Evaluation of streptomycete actinobacterial isolates as biocontrol agents against royal poinciana stem canker disease caused by the fungal pathogen Neoscytalidium dimidiatum

In this study, 47 actinobacterial isolates were obtained from rhizosphere soils of royal poinciana in the United Arab Emirates (UAE). Three Streptomyces strains exhibited powerful in vitro antifungal activity against Neoscytalidium dimidiatum (the causal agent of stem canker disease of royal poinciana). The antifungal action of Streptomyces rochei UAE2 and Streptomyces coelicoflavus UAE1 was mainly correlated with antibiosis and cell-wall-degrading enzymes production, respectively. Streptomyces antibioticus UAE1 was, however, associated with both mode of actions. These isolates suppressed lesion development on fruits inoculated with N. dimidiatum in the apple fruit bioassay. Under greenhouse conditions, individual biocontrol agents (BCAs) showed greater efficacies against stem canker when applied before N. dimidiatum than after the pathogen. The curative effects of Cidely® Top (a chemical fungicide) and S. antibioticus UAE1 on disease symptoms were comparable on royal poinciana. Prior to pathogen inoculation, S. antibioticus UAE1 was capable to suppress disease symptoms and prevent the pathogen spread, suggesting a relative superiority of S. antibioticus UAE1 over all other tested treatments. This was evident in the significant (P < 0.05) reduction in the number of defoliated leaves and conidia counts of N. dimidiatum in S. antibioticus UAE1-treated seedlings compared to the other two BCA or chemical treatments. In conclusion, the multiple mode of actions in S. antibioticus UAE1 can be particularly effective to produce synergistic actions against the fungus. This study is the first to explore the potential to use both biocontrol and fungicides to further develop integrated disease management against stem canker in royal poinciana.