Influence of CaCl2 on Penicillium digitatum, grapefruit peel tissue, and biocontrol activity of Pichia guilliermondii

Interactions between CaCl2, grapefruit peel tissue, Penicillium digitatum, and the yeast antagonist Pichia guilliermondii strain US-7 were investigated. Application of 68 or 136 mM CaCl2 to grapefruit surface wounds reduced the incidence of green mold caused by Penicillium digitatum by 43 to 52%. In laboratory tests, a cell suspension (10(7) cells/ml) of Pichia guilliermondii containing either 68 or 136 mM CaCl2 reduced the incidence of green mold from 27 to 3%. In large scale tests, dip application of 136 mM CaCl2 with US-7 (10(7) cells/ml) significantly decreased the number of wounds infected by Penicillium digitatum. CaCl2, with or without yeast cells, stimulated ethylene production in grapefruit tissue. Increasing concentrations of CaCl2 resulted in decreased spore germination and germ tube elongation of Penicillium digitatum. Pectinolytic activity of crude enzyme preparations of Penicillium digitatum was also inhibited by the presence of increasing concentrations of CaCl2. US-7 exhibited a strong ability to maintain cytosolic Ca2+ homeostasis at levels that did not exceed 1.4 micromolar when exposed to 150 mM CaCl2. On the other hand, strain 114 of Debaryomyces hansenii, which failed to give any protection against infection by Penicillium digitatum, showed reduced capacity to maintain Ca2+ homeostasis. The effect of calcium in reducing infection of grapefruit wounds by Penicillium digitatum could be due to direct effects on host tissue (making cell walls more resistant to enzymatic degradation) or the pathogen (interfering with spore germination, growth, and inhibition of fungal pectinolytic enzymes). Alternatively, the ability of US-7 to maintain calcium homeostasis may allow it to grow or assist in its competitive ability in a microenvironment that, because of high levels of calcium ions, is inhibitory to growth of the green mold pathogen.