38 Disease and Death of Trees — Generalities 
portions of the leaf surface and thus simply reduce the 
assimilating area and lessen the feeding capacity of the tree; 
thereby the growth of the tree is diminished in proportion to 
the leaf area destroyed. Since the leaves fall and are 
renewed the next season, the trouble can be continued only 
by a new infection; in other words, only one season’s growth 
and function is involved. 
There are, however, a few fungi which not only reduce 
the assimilating leaf area but even cause the premature 
fall of the entire leaves, and these fungi are, of course, the 
more injurious. 
The most common leaf fungi are those producing the spot 
diseases, the foliage being covered by spots varying in color, 
size, and shape, which mark the destroyed tissue. Some- 
times the destroyed tissue dries out, becomes brown, and 
breaks out under action of rain and wind, the leaves remain- 
ing perforated as by shot. 
Another class of fungi produce the so-called powdery and 
other mildews, recognized by a whitish or else black spider- 
web-like growth on the surface (the fruit-bodies), which 
becomes powdery. 
While the spot diseases and mildews affect mainly the 
broad-leaved trees— maples, chestnuts, oaks, and many 
others — the rusts, characterized by collections of yellowish 
red, somewhat powdery fruit bodies are more frequent on 
conifers. These attack also stems and branches, causing 
peculiar swellings or knots and blister-like patches. 
Here also belongs the disease known as juniper apple, 
which appears on the red cedar with a fruit body of slimy 
or gelatinous, yellow or brown, conical masses. 
On the whole, all these fungi are of minor importance and, 
if the trees are otherwise kept in vigorous condition, their 
damage is in most cases readily repaired, 
