Abstract
This study was conducted to investigate the antagonistic activity of endophytic and rhizosphere fungi isolated from a medicinal plant, Sesuvium portulacastrum, against Pythium aphanidermatum, the cause of damping off of cucumber. A total of 40 endophytic and 19 rhizosphere fungi were isolated from S. portulacastrum. Three endophytic isolates and two rhizosphere isolates gave >50% suppression of P. aphanidermatum in the in vitro dual-culture tests. Scanning electron microscopic studies at the inhibition zone showed hyphae wall damage and abnormal mycelial growth of the genus Pythium. Molecular analysis identified the antagonistic endophytes as Aspergillus insulicola (isolate A435), A. insulicola (A419), and Aspergillus melleus (A412) and the rhizosphere antagonists as Aspergillus terreus (A213) and Aspergillus luchuensis (A116). Except for A116, the culture filtrates of the other antagonists significantly increased the electrolyte leakage from Pythium mycelia, whereas ethyl acetate extracts of A435, A412, and A213 showed significant growth suppression. All five antagonists were able to produce varying amounts of cellulase and β-glucanase enzymes. However, A435, A412, and A213 showed significantly higher cellulase activity, whereas A435 and A116 showed the highest β-glucanase activity. Controlled glasshouse growth experiments showed that isolates A435 and A116 resulted in up to 70% control of damping off, whereas isolates A412 and A213 showed 30 to 40% damping-off control. The antagonists A435, A116, and A213 also contributed to increased cucumber shoot length as well as shoot and root dry mass. The synergetic effects of metabolites and hydrolytic enzymes could be the reason for the variation between isolates in the antagonistic activity and cucumber growth promotion. This study reports for the first time A. insulicola, A. melleus, and A. luchuensis as potential biocontrol agents against P. aphanidermatum-induced damping off of cucumber.
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