Coniothyrium bartholomaei, a fungus, has been found on yucca in Oregon.  Generally, weather or irrigation that keeps leaves wet for extended periods favors the disease.

Symptoms:  Coniothyrium-small spots start out purple but become zoned with a center that turns light grayish brown. Concentric rings of tiny, black, fruiting bodies (pycnidia) develop in the spots. Spots generally are 1 inch or more in diameter and have a purple border. They coalesce on severely infected leaves, which eventually die. It is generally more severe on lower portion of the plant. Cultural control recommends removing and destroying infected leaves. Chemical control recommends applications in the spring during new leaf growth: Bonide Liquid Copper Fungicide at 0.5 to 2 fl oz/gal.

Hope this helps!

https://pnwhandbooks.org/plantdisease/host-disease/yucca-leaf-spot

Hope it works for you. Remember to avoid wetting foliage when irrigating also.

Thanks for contacting Ask an Expert.