| What you describe sounds like sooty mold. It is caused indirectly by insect feeding - usually scales or mealybug, but sometimes aphids. These insects suck the plant juices and excrete a liquid called honeydew. The sugar content of this excrement is just high enough to encourage fungal growth. That's what you're seeing on the leaves of your plants. There are several control options: if the plant isn't too large, you can use a gloved hand and squish the adult scales. Or you can scrub the scales off with a soft brush, then rinse with soapy water. Or, you can apply a dormant oil to effectively suffocate the insects. Apply dormant (horticultural) oil to all surfaces of the plant to get good control, but don't spray if the air temperature is below 40F or above 65F. Be sure to read the label, and apply exactly according to directions to avoid damage to your plants. |
