Crinipellis perniciosa
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Crinipellis perniciosa is a fungus that causes "Witches' Broom Disease" (WBD), which damages cocoa production in the Americas, and is consequently a major bane for makers of chocolate products. An infected crop may lose up to 90% of its harvest.
It exists in two characteristic phases: biotrophic (expanding and infecting) and saprotrophic (dying, and producing spores). The biotrophic stage, and what triggers its switch to a saprotrophic stage, are still not understood.
The spores of this fungus are spread by wind, but must land in water in order to germinate. As a consequence, it mainly spreads during rainy periods. Control is very difficult, and requires extensive work to remove. The most critical part of treatment (pruning of diseased material) can only be conducted during dry periods, or one risks spreading the disease further. Due to economics, most discarded brooms are usually left on the ground, leaving a risk of reinfection of trees the subsequent year. However, spraying the ground with a light coating of oil can prevent the spores from becoming airborne again.