Q.My donkey tails developed brown spots
I have several donkey tails that I have raised (many from propagating) over many years. Never had a problem with them but recently one of the healthiest ones has brown spots on it. I keep in a pot on my patio (northwest corner) and it gets direct sun. I live in the San Francisco Bay area which has a mild temperature in summer, rarely above 70-75. Any suggestions?
Dry brown spots are a sign of under watering.
If the spots are soft it is most likely leaf spot disease.
This can be treated with a fungicide.
Adding circulation among your plants is a good idea.
https://www.gardeningknowhow.com/ornamental/cacti-succulents/burros-tail/burros-tail-care.htm
https://www.gardeningknowhow.com/plant-problems/disease/plant-leaf-spots.htm