Those look like scale (a type of insect). Unfortunately I can't tell you how to treat it, as I have no experience with yuccas. If the regulars here can't help you, ask on GW's Cacti & Succulent forum.

I agree...scale. Isolate it if you have other plants so it doesn't spread. I have always had good luck with Safer Insecticidal Soap. Others may have other solutions. Good luck.

Nancy

Very lucky it is a Yucca, and not a smaller leaved plant.

Make a solution with dish soap and water, and spray it down really well. You want it sudsy. Use a rag and clean the leaves like you are washing the dishes. Wash all the bugs off the leaves. Then rinse all the soap and dead bugs off with a showerhead or hose. After the plant has dried, use a spray with neem oil to cover the whole plant. Let it dry. Neem oil is stinky but it is natural and safe, and the smell dissipates when it dries. It will work to starve any bugs that are left on the plant (it makes them stop eating) and will repel insects from settling on the plant. Repeat the neem oil treatment at least once; or even better, periodically. Once a week maybe. You can use it once every few weeks for prevention of further infestation. I like to use a combination neem oil insecticidal soap. And yes, you want to keep it away from other plants.

This post was edited by Lauraeli on Wed, Dec 31, 14 at 14:28