| Popis: |
Young shoot (flushes) and pod extracts of two cocoa clones (SCA-6, SCA-12), which had demonstrated field resistance to the Para strains of Crinipellis perniciosa (Stahel) Singer, were found to inhibit basidiospore germination. Basidiospores in flush and green pod extracts of highly susceptible clones (ICS-39, UF-677) germinated slowly and abnormally, often becoming swollen and distorted, and produced short swollen irregular germ-tubes. However, in extracts of woody stems and ripe pods, the basidiospores were usually swollen but failed to germinate. Flush extracts of mildly susceptible clones had an intermediate effect, with both inhibition and modification of germination. Using these criteria, basidiospore germination in extracts of inoculated cocoa seedlings was compared with their reaction to infection. Immune and resistant plant extracts either completely inhibited germination or had no effect, positively or negatively. Extracts of highly susceptible seedlings initially promoted abnormal germination. Germination in susceptible plant extracts was varied being inhibited, modified or unaffected. It is concluded that young tissues of susceptible cocoa cultivars contain excess of a heat-stable metabolite or modifier (possibly a reducing sugar) which promotes pathogenicity by inhibiting dikaryotization, and that resistant cultivars contain either insufficient modifier to maintain the fungus in the monokaryotic state or excess of an inhibitor (possibly a bound monophenol) which provokes spore plasmolysis. The significance of these findings is discussed in relation to breeding work and host-pathogen interactions. |
