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Enhancement of Biocontrol Activity of Cryptococcus laurentii by Silicon and the Possible Mechanisms Involved

Exogenous application of silicon (Si) in the form of sodium metasilicate reduced disease development caused by Penicillium expansum and Monilinia fructicola in sweet cherry fruit at 20°C. The inhibition of fruit decay was correlated closely with Si concentrations. Silicon at concentrations of 1%, in combination with the biocontrol agent Cryptococcus laurentii at 1 × 10⁷ cells per ml, provided synergistic effects against both diseases. Population dynamics of C. laurentii were stimulated by Si 48 h after the yeast treatment in the wounds of sweet cherry fruit. Silicon strongly inhibited spore germination and germ tube elongation of P. expansum and M. fructicola in vitro. Based on results with scanning electron microscopy, growth of both pathogens was significantly inhibited by Si in the wounds of sweet cherry fruit. Compared with the wounded water control, Si treatment induced a significant increase in the activities of phenylalanine ammonia-lyase, polyphenoloxidase, and peroxidase in sweet cherry fruit but did not increase the levels of lignin. Application of Si activated a cytochemical reaction and caused tissue browning near the site of wounding. Based on our studies, the improvement in biocontrol efficacy of antagonistic yeast when combined with Si may be associated with the increased population density of antagonistic yeast by Si, the direct fungitoxicity property of Si to the pathogens, and the elicitation of biochemical defense responses in fruit.