Abstract:
Fungal diseases such as Fusarium head blight (FHB) are significant biotic stressors, negatively
affecting wheat production and quality. This study explored the antifungal activity of the
metabolites produced by the bacterial symbionts of entomopathogenic nematodes (EPNs) against
FHB-causing Fusarium sp. Fusarium graminearum. To achieve this, the symbiotic bacteria of nine EPN
isolates from the EPN collection at the Agricultural Research Council-Small Grains (ARC-SG) were
isolated from the cadavers of Galleria mellonella (Lepidoptera: Pyralidae) larvae after infection with
EPNs. Broth cultures (crude) and their supernatants (filtered and autoclaved) of each bacterial isolate
were used as bacterial metabolite treatments to test their inhibitory effect on the mycelial growth and
spore germination of F. graminearum. Mycelial growth inhibition rates varied among both bacterial
isolates and treatments. Crude metabolite treatments proved to be more effective than filtered and
autoclaved metabolite treatments, with an overall inhibition rate of 75.25% compared to 23.93%
and 13.32%, respectively. From the crude metabolite treatments, the Xenorhabdus khoisanae SGI 197
bacterial isolate from Steinernema beitlechemi SGI 197 had the highest mean inhibition rate of 96.25%,
followed by Photorhabdus luminescens SGI 170 bacteria isolated from Heterorhabditis bacteriophora SGI
170 with a 95.79% mean inhibition rate. The filtered metabolite treatments of all bacterial isolates
were tested for their inhibitory activity against Fusarium graminearum spore germination. Mean spore
germination inhibition rates from Xenorhabdus spp. bacterial isolates were higher (83.91 to 96.29%)
than those from Photorhabdus spp. (6.05 to 14.74%). The results obtained from this study suggest that
EPN symbiotic bacterial metabolites have potential use as biological control agents of FHB. Although
field efficacy against FHB was not studied, the significant inhibition of mycelial growth and spore
germination suggest that the application of these metabolites at the flowering stage may provide
protection to plants against infection with or spread of F. graminearum. These metabolites have the
potential to be employed as part of integrated pest management (IPM) to inhibit/delay conidia
germination until the anthesis (flowering stage) of wheat seedlings has passed.
