My yucca started developing black and white spots on the leaves a few months ago. I thought it was some sort of pest and sprayed all the leaves with this horticultural spray (https://www.gardeningknowhow.com/plant-problems/pests/pesticides/homemade-soap-spray.htm) and wiped the remaining leaves with alcohol wipes. New growth started to appear and I thought I was in the clear. But then they started to develop again after some humid and rain stormy days. What is this? And how can I save my yucca?