"Black Moldy Stuff" on Plant Leaves

Advice for the Home Gardener from the Help Desk of the
UC Master Gardener Program of Contra Costa County

Client's Request (originally via email): I recently noted a large amount of black growth on the top side of leaves of three plants/bushes. These are old bushes, about 6 ft tall with branches about 2 inches in diameter. The growth is flat, sticky on the top of the limbs and leaves. The bottoms of the leaves have small brown dots as well as black dots. I sprayed with a fungicide but it had no effect. The bushes are probably 30 years old. I am thinking of removing them to protect our other plants. Do you have other suggestions?

Help Desk Response (via email after phone conversation followup to original email request) Thank you for contacting the UC Master Gardener Help Desk. It was nice to speak with you about your unidentified plant with “black stuff” on the leaves. As we discussed, your plants have sooty mold covering the tops of the leaves. It's growing on the honeydew that was excreted by insects living under the leaves above. From your description, those insects are most likely scales of some kind.

The sooty mold is really no problem for the plant, except in extreme cases where it blocks the sun and stops photosynthesis. It won't spread to other plants that don't have honeydew on them. Here's a link to more information about it: http://ipm.ucanr.edu/PMG/PESTNOTES/pn74108.html. 

The scale insects are sucking “plant juices” and can weaken the plant. Without knowing what kind of plant it is, or seeing the actual insects, we can't identify the type of scale, but management is pretty much the same for all. Scales hatch from eggs that are often under the immobile shells of the females (probably what you are seeing). The young scale ('crawlers' at that stage) can move around the plant but then settle down in one place where they stay to maturity. 

Many species are well controlled by natural enemies, but ants can protect the scales against the predators, so take measures to control the ants. Spraying horticultural oil during the dormant season or early summer when crawlers are moving may be all the treatment you need. 

Information about scales and their control: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7408.html  and
Information about ants and their control: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7411.html

Please let us know if you have more questions.

Help Desk of the UC Master Gardener Program of Contra Costa County (SEH)

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Note: The  UC Master Gardeners Program of Contra Costa's Help Desk is available year-round to answer your gardening questions.  Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA  94523. We can also be reached via telephone:  (925)646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/ MGCC Blogs can be found at http://ccmg.ucanr.edu/HortCoCo/ You can also subscribe to the Blog  (//ucanr.edu/blogs/CCMGBlog/).