180 THE FUNGI WHICH CAUSE PLANT DISEASE 



eluding species of Avena, Festuca, Hordeum, Phleum, Poa, Sac- 

 charum, Secale, and Triticum. 



The asci are peculiar in that they usually contain undifferenti- 

 ated granular protoplasm, not spores, though in some cases the 

 spores, normally 8, are present. Wolff ^'^ found that after a few 

 days in water the undifferentiated ascoplasm developed spores 

 which proceeded to normal germination. 



This species on grasses shows no morphological differences, yet 

 inoculation tests have revealed in it numerous biologic varieties. 

 Reed*' summarizes the results of his own work together with 

 that of Marchal ^"' and Salmon ^"^ as follows: 



"So far as tested, all species of Avena are susceptible to the 

 oat mildew. All species of Triticum are likewise susceptible to 

 the wheat mildew. We find, however, that certain varieties of 

 Triticvun dicoccum are practically immune to the wheat mildew. 

 Other varieties of this same species are entirely susceptible. Some 

 species of Hordeum are immime to the barley mildew, and the 

 same seems to be true of certain species of Secale with reference 

 to the rye mildew. 



"To these general statements there are two possible exceptions. 

 Marchal states that the oat mildew will infect Arrhenatherum 

 elatius. Salmon, however, obtained a negative result with the 

 oat mildew on this grass. The evidence is not conclusive either 

 way. The other exception is that, according to Salmon, conidia 

 from wheat can infect Hordeum silvaticum. 



"It would seem then that under normal conditions there are 

 well-defined forms of Erysiphe graminis occurring respectively 

 on the species of each of the four cereals." 



It is thought that some hosts may act as bridging species and 

 enable the parasite to pass from one host to another to which it 

 could not pass directly. 



Uncinula L6veill€ (p. 175) 



Perithecia globose to globose-depressed; appendages simple or 

 rarely once or twice dichotomously forked, uncinate at the apex, 

 usually colorless, rarely dark brown at base or throughout; asci 

 several, 2 to 8-spored. 



There are eighteen or twenty species. 



