My desert rose was flowering beautifully and almost overnight leaves turned yellow. It's early September - no cold weather yet.
Yellow Leaves on Desert Rose
Certified GKH Gardening Expert
Yellow leaves generally indicate a watering issue, to much or to little.
Adding water if the soil is dry. If the soil it soggy this can also cause root rot.
Inspect the plant for insects as this can also cause yellowing leaves.
A good treatment plant for insects or fungus is Neem Oil.