Pustules and brown spots Agave attenuata

[kunterbunt5]

Hello,
Unfortunately I have problems with Agave attenuata. I cultivate various agave species, including Agave attenuata. I have serval plants, which I have grown from seed, and also Agave attenuata "Ray of Light", which I bought a few years ago. On a plant grown from seed I first noticed pustules, brown spots and yellowish areas near the base of the leaves. I had this plant for several years and the pustules usually appeared in the winter months and disappeared again in the summer and only on this plant. Until now I did not think the phenomenon was a contagious disease. This year, unfortunately, my "Ray of Light", was affected :-(. All plants are pottet and inhouse for the winter. Temperature ca. 68 F and a place on the windowsill.
Does anyone know about this disease and can tell me what to do about it? I would hate to get rid of the Ray of Light, but of course I don't want this disease to spread. The first infected plant is destroyed.

Thank you very much for your help in advance!