I planted my yucca about 3 months ago. It has never really looked good. Over the past month most of the leaves are turning yellow, and new center leaves have black spots on them. Plant is appox. 20 inches tall and is starting to lean bad, even though is in direct sunlight. I planted as per directions from landscaper that I purchased from.
Certified GKH Gardening Expert
I would start by getting your soil tested. You sound like you may have a nitrogen or 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 the natural soil of your yard cannot support them. 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. If you find pests, treat with neem oil.