372 



THALLOPHYTES 



estimated at about $25,000,000. So serious 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 



FIG. 322. Pus- 

 tules on the bark of 

 a Chestnut caused 

 by the Chestnut 

 Blight Fungus. 

 From Bulletin 380, 

 U. S. Dept. Agri- 

 culture, 1917. 



FIG. 323. Powdery Mildew 

 on an Apple leaf. The light 

 areas are due to the presence 

 of many superficial hyphae. 

 From Bulletin 185, Maine Agr. 

 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 spores) 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 



