April, 1921] Powdery Mildews of Ohio 2ia 



The powdery mildews should be collected as soon as the 

 perithecia are mature, and if possible, while the mycelium is 

 still fairly conspicuous. Of course the combination can not be 

 had in species which have very evanescent mycelia. Finally, 

 the collecting should be done before wind and rain have caused 

 nearly all of the mature perithecia to disappear, or these fruits 

 have become so ripe that they fall off shortly after being 

 collected. More than once, such material collected in quantity 

 for study in the laboratory has proved satisfactory soon after 

 being brought in from the field, but has been found so nearly 

 devoid of perithecia by the next year as to be practically useless 

 for any purpose. 



For successful study, the conidial stage must be taken in its 

 prime (Fig. 2), and this is before the perithecia are mature. 

 Young mycelia with such conidial conditions may be found at 

 any time from the middle of June until late autumn. By seeking 

 silvery-white, often glistening stages, devoid of perithecia, any 

 species with a reasonably conspicuous mycelium will serve for 

 this purpose. Erysiphe graminis on grasses, the species on roses, 

 and some forms on plaintain are likely to give good conidial 

 conditions late in the season, while quite as good material may 

 be found on asters and other hosts. 



Mr. E. E. Duncan made the collections recorded from 

 Montgomery County. All others were made by the writer unless 

 otherwise stated, in the additions to hosts, distribution, and 

 species newly reported for Ohio to follow. 



Sphaerotheca humuli (DC.) Burr. 



Butler, Hamilton, and Montgomery Counties. This seems 

 to be the species that causes injury to our roses most commonly 

 throughout Ohio and other portions of the United States. 

 Spcerotheca pannosa (Wallr.) Lev. is said by Salmon to grow 

 commonly on the same hosts in Europe, but to be replaced 

 largely on the roses in America by the other species. Someone 

 might well try to ascertain the facts regarding the cause of the 

 disease of our roses. 



Sphaerotheca humuli fuliginea (Schlecht.) Salm. 



Montgomery County. Known on three or four hosts from 

 about as many localities in Ohio. 



