Effects of Ca2+ and Mg2+ on Botrytis cinerea and Penicillium expansum in vitro and on the biocontrol activity of Candida oleophila

Calcium salts have been reported to play an important role in the inhibition of postharvest decay of apples and in enhancing the efficacy of postharvest biocontrol agents. Therefore, the present study was conducted in order to examine and compare the effects of calcium and magnesium salts on the germination and metabolism of the postharvest pathogens Botrytis cinerea and Penicillium expansum, and to determine the effects of these salts on the biocontrol activity of two isolates (182 and 247) of the yeast Candida oleophila. Increasing concentrations of CaCl2 (25-175 mM) resulted in decreased spore germination and germ-tube growth of both pathogens. The greatest effect was observed in the case of B. cinerea. The inhibitory effect could be overcome by the addition of glucose to the germination medium. MgCl2 (25-175 mM) had no effect on germination or germ-tube growth of either pathogen, indicating that the calcium cation rather than the chloride anion was responsible for the inhibition. The pectinolytic activity of crude enzyme obtained from the culture medium of both pathogens was also inhibited by 25-175 mM CaCl2, with the greatest effect on the crude enzyme from P. expansum. Biocontrol activity of isolate 182 was enhanced by the addition of 90 or 180 mM CaCl2, whereas there was no effect on the biocontrol activity of isolate 247. This was apparently due to the inability of isolate 247 to proliferate in apple wounds. It is postulated that enhanced biocontrol activity of isolate 182 of the yeast C. oleophila in the presence of Ca2+ ions is directly due to the inhibitory effects of calcium ions on pathogen spore germination and metabolism, and indirectly due to the ability of isolate 182 to maintain normal metabolism in the presence of 'toxic' levels of calcium.