Saturday, February 15, 2014

Smut fungi

On my way to the 3rd street promenade, I happened upon a Kusamaki tree (Podocarpus macrophyllus) that had its leaves covered with a dark soot-like material.  The sooty debris is actually a multicelular fungus in the taxonomic class Ustilaginomycetes, better known as "smut" fungi.  This particular smut has a fantastic backstory; do you see the white-covered "dots" along the bottom of the leaves?  These are female mealybugs (Pseudococcidae), which are phloem-feeding scale insects.  Male Pseudococcids are very short-lived and sexually dimorphic (so they look more like tiny little wasps with a single pair of wings!).  Due to their all-liquid diet, female mealybugs excrete large amounts of fluid rich in carbohydrates called "honeydew", which then attracts a diverse assemblage of ant species.  In exchange for the honeydew services, the ants defend mealybugs from predatory insects, like parasitoid wasps.  However, when ants are absent, the untended mealybugs will continue to drip honeydew leaving a sweet glassy film that then elicits the growth of the smut fungi!  The smut is antagonistic, eventually precipitating harm to both the mealybugs and host plant.  Here's a great paper by Johnson et al. 2001, which details that these symbiotic ant-tending interactions are at least 20 million years old!