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Cassia trees can suffer from bacterial leaf spot, fungal leaf spot, and anthracnose, all of which cause shot holes with black dots in the center of yellow rings, eventually leaving holes. Bacterial and fungal leaf spots start with small, water-soaked or brown spots that enlarge and develop yellow halos, while anthracnose causes dark, sunken lesions. Insects can also cause similar damage. To manage these issues, remove affected leaves, ensure good air circulation, avoid overhead watering, and use fungicides or insecticides if necessary. Bacterial leaf spot is often not curable and must be managed by removing infected parts and practicing good hygiene. I wouldn't fertilize in times of stress, though. This will add unnecessary stress while it is already under stress.

You can find your local extension service, where they can do more testing, with the search bar on the bottom of this page:

https://www.gardeningknowhow.com/garden-how-to/info/what-is-extension-service.htm

These articles will offer tips on treating these diseases:

https://www.gardeningknowhow.com/plant-problems/disease/bacterial-leaf-spot.htm

https://www.gardeningknowhow.com/plant-problems/disease/plant-leaf-spots.htm

https://www.gardeningknowhow.com/plant-problems/disease/alternaria-leaf-spot.htm