Yellow Leaves and Black Spots
My poor yuccas looks 'yucky' right now, and I sure hope I'm not losing them. There have yellowing leaves with lots of black spots. Also, my Rosemary bushes have lots of dying limbs right in the center (the rest of the bushes look healthy). One more thing, my Mountain Laurel trees have started yellowing too. I know they have not been overwatered. When they were healthier, the leaves were a beautiful forest green. They are not full grown, just about 2 feet tall right now.
Certified GKH Gardening Expert
I would start by getting your soil tested. You sound like you may have a yard wide nitrogen deficiency or an iron deficiency. The soil in the container that they were planted in when you bought them has probably kept them healthy up until now, but now that this has been depleted, the natural soil of your yard cannot support them. If they have been planted for a while, too much rain may have washed away the nutrients. Some fertilizer, regardless, cannot hurt.
Check the undersides or the leaves of the plants for pests as well. Most pests that might possibly cause this kind of damage hide on the bottom of leaves.