Yerba Santa's Life Long Nemesis: Sooty Mold

    I have always enjoyed watching plants, but I have never specifically observed certain plants. I simply appreciated their “greenness”, not looking at their specific characteristics. Maybe I was just too busy, or maybe I did not care. But going out to the Bernard Field Station (BFS for short), I started to enjoy watching plants lately, sitting down and observing their characteristics for hours (Fig. 1.). Specifically, I have been obsessed with a certain plant: Yerba Santa (Eriodictyon crassifolium). To clarify, though all plants in the genus Eriodictyon are called Yerba Santa, we have one species at the BFS.

Fig. 1. An image capturing the lush scenery of Bernard Field Station, showing a variety of plants. The author frequently visits the field, taking time to sit down and observe the diverse plant life present in each environment.

    The BFS is an academic resource within the Claremont Colleges Campus, spanning approximately 86 acres. The BFS has a variety of plant communities that make it useful for scientific research and also make it interesting to explore. The most notable part about the BFS is that it has gone through recovery following both natural and human-caused disturbances. While walking around the BFS, I couldn't help but notice something: Yerba Santa exists everywhere. I took notes on various plants from different environments and discovered that Yerba Santa thrives regardless of the environment I visited (Fig. 2). When I first encountered it around January, Yerba Santa appeared dry and dying, but as spring arrived, it transformed into a brand new plant, ready to spread. This immediately grabbed my attention: what is this plant, and why does it look so dead?

Fig. 2. Author's Note: The author has provided brief notes for each plant species found within each environment. You can find the description of Yerba Santa in the note.

    Yerba Santa is an evergreen aromatic shrub, typically reaching a height of 3 to 4 feet. It boasts long, leathery, dark green leaves covered in sticky resin (note: avoid tasting their leaves, as they are quite bitter). During the spring, they bloom white, lavender, or purple flowers. Due to their durability in arid environments, Yerba Santa thrives along the entire west coast of America. Interestingly, the name “Yerba Santa” is the Spanish word for “holy weed” or “holy herb.” Reflecting this name, Yerba Santa is often utilized in herbal medicine, particularly for treating respiratory illnesses, colds, and stomachaches when brewed as a tea. (Though I've never tried it, I wonder if it tastes similar to Yerba Mate.) However, my fascination with Yerba Santa extends beyond these traits, as it harbors a nemesis: sooty mold.

Fig. 3. Yerba Santa Leaf Covered by Sooty Mold. A white paper background has been used to clearly show the infection of sooty mold on Yerba Santa's leaf.

    Take a closer look at the leaf of Yerba Santa, especially the older ones. You'll notice that the leaf is very dry and covered with multiple black dots (Fig. 3). These dots are sooty mold, a fungus that grows on the secretions of insects like aphids, scale, mealybugs, and whiteflies. While some people may find this unpleasant, I find it to be the most interesting aspect of Yerba Santa. The leaf is completely covered by sooty mold, which even made me think the plant is dead! However, it was not.

Fig. 4. New Yerba Santa Leaf. Unlike the old leaves, this leaf displays a bright green color and shows no signs of sooty mold infection.

    Over the weeks I have been observing, Yerba Santa shed its old leaves and bloomed ample fresh, green ones, without being infected by sooty mold (Fig. 4). It seemed as if Yerba Santa was shedding its old clothes. In fact, after reading more about Yerba Santa and Sooty mold, I discovered that sooty mold does not directly harm Yerba Santa; it can indirectly harm it by blocking sunlight, reducing the photosynthetic output of the leaves, and accelerating senescence and leaf drop. Sooty mold, the nemesis of Yerba Santa, tirelessly pursues it, while Yerba Santa swiftly escapes by generating new leaves and completing photosynthesis.

    Through my experiences at the field station, I have learned to appreciate this plant. Each plant has its own unique characteristics that can only be understood by observing and inspecting it thoroughly. Also, it is interesting to learn how plants can be affected by fungus, and how this can be caused by insects. Now, whenever I encounter a plant that captures my attention, I am prepared to grab my camera and notebook, ready to delve deeper into understanding plants and their interactions with other things such as insects, fungi, and maybe other plants.


Media Credit:
All of the photos have been taken by the author

Further Reading:

Eriodictyon californicum; Yerba Santa (CalPhotos Photo Database)

    https://calphotos.berkeley.edu/cgi/img_query?seq_num=632854&one=T

Plant of the Week: Yerba Santa (United States Department of Agriculture)

    https://www.fs.usda.gov/wildflowers/plant-of-the-week/eriodictyon_sp.shtml

What’s that black mold on my plant? (University of Florida Blogs)

   https://blogs.ifas.ufl.edu/highlandsco/2020/03/25/whats-that-black-mold-on-my-plant/

Yerba Santa (Friends of Edgewood)

   https://friendsofedgewood.org/yerba-santa

Thickleaf Yerba Santa (California Native Plant Society)

   https://calscape.org/Eriodictyon-crassifolium-(Thickleaf-Yerba-Santa)