Question
We recently moved into a house with what we think is a sycamore tree (with maroon-ish broad leaves shaped similarly to a maple tree) in the front yard.  In the past months it has developed what appears to be a white fungus on many of its leaves---sometimes very thick.  The fungus seems to be spreading to some of our other plants (a peony plant, and a small boxwood type bush).  Any help would be appreciated.

Answer
Sounds like powdery mildew.

Powdery mildews, as the name implies, often appear as a superficial white or gray powdery growth of fungus over the surface of leaves, stems, flowers, or fruit of affected plants. These patches may enlarge until they cover the entire leaf on one or both sides. Young foliage and shoots may be particularly susceptible. Leaf curling and twisting may be noted before the fungus is noticed. Severe powdery mildew infection will result in yellowed leaves, dried and brown leaves, and disfigured shoots and flowers. Although it usually is not a fatal disease, powdery mildew may hasten plant defoliation and fall dormancy, and the infected plant may become extremely unsightly.

In many cases, powdery mildew diseases do little damage to overall plant health, and yearly infections can be ignored if unsightliness is not a major concern. For example, lilacs can have powdery mildew each year, with little or no apparent effect on plant health. On some plants, powdery mildews can result in significant damage. Thus, fungicides must be used to achieve acceptable control. For best results with fungicides, spray programs must begin as soon as mildews are detected. Spray on a regular schedule, more often during cool, damp weather. Use a good spreader-sticker with the fungicides. Be sure and cover both surfaces of all leaves with the spray. Fungicides generally recommended for powdery mildew control include: Triadimefon (Bayleton, Strike); Triforine (Funginex), Thiophanate-methyl (Cleary's 3336, Domain) and Propiconazole (Banner).

I would spray the lower plants but the tree will be hard to spray and the leaves will be dropping soon anyway so i would not try and spray it. On many trees, the disease causes little or no damage, therefore, control is not necessary.