Summary. Sheath blight caused by Rhizoctonia solani is one of the most serious rice diseases worldwide.
The disease is currently managed only by the excessive application of chemical fungicides which are toxic
and not environmentally friendly. Therefore, greater emphasis should be given to biological control as
being both safe and effective. Trichoderma species are ubiquitous fungi in the soil and have an antagonistic
activity against several soil-borne plant pathogens including R. solani. The present study was undertaken
to evaluate the potential of indigenous Trichoderma strains from Mazandaran province, Northern Iran (a
Mediterranean region on the southern coast of the Caspian Sea) against R. solani AG1-IA in vitro, and
against sheath blight disease in the glasshouse, in order to find biocontrol isolates for application in the
field. More than 200 Trichoderma strains were isolated from the soil, plant debris and the phyllosphere in
rice fields. Strains were first screened for their antagonism to R. solani by in vitro antagonism tests inclu-
ding dual culture, antibiosis, the effect of Trichoderma strains on the production and viability of R. solani
sclerotia, and hyperparasitism on microscopic slides. According to the in vitro experiments, several strains
belonging to T. harzianum, T. virens and T. atroviride showed excellent biocontrol. These potential anta-
gonist strains were further evaluated for their effectiveness in controlling sheath blight under glasshouse
conditions. Among the 55 selected strains, seven significantly controlled the disease. T. harzianum AS12-2
was the most effective strain in controlling rice sheath blight, better even than propiconazole, the most
commonly used fungicide in Iran.