Evaluating the Effects of Soil-Borne Streptomyces spp. on Tomato Growth Indices under Biotic Stress Condition Caused by Phytophthora nicotianae

Phytophthora nicotianae is a destructive plant pathogen with a worldwide distribution that causes root, fruit, leaf, and crown rot. Phytophthora species are the main reason for various diseases in a wide range of plant species including vegetables. Symptoms include fruit, stem, crown, and root rot. They lead to plant host death and in most cases, the economic losses caused by P. nicotianae are very high due to the diversity of the host range and its damage, therefore, controlling this pathogen is a constant challenge.

In order to biologically control the disease, three Streptomyces isolates including M6, M8, and M215 were selected from a large number of soil-inhabitant actinomycetes isolated from Kerman province, which showed effective anti-Oomycete activity against P. nicotianae. To better understand the inhibitory mechanism of these isolates, in vitro dual-culture assay, plant growth promotion, and pathogen control effects were investigated through the application of antagonists under laboratory and greenhouse conditions. Molecular identification of the most promising isolate was also achieved.

The antagonistic activity of three isolates including M8, M6, and M215 against P. nicotianae in tomato plants (super chef variety) was evaluated. Among all, based on the statistical analysis under both laboratory and glasshouse conditions, the M8 isolate had an inhibitory effect on the crown and root rot of tomato. The comparison of the effects of antagonists on the tomato plant yield indices revealed that M8 isolate had the greatest effect on increasing the growth indices of the tomato plant (super chef cultivar) including plant height, the number of flowers, shoot, and root dry weight. The M8 isolate was identified by the sequence analysis of a small ribosomal RNA subunit (16S rRNA). Based on the results of the present study, Streptomyces venetus CMU-AB225T strain had the highest similarity (96.08%) with the M8 isolate.

The results showed that M8 isolate can be considered as a beneficial microorganism to produce some useful biofertilizers and bioactive materials to control tomato crown and root rot biologically. Some biocontrol agents such as T. harzianum and T. asperellum within composts can be effective in reducing the symptoms of Ph. nicotianae in pepper seedlings. The production of any product based on the S. venetus (M8) requires fulfilling further research steps such as the interaction of the bacterium with other beneficial soil microorganisms and its impact on the population of beneficial soil microbes, evaluation of the antimicrobial activity of M8 against other plant pathogenic fungi, fungal-like microorganisms and bacteria. Moreover, biosafety considerations and measures have to be investigated.