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Influence of CaCl₂ on Penicillium digitatum, Grapefruit Peel Tissue, and Biocontrol Activity of Pichia guilliermondii

First, third, fourth, and seventh authors: Department of Postharvest Science, ARO, the Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel; second author: USDA-ARS, Appalachian Fruit Research Station, 45 Wiltshire Road, Kearneysville, WV 25430; and fifth and sixth authors: Department of Bacteriology, Hebrew University of Jerusalem, Hadassa Medical School, Jerusalem, Israel

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 (10⁷ 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 (10⁷ 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 μM 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.