Rhytisma
A genus of Fungi which make their appearance upon the leaves and branches of Maples, certain Willows, and a few other plants, in the form of shining, deep black patches, rising a little above the general level of the part bearing them. The most generally known species is that which gives rise to the large black spots so common in autumn upon the leaves of Acer campestre, or Field Maple, and of Acer Pseudo-platanus, the Sycamore of England, the Plane of Scotland. These spots, in their common form, are often 1/2in. or more in breadth, and are of a uniform tint. The Fungus is known as R. acerinum. A variety called R. punctatum differs from this in having the spots broken up into a number of small black specks instead of a uniform black patch. In autumn, there is no sign of reproductive organs on the Fungus; but if a patch is examined in spring, after the leaf has lain on the moist ground all winter, there are found imbedded in the mass numerous asci, each inclosing eight slender spores.
The remedy is easy, and consists in the careful removal of the diseased leaves, as is done in well-kept gardens and pleasure-grounds. In such places, this disease is of rare occurrence compared with its abundance in most other localities. Its presence renders the leaves unsightly, but is not dangerous to them.