Submitted to: Physiological and Molecular Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 2, 2002
Publication Date: November 1, 2002
Citation: Richard R. Stange,Sharon L. Midland,James J. Sims,T. Greg McCollum
Interpretive Summary: Most disease causing organisms cause disease only on one or a few related species. This is called host specificity. If we knew the underlying causes of this phenomenon, we would be better able to control disease. To get some answers, we used three related fungi, two that cause disease on citrus fruit (Penicillium digitatum & P. italicum) and one that does not (P. expansum). The growth of these fungi on extracts from the peels of seven species of Citrus fruit was determined. All of the citrus peel extracts stimulated growth of the disease causing fungi, but did not stimulate growth of P. expansum. Working with grapefruit, we determined that much of the stimulatory activity was due to a single component in the extract, a compound called nootkatone. This suggests a new option for disease control, if the concentration of stimulatory components in the host could be reduced, the host should be less susceptible to disease. As we understand better how disease causing microbes function, we will be better able to control diseases.
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 synergistic effect. The primary growth stimulating component in the hexanes fraction was identified as nootkatone. Growth of P. digitatum and P. italicum is stimulated by components present in citrus peel.