114 THE PHYCOMYCETES 



Classification. No recent comprehensive monographic treat- 

 ments of downv mildews exist. That of Berlese (1903) now has 

 a Hmited usefulness. Wilson's (1907, 1914) accounts deal only 

 with approximately 40 North American species. Gaumann's 

 (1923) monograph of Peronospora treats of 140 species occurring' 

 in Switzerland. He divided the oenus into two subgenera, 

 Leiotheca and Calotheca, on the basis of whether the oospores 

 have smooth or verrucose walls, respectively. Each subgenus 

 contains 2 groups. In Leiotheca the parasiticae include 36 thick- 

 \Aalled species, and the effusae, 72 thin-walled species. In Calo- 

 theca the verrucosae include 5 species with hemispherical warts, 

 and the reticulatae, 28 species wdth a netw^ork of anastomosing 

 ridges. Species are separated largely on the basis of biometrical 

 measurements of sporangia and of the results of cross-inoculation 

 trials. 



In identifying downy mildews the mycologist depends almost 

 wholly upon the use of a host index. Such a procedure has little 

 to commend it and indicates the urgent need of a usable mono- 

 graph. 



General considerations. It may be of more than passing in- 

 terest to point out that all the dozen or more known species of 

 Sclerospora, with one doubtful exception, appear to be confined 

 to grasses. Sclerospora gi-aminicola attacks not only corn and 

 suorar cane but also a larg^e number of other grrasses. Several 

 Other species are known to attack corn in the tropics. Sclero- 

 spora viacrospora also attacks a wide variety of grass species, in- 

 cluding rice, oats, and wheat. Some species are known only in 

 the conidial stages and appear to lack the ability to form oospores. 



Bremia lactiicae, common on lettuce, seems to be a monotypic 

 species. 



Downy mildews are of rare occurrence on trees and shrubs. 

 Feronoplasmopara celtidis on hackberry (Celtis spp.) and P. 

 meliae on chinaberry (Melia azadarrach) are notable exceptions. 



The discovery in 1882 of the fungicidal value of Bordeaux mix- 

 ture by Millardet (1933), marking the beginning of spraying, is 

 a landmark in the history of plant pathology. This discovery 

 may be attributed to two circumstances: (1) the outbreaks of 

 downy mildew in the vineyards of France, and (2) the propen- 

 sity of boys, on certain occasions, to pilfer fruit. 



It is an interesting coincidence that the use of volatile fungi- 



