Yuccas are classed as a succulent ? not sure
if so , you may have blackspot , i get the same thing on my Epiphylliums ,
it is usualy caused due to too much water , i had to mix up an enviro friends spray of sulphur and lime and oil i think it was..which dries out the leaf and protects it from future infestations (up to 3 months).
the black spot will dry up and just fall out , leaving a hole in your leaf.
Better than the plant dying i guess.
if you're interested i can post the mixture...but it will send the plant blue
, only for a short while.
tell you a story..
i watered my epiphylliums one afternoon.. and left the hose in my shade house and went to do something...
i forgot to turn off the hose at the tap , and during the night the hose come apart slightly at the join..
now we all know cacti do not like too much water..
well the next morning i get up and go outside , only to find a rivver of water running down my driveway...my epi's had copped water all night long...
within a week..they all started to rot.. all the new spikes were rotting , and they were due to flower in a months time
this spray was the only thing that saved them as it also stops rott.
this september i should get my flowers