Differential effects of citrus peel extracts on growth of Penicillium digitatum, P-italicum, and P-expansum

Most pathogenic species of Penicillium have a limited host range, suggesting unique adaptations to particular hosts. P. digitatum and P. italicum are primarily pathogens of mature citrus fruit, while P. expansum has a broad host range, but does not infect citrus. One possible basis of host specificity is that the pathogens are physiologically adapted to grow in the presence of the many bioactive compounds present in the citrus peel. To test this, peels from the fruit of seven Citrus sp. were extracted with 80 % ethanol, concentrated and standardized. Growth of P. digitatum, P. italicum. and P. expansum on potato dextrose broth, alone, or plus extract, was quantified by measuring conversion of the vital stain, thiazol blue. All citrus peel extracts stimulated growth of P. digitatum and P. italicum at greater dilution and to a greater degree than P. expansum. 'Marsh' grapefruit extract was separated into hexanes, ethyl acetate and aqueous partitioning fractions. The aqueous fraction was active at the greatest dilution tested, the hexanes fraction was active only at the highest concentration tested, while the ethyl acetate fraction had no stimulatory activity. When the aqueous and hexanes fractions were combined, growth increased tremendously, indicating a synergistic effect. The primary growth stimulating component in the hexanes fraction of grapefruit was identified as nootkatone. Growth of P. digitatum and P. italicum is stimulated by components present in citrus peel. Published by Elsevier Science Ltd.