I was following your posting and surely thought your yucca was a different species. The Baccata here grows wild in the mountains and colder mesa's around Albuquerque. Here they grow in zones 7a, and more like 6b and colder. They grow fine here in 7b/8a Albuquerque, but south Texas is a whole other ballgame. I have not seen that damage and that is why I did not comment. I would try a systemic like Don said.