Remove infected leaves. Destroy all fallen leaves before growth begins in the spring. Improve air circulation. Sulfur fungicides may help, too. Follow instructions carefully.
Mosaic virus causes yellow striping or mottling of the leaves and flowers.
Control aphids that transmit this virus. Discard all infected plants. I love magnolias. A friend game me a magnolia seedling, and I put it in a 6-inch pot with regular dirt and am watering it once a week with diluted Miracle Gro. It seems to be doing OK, but a few bottom leaves turn brown.