Abstract

All wild types and mutant strains of Trichoderma harzianum gave high efficacy of mycelial growth inhibition of Phytophthora parasitica in dual culture tests. Comparison between wild types and mutant strains showed that all mutant strains provided higher efficacy of inhibition compared with the wild type strains. Strain T-35-co4 gave the highest inhibition rate (81.5%). Under plant growth cabinet conditions, all treatments, either individuals or combinations of strains of T. harzianum (except an individual strain T-152-wt), significantly decreased severity of tangerine seedling disease in comparison with the control. Three treatments of combination strains (T-35-co435-co5, T-35-co4152-wt and T-152-wtCB-Pin-01) were able to decrease disease severity more than their individual strains, especially treatment T-35-co4152-wt, which showed only 17.5% of disease severity compared with the control (87.5%). In glasshouse conditions, all treatments, either individuals or combinations of strains, significantly decreased the severity of tangerine seedling disease. Two combinations of strains, T-35-co435-co5 and T-35-co4152-wt, gave the lowest percentage of disease severity at 28.0% and 29.5%, respectively. All treatments with T. harzianum showed a high percentage, 60-100%, of root colonization of tangerine seedlings; all were recovered from their rhizosphere soils. Moreover, they clearly decreased populations of P. parasitica in comparison with the control. In particular, in a treatment with combination strains of T-35-co4 and T-35-co5, the recovered population of P. parasitica was 1.9×10 cfu g ^-1 rhizosphere soil, the lowest amount in this study, while that of the control was 8.4×10 ² cfu g ^-1.