372 THALLOPHYTES 
estimated at about $25,000,000. So scrious is this disease that 
legislatures have made special appropriations for fighting it. 
The spores are carried by the wind and sometimes by birds 
and insects. When the spores reach the bark of the Chestnut, 
they develop hyphae which penetrate and kill the phloem and 
cambium. The dead bark soon becomes warty with yellowish- 
brown pustules in which summer spores in great numbers are 
Fic. 322. — Pus- 
tules on the bark of 
uw Chestaut caused T'tg. 323. — Powdery Mildew 
by the Chestnut on an Apple leaf. The light 
Blight Fungus. areas are due to the presence 
From Bulletin 380, of many superficial hyphae. 
U. 8. Dept. Agri- From Bulletin 185, Maine Agr. 
culture, 1917. Exp. Sta. 
produced (Fig. 322). The summer spores are extruded in 
threads and spread the disease to other trees. In autumn these 
same pustules develop deeply buried perithecia in which the 
ascospores (winter sporcs) develop. The ascospores germinate 
the next spring and when carried to other trees start the disease 
anew. The mycelium in an affected tree renews its activity each 
year and thus continues to spread, usually downward, until the 
