Fruity-scented antifungal volatiles from endophytic Geotrichum candidum PF005: Broad-spectrum bioactivity against stored grain pathogens, mode of action and suitable formulation for mycofumigation

Abstract

Geotrichum candidum PF005, an endophytic yeast-like fungus emits fruity scented volatile organic compounds that exhibit significant antifungal activity. In the present study, 13 out of 28 fungal isolates obtained from stored grains of rice and wheat, were found to show growth inhibition of 41% to 87% in the presence of the volatile mixture, implying its broad-spectrum bioactivity. Two well-known phytopathogens, Rhizoctonia solani and Curvularia oryzae were selected as test fungi to elucidate the mechanism of action of the volatiles. R. solani, when exposed to these volatiles, exhibited reduced or no sclerotia formation and restricted development of aerial mycelia; whereas, C. oryzae showed sparse sporulation with reduced pigmentation. Besides, the volatile mixture affected the chitin distribution and mycelial/spore morphology in R. solani and C. oryzae. An increased content of malondialdehyde by 1.33- and 1.65-fold in R. solani and C. oryzae, respectively was an indication of elevated oxidative stress, which was further exemplified by staining of pathogen mycelia using two reactive oxygen species-specific dyes. In order to employ the antifungal volatile mixture as potent mycofumigant against the phytopathogens, five different solid-state media formulations (viz., FN1 to FN5) were developed using inexpensive substrates on which the PF005 was grown as usual culture or immobilized in calcium alginate beads. The FN5 exhibited maximum growth inhibition of 86% and 92% in R. solani and C. oryzae, respectively due to maximum volatile production, with ethyl isovalerate being the major identified compound. It is noteworthy that the volatiles neither altered the nutritional values of cereal grains nor exhibited significant toxicity against plant tissues or animal cells, reassuring the safe use on consumables. Taken together, this study paves the way for the utilization of G. candidum PF005 to control the stored grain pathogens through mycofumigation.