35 



rye and wheat. Rxpcriments have been made which go to establish the 

 fact that a distinct race of this mildew is now found on the species and 

 varieties of each of the four genera of cereals, — A vena ( oat ) , Hordeum 

 ( barley ) , Secale ( rye ) , and Triticum ( wheat ) . The mildews which at- 

 tacks one of these genera ^^dll not pass on to a species belonging to any 

 of the others. It has even been established that some species of a genus 

 may be immune to the mildew which attacks other species of the same 

 genus. 



The genus Krysiphe has received more attention in this matter than 

 any other, and while some experimentis have been carried out with the 

 others, the data is not verv satisfactory. 



Powdery Mildew of the Cherry (Podosphaera oxyacanthae). a. winter spore case, b. ascus. 

 c. and D. the formation of summer spores. 



FORMS OF ECONOMIC IMPORTANCE. 



Although I have examined m.ost of the common species in the labor- 

 atory it is not my intention here to go into a detailed description ol 

 them, but merely to mention, with notes on their control, a few ot the 

 more economic forms. 



The following is a key to the genera: — 

 A. Perithecia with a single ascus. 

 1 



Appendages simple, flexuous, 

 and undivided at tip, 



Appendages once or more 

 dichotomously divided at tip. 



B. Perithecia with several to man\ asci. 



I. Appendages never more than 

 slightly swollen at base. 



Sphaerotheca. 

 Podosphaera. 



