples obtained from lmbu Agricultural Training Center (ATC) farm, identify and to test them in vitro and in vivo for their ability in control of FOP. The molecular identification and characterization of the Trichoderma isolates was based on the ITS region. The ability of the isolates to antagonize FOP was evaluated in vitro using dual culture assays and through use of volatile and nonvolatile metabolites produced by the isolates. The two local isolates T. koningiopsis (TI8) and T vir/de (1'35) that showed the highest inhibitory effect against the pathogen at 28.56% and 27.42% respectively were used as seed treatment on bean variety rnwezi moja and tested against the pathogen in the greenhouse experiment. The inhibition tests effects were analyzed using the analysis of variance (ANOVA) and the significance of the means was evaluated by Turkey HSD and Duncan's multiple range tests for in vitro experiments and greenhouse experiment respectively. The local isolates of Trichoderma were identified as; Trichoderma hamatum ('1'6), Trichoderma koningiopsis (T 18), Trichoderma hamatum (T21), Trichoderma viride enS), Trichoderma spira!e ('1'41) and Trichoderma asperellutn (ECO-T). In dual culture experiments the commercial ECO-T and the five local Trichoderma isolates showed substantial inhibition of the growth of FOP. Use 'of non-volatile and volatile metabolites

produced by the Trichoderma isolates also confirmed that there was production of inhibitory substances by all the isolates against FOP. In the greenhouse studies seeds inoculated with Trichoderma reduced infection of bean plants with FOP and resulted in increased plant growth relatively compared with untreated control. All the plants from seeds that were treated with T. koningiopsis (1' 18), 1'. viride (1'35), ECO- T and the combination of 1'. koningiopsis (T18). T. viride (1'35) showed significant resistance against FOP in the greenhouse. T. viride (T35) achieved the highest reduction or post-emergence damping-off at 50%; T. koningiopsis ('1'18), ECO-T and the combination of 1'. koningiopsis ('1'18) + T. viride ('1'35) achieved reductions of 35%, 43.3% and 35% respectively. Combination of 1'. koningiopsis ('1'18). T. viride ('1'35) achieved the highest increase in dry weight of shoots (680%) and dry weight of roots (166.6%) which were significantly different (P::;0.05) compared to T. koningiopsis (T18), T. viride ('1'35) and ECO-T with increases in dry weight of shoots (230%, 210%, 80% respectively) and dry weight of roots (33.3%, 133.3%, 33.33% respectively). The local Trichoderma isolates T. koningiopsis ('1'18) and T. viride ('1'35) are effective and promising biocontrol agents against FOP in beans.