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Macrophomina phaseolina (Tassi) Goid remains the prevailing causal agent of charcoal rot disease that significantly suppresses the yield of a variety of oilseed crops. Its wide host range and ability to survive under arid conditions, coupled with the ineffective use of fungicides against it, have spurred scientific endeavours for alternative avenues to control this phytopathogen. Hence, the present study aimed to provide empirical evidence of the efficacy of three fungal isolates (T2, T10 and T12) of Trichoderma harzianum as biological control agents against charcoal rot in soybean (Glycine max L.). The results of the in vitro studies revealed that all three fungal isolates significantly inhibited the growth of M. phaseolina phytopathogen, with T12 showing considerably higher inhibition effect than T2 and T10 isolates. T12 inhibited the growth of M. phaseolina in the dual culture (72.31%) and volatile production (63.36%) assays, and the hyperparasitism test indicated cell lysis following the interactions with T12 mycelia. T12 isolate was mostly effective in field experiments, observable in the attained minimum plant disease indices both in the soil incorporation (11.98%) and seed inoculation (5.55%) treatments, in comparison to isolates T2 and T10. Moreover, the stem and root lengths, as well as the seed weight, were considerably increased, as compared to the control. Hence, the findings reported in the present study supported the applicability of T12 isolate as possible alternative to fungicides for the control of charcoal rot in soybean.

Charcoal rot, caused by Macrophomina phaseolina (Tassi) Goid., is an economically important disease of oilseed plants in Northern Iran. Seventy isolates of M. phaseolina were obtained from different hosts, including soybean ( Glycine max L.), and sunflower ( Helianthus annuus L.) in the northern oilseed planting regions of Iran. RAPD-PCR amplification profiles, by using of six random OPA primers (kit A) showed polymorphisms among the isolates. The primer OPA-13 (5'_ CAGCACCCAC_3') amplified the genomic DNA of all isolates of M. phaseolina producing 4-12 bands of sizes between 0.25 and 2.3 kb. Unweighted pair-group method with arithmetic mean (UPGMA) analysis classified the isolates into the nine major groups with 64% similarity. Pathogenicity of isolates was evaluated at seedling stage of soybean, sunflower, and maize ( Zea mays L.) plants under in vitro conditions. None of the isolates were pathogenic on corn, while all of the isolates infected soybean and sunflower seedlings. Isolates were more virulent on soybean than sunflower. The disease index on sunflower and soybean varied between 19-24 and 27-30 respectively. These results indicated a significant pathogenic and genetic variability within the Iranian isolates of M. phaseolina. Cultivation with crop rotation was probably tended to induce less diversity of the pathogen isolates.