During fall and winter, it's common for Yerba Santa to have their leaves conspicuously covered with a black sooty mold. This is apparent in the photo, though a few relatively new leaves there are green, glossy, and ''soot free''.

In some locales the high degree of sooty mold infection on Yerba Santa leaves can appear epidemic, and give the impression that entire populations are in danger of imminent demise. Thankfully the plants usually spring back the following year appearing relatively unharmed. I've often wondered about the details of this phenomenon...so I decided to research it a bit.

According to various references (e.g this UC extension web page...and a fascinating, highly detailed research paper on sooty molds) the mold does not directly attack the plant...that is, its hyphae do not penetrate the leaf to digest plant tissue. Instead, the sooty mold fungi are sustained by digesting sugars from ''honeydew'' that accumulates on the leaf surfaces, and which originates from the liquid exudates of various plant-feeding true bugs (e.g. hemipterans such as aphids, leafhoppers, whiteflies, mealy bugs, scale bugs, psillids, etc).

The main negative impact for the plants is that the sooty mold simply blocks sunlight and thus reduces the photosynthetic output of the leaves, which accelerates senescence and leaf drop, and reduces overall food energy available to the plant. The references indicate that washing off honeydew residues is sufficient to eliminate sooty molds, and rains can partially reduce infestations in natural habitats.

Apparently it's very difficult to identify species of sooty molds, and there are usually a number of different species (and their sexual and asexual synanamorphs) present in any given infection. Some fungal species names associated with Eriodictyon leaves are Coniothecium eriodictyonis, Heterosporium californicum, and Heteroconium glutinosum.

I was surprised to learn of the crucial ecological role of insect 'honeydew' in this process. I've seen afflicted patches of Yerba Santa blackened from top-to-bottom but not obviously infested with bugs...nor lying within the ''honeydew fall-out zone'' of taller surrounding plants. In fact, I've never found any hemipteran bugs on the leaves (just beetles, e.g. Trirhabda). But many of the bugs involved here are cryptic and miniscule...and they're likely only present for a relatively short time, while their sticky exudates persist much longer.

The Discover Life Eriodictyon californicum page lists numerous bee and bug associates, including:

4 species of whitefly (family Aleyrodidae); 1 species of leafhopper (genus Macrosteles); 1 species of treehopper (genus Tortistilus, formerly part of Ceresa); 2 species of planthopper (genera Dictyssa and Neaethus); and 2 species of mealybugs (family Pseudococcidae)

In the future I'll have to keep my eyes open for these hemipterans in the vicinity (and generally above) infected patches of Eriodictyon californicum.