THE FUNGI WHICH CAUSE PLANT DISEASE 



261 



possesses an Alternaria conidial form. Following Diedicke, the 

 forms given below would be recognized. 



P. bromi Died. 



Perithecia brown, hairy; asci 189-288 x 34-59 n, saccate, thin- 

 walled; spores 2-seriate, golden-brown, 4-celled, 48-83 x 19-33 n. 



Conidia (=Helminthosporium bromi) on brownish spots, 108- 

 150 X 13-20 li, 5 to 7-celled, dark colored. On Bromus. 



P. gramineum Died. 



Conidia (=Helminthosporium gramineum) ; conidiophores short, 

 subflexuose, light-brown; conidia solitary, elongate-cylindric, 4 to 

 7-celled, 15-19 n wide and of variable length. 



The mycelium invades the tissue -causing long brown spots. 

 These later become covered with an abundance of conidiophores 

 which emerge through the stomata. Potter also reports in- 

 vasion and complete occupation of ovaries by the mycelium. 

 Sclerotia-like bodies are 

 formed on leaves and 

 stems. They were first 

 seen in artificial cul- 

 tures of the fungus by 

 Ravn '^"■^ and have been 

 since found in nature 

 (Noack 24^). 



The conidiospores 

 have been shown to be 

 long-lived, and spring 

 infection begins largely 

 from conidia carried 

 over winter on seed. 

 Extensive study was made of the conidial form by Ravn who 

 found the mycelium to be of two kinds, one afirial and hyaline, 

 the other strict and dark. It grew well on acid or neutral media. 



Careful infection experiments (Ravn) proved the pathogenicity 

 of H. graminum for barley but showed it incapable of infecting 

 oats, rye or wheat. 



Ravn regards the disease produced by H. gramineum as often 

 general, not local, in that the mycelium may invade the growing 

 points, resulting in infection of all the leaves. 



Fig. 192. — P. trichostoma. 1, group of asci, 2, a 

 single spore at the apex of an ascus. After 

 Diedicke. 



