GRASSES OF IOWA. 



215 



The disease appears about the time when the grain begins 

 to turn. Either the entire head or s:me part of it ripens sud- 

 denly and prematurely. If the head is partially destroyed the 

 lower part will be green. Affected heads or parts of the same 

 have a whitish appearance instead of a golden yellow. The 



Fig. 102. Gray spot disease of crab grass (Piricularia grisea). (King.) 



disease usually starts at the upper end, although it may begin 

 at the lower end. The mycelium is whitish or varies from 

 yellow to orange, divided, torulose. The spores are borne on 

 erect threads arising from the mycelium, terminal or lateral, 

 crescent-shaped, at first one-celled, finally divided into two or 

 more cells. The color of the spores is whitish or, in masses, 

 orange or pink. In germination each cell produces a germ 

 tube. The several-celled spores may break up into conidia. 



The Fusarium heterosporum Nees, is common in parts of Ger- 

 many, and Tubeuf quotes Frank as stating that the destruc- 



