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 

 bv 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 m.ost common leaf fungi are those producing the spot 

 diseases, the foliage being co\-ered 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 

 pecuHar 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. 



