The saucer magnolia is also known as Magnolia x soulangiana. In landscape design, it is usually used as a specimen tree because of its lovely blossoms. Normally, this specimen is pest and disease free. Although uncommon, it can be attacked by certain pests like magnolia scale, and the tulip-poplar weevil (sassafras weevil). Possible diseases are: leaf spot (fungus), cankers and verticillium wilt. Leaf spots are caused by bacteria or fungi.
Identification of Leaf Spot
Leaf spot will appear on older leaves, generally in damp weather. The spots will be white and powdery. These spots may also appear on stems, flowers and buds. In some instances the fungus covers the entire leaf.
Not a Serious Problem
Fungal leaf spot is not a problem when it comes to the saucer magnolia. But leaf spot caused by bacteria can be. The Ohio State University Extension says, "Bacterial leaf spot has the potential to be serious, especially in nursery situations with overhead watering. Any practice directed toward maintaining dry foliage is beneficial."
If Your Tree Gets Fungal Leaf Spot
If your tree gets fungal leaf spot, the infected leaves will eventually fall off. Normally, fungal leaf spot does not require any chemical application. When the leaves fall off--just rake them up and dispose of them. (Do not put the leaves in your compost bin.)
Preventing Leaf Spot
There are no preventative measures that you can take against fungal leaf spot, since the fungus forms during damp weather. But you can make sure you water early in the day so that the leaves of the magnolia have time to dry before nightfall. And avoid excessive overhead sprinkling.
Overall, the saucer magnolia is an excellent specimen (for the most part pest and disease free). Fungal leaf spot is not a serious problem for this beautiful tree, certainly not anything that would make you avoid planting a saucer magnolia in your landscape. It also requires little pruning, which is another favorable quality. The lovely display of spring flowers makes the saucer magnolia a most desirable planting.