Biocontrol of Agroathelia rolfsii associated with stem rot disease in tomato (Solanum lycopersicum L.) and growth promotion using compost-associated actinobacteria.

Stem rot disease (Agroathelia rolfsii) biocontrol is an environmentally safe alternative that could potentially decrease disease severity and limit plant and yield losses. In the present investigation, 11 actinomycetes isolates, recovered from disease-suppressive composts, were tested as whole cell suspensions and cell-free culture filtrates for their ability to suppress tomato stem rot and to stimulate plant growth. Five isolates (namely A5-3, A2-4, A3-4, A4-4 and A5-4), applied as cell suspensions, were found to be the most effective in suppressing disease severity by 37.5-56.2% compared to the untreated control and 25-56.2% using their cell-free culture filtrates. The in vitro antifungal activity of isolates tested and their filtrates were estimated at 58.8-88% and 59-91.3% decrease in fungus mycelial growth, respectively. As for their growth-promoting ability, tomato plants treated with A5-3, A2-4, A3-4, A4-4 and A5-4 isolates were 20-89.1% and 10.3-79% higher than A. rolfsii-inoculated and pathogen-free controls, respectively. Inoculated and uninoculated plants treated with filtrates showed significant increments in their growth parameters by 18.2-91.9% and 15.3-93.4% over control, respectively. The most bioactive isolates against target pathogen were affiliated, based on 16 S rDNA gene sequencing, to Streptomyces, Saccharomonospora and Micromonospora genera. All these isolates were shown able to produce indole-3-acetic acid. Streptomyces sp. (A5-3) and Streptomyces sp. (A5-4) displayed chitinase, protease and lipase activities together with phosphate solubilization and nitrogen-fixing abilities. Streptomyces sp. (A5-3) displayed the greatest amylolytic activity and ability to solubilize zinc and to produce siderophores and hydrogen cyanide. This investigation demonstrated that actinomycetes recovered from disease-suppressive composts can be explored as potential sources of bio-active compounds with antifungal and bio-fertilizing abilities useful for the improvement of tomato growth and health.