THE FUNGI WHICH CAUSE PLANT DISEASE 



329 



Heteroecism.'^" All of the examples just given are autcecious, 

 i. e., all known spore forms are found on the same species of host 

 plant. In many other rusts, however, heteroecism prevails, i. e., 

 one stage of the fungus is found on one species of host and another 

 stage upon another host; rarely three host plants are involved in 

 the cycle. Aside from the rusts only one other fungus (Sclerotinia 

 ledi) is known to show heteroecism. 



HetercEcism has been experimentally proved in some one hun- 

 dred and fifty cases and may be assumed to exist in many hun- 

 dreds of cases not yet investigated. 



Examples of heteroecism are as follows: 



Stages II, III. 

 Wheat 



Corn 

 Pea 



Red cedar (III) 



Stages 0, I. 

 Eu-type, Puccinia graminis, Berberis 



" rubigo-vera, Boraginacese 

 " sorghi, Oxalis 



Uromyces pisi. Euphorbia 



Opsis-tjT)e, Gymnosporangium 



macropus, Apple 



It frequently 

 happens that part 

 of the life cycle is 

 passed upon a mo- 

 nocotyledonous 

 plant, the remain- 

 der upon a dicoty- 

 ledon. In such 

 event it is more 

 often the II and 

 III stages that are 

 on the monocoty- 

 ledon while the O, 

 I stages are on the 

 dicotyledon; exam- 

 ples of this are af- 

 forded in the nu- 

 m e r o u s rusts of 

 grasses, sedges and rushes. In one group the pycnia and the 



Fig. 242. — Urediniospore of P. asparagi germinating on 

 surface of plant, and separate spores. After Smith. 



