Q.Small Chrysanthemum Bushes Have Developed What Looks Like Blight ??
I have several of these dotted around garden never had problems over 9 years. Now just two at the front have got leaves turning brown and plant looking very unhealthy.

Certified GKH Gardening Expert
Unfortunately, your photos did not come through. I am unable to see the damage. In the meantime, these articles will provide reasons why this could be happening:
https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/yellow-chrysanthemum-leaves.htm
https://www.gardeningknowhow.com/ornamental/flowers/chrysanthemum/mum-plant-disease-and-pests.htm