In this study, the effects of five isolates of Trichoderma harzianum fungal species on Lettuce plant growth rate under greenhouse conditions and soilless culture were evaluated in the CRD in four replications. To conduct the research , only the 10% concentration of the extract per total amount of irrigation (water + chemicals) of each pot was used. Wet and dry weights of shoot and root were used as growth indices in this experiment. The test results showed that the isolates had different effects on the lettuce plant growth indices, and indicated a significant statistical difference at 1% level using the LSD test. Among the isolates, the TBi isolate showed the strongest growth effect with the following features: 66.66% increase in fresh shoot weight, 45.21% increase in dry shoot weight, 87.81% increase in fresh root weight, and 51.12% increase in dry root weight. The possible use of isolated strains as plant growth promoting factors is discussed in this paper.

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