Question
Does the tree have to be taken down if you have white cotton spots on the underside of the leaves?

Answer
These sound like insect galls and are really not a major treat to the health of the tree. There is not an insecticide that can be sprayed on the leaves to control the galls but when the leaves fall rake them up and remove them from the area this will reduce the population for next year. The tree will be just fine no need to remove the tree.

A second thought is this could be aphids. Try touching the cotton spots and if they move these are aphids. Aphides suck the plant juices from the leaves and secrete a substance called honeydew which is sticky since it is high in sugar content. IF these are aphids you can treat the tree with an insecticide called Bayer Advanced Tree and Shrub Insect control. This is applied around the tree to the soil and the roots will carry the insecticide to the leaves and when the aphids suck the plant juices they are killed. Here is a web link to this product. http://www.bayeradvanced.com/tree-shrub-care/products/12-month-tree-shrub-insect...