Thanks for contacting ting us. Can you please tell me what kind of magnolia you have, how long it has been in the pot, and what your watering routine is? Also, it would be helpful to have  close photo of the plant, and where you are located.

Magnolia ‘Susan’ has been in it’s present pot for 15 years.
it is watered well about once a week (with rainwater from a water butt), sometimes with a sequestered seaweed feed.
 I live in the north of England, but we have had about 4/5 weeks of very warm dry weather, so this last week it has been watered with tap water.

Hello gardener, Thank you for the photos. It appears your magnolia may have some sort of fungal disease. These are not generally fatal, but the can get worse if left untreated.

The controls involve mostly good sanitation. Since these pathogens reside in the soil, promptly remove all debris that falls from the plant. (You may wish also to remove any leaves still on the plant that are affected.) Do not compost this debris, but put it in your trash. 

Make sure the tree receives a good flow of air. You may need to move it away from the building to increase air circulation. If the foliage becomes dense, prune judiciously, again to improve the air flow. 

You can purchase fungicidal sprays that may help check the disease and prevent secondary problems. Be sure to carefully follow the manufacturer's instructions for use.

Thank you very much for your helpful advice. I have already acted on it, so hopefully this will stop the fungal disease from spreading.