A collection of 105 strains of actinomycetes isolated from the soils of Riyadh, Saudi Arabia were evaluated for their ability to inhibit plant pathogenic Fusarium oxysporum, Alternaria alternata, Aspergillus niger, and Aspergillus flavus in vitro. About 61.90% isolates were able to inhibit the growth of A. alternata, followed by 61.8, 52.24 and 50.9% inhibiting A. flavus, F. oxysporum and A. niger respectively. Subsequently, these isolates were tested for their ability to control Alternaria blight on tomato plant (Lycopersicon lycopersicum) grown in sterilized peat moss soil. Broth and crude extract of all except, DS-8(32), DS-6(48) and DS-6(33) isolates of actinomycetes have shown effect on plant disease severity. Over all analysis of data showed that strain AS-2(29) was the most effective isolate.Broth and crude extract of this strain had shown maximum zone of inhibition against F. oxysporum (11 and 18 mm respectively) and A. alternata (15 and 17 mm respectively). Plants treated with this strain were healthy and green and only a few older leaves showed the blight symptom and growth was found at par with control plants. However, strain DS-6(33) has not shown any antifungal activity and plants succumbed to infection of A. alternata, whereas, broth of DS-8(24) and DS-1(23) showed strong antifungal activity against A. alternata (20 and 19 mm zone of inhibition). However, crude extract of these strains has not shown any antifungal activity and plants treated with these strains were healthy and only older leaves were blighted. Extract but not broth of DS-1(20) showed strong antifungal activity against F. oxysporum and A. alternata (15 and 16 mm respectively) and the growth of the plant was similar to that of the control plants in pot trial strains varied in their antifungal activity against one or both plant pathogenic fungi. Based on the cultural, morphological and physiological characteristics, most of the strains were identified as the different isolates of the genus Streptomyces. In the further study, Streptomyces species as biological control agents would be offered a much needed alternative to the use of synthetic agrochemicals.
Key words: Actinomycetes, screening antifungal activity, A. alternata, green house pot trial, scanning electron microscope, tomato blight disease.
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