Effects of single and combined inoculations of selected Trichoderma and Bacillus isolates on growth of dry bean and biological control of Rhizoctonia solani damping-off
Six Trichoderma isolates, Trichoderma atroviride strain 3A, T. atroviride strain 6, Trichoderma harzianum, strain SY, Trichoderma pseudokoningii, an unidentified strain Trichoderma sp. strain 2F and T. harzianum strain kmd and three Bacillus subtilis isolates, B. subtilis B69, B. subtilis B77 and B. subtilis B81, were tested in vivo, singly and each in combination for growth promotion of dry beans and biological control of damping-off caused by Rhizoctonia solani in cucumber. All fungal and bacterial isolates were applied as seed treatments in greenhouse and rhizotron studies. Greenhouse trials showed that combined inoculations of T. atroviride strain 6 and B. subtilis B69 gave the highest growth promotion of bean in terms of seedling dry biomass (43.0% over uninoculated control). Rhizotron studies supported these findings, where it was shown that root biomass and root area were increased. However, results obtained for bean yield trials were inconsistent and had no correlation with the seedling trials (P = 0.87 and P = 0.35). No increase was obtained in protein or fat content of bean seed for any of the selected isolates and/or their combinations tested in two separate greenhouse yield trials. In the biological control trials, single inoculations of T. harzianum strain kmd, T. atroviride strain 3A and T. harzianum strain SY gave the highest percentage survival of cucumber plants in the greenhouse. None of the Trichoderma plus Bacillus combinations were better than the single inoculations of T. harzianum strain kmd, T. atroviride strain 3A and T. harzianum strain SY. The performances, particularly of B. subtilis B69 and B81 were enhanced when combined with T. atroviride strain 3A, T. atroviride strain 6, T. harzianum strain SY or T. harzianum strain kmd. The performance of each of the Trichoderma and Bacillus combinations was better than the Bacillus isolates used alone. This study showed that there was potential in using mixtures of Trichoderma and Bacillus for improving plant growth and disease control.
Key words: Bacillus, dry bean, plant growth promotion, Trichoderma.