May 1, 1920 Philippine Downy Mildew of Maize 119 



predominance of the conidial stage, no oospores having been found con- 

 nected with any save Scleras pora sacchari, with which, indeed, the rela- 

 tionship is not very well established. 



Furthermore in all these species the conidiophores are large and 

 prominent with a differentiated basal cell, stout main axis, and extensive 

 dichotomous system of branches comprising large primary, secondary, 

 tertiary, and even quaternary branches. The germination of the conidia 

 also is invariably by means of hyphae. 



In contrast to these species the cosmopolitan Sclerospora graminicola 

 (Sacc.) Schroet., the type on which the genus was established, is char- 

 acterized by the predominance of the oogonial stage, the conidial phase 

 being comparatively rare; by its smaller inconspicuous conidiophores, 

 which lack a differentiated basal cell and give rise to few short primary 

 or at times secondary branches only; and by the regular germination 

 of the "conidia" by zoospores. 



Such marked and essential differences certainly appear to indicate 

 that these oriental species should be separated from the type as a different 

 genus; but, in the opinion of the writer, such a step can not be made with 

 justice until more is known of the conidial stage of Sclerospora graminicola 

 and of the oogonial stage of the oriental forms. 



Moreover, whether Sclerospora graminicola var. andropogonis-sorghi 

 Kulk. should be included with the oriental group by virtue of its well- 

 developed conidiophores and the germination of the conidia by hyphae, 

 as Ito (9) suggests, also depends on further knowledge of the points 

 just mentioned. 



When one considers the great variations in effect on the host and even 

 in such essential features as the characteristics of the conidiophores and 

 conidia, which have been found by the writer to occur in Sclerospora 

 philippinensis under different conditions of the environment at different 

 stages of its "development and on various hosts, one can not avoid a 

 suspicion that these oriental forms may in reality be a single species. 

 It is not inconceivable that such may be the case and that the variation 

 in effect on the host, the susceptibility of different plants in different 

 places, and the variations in structure of the causal organism may all 

 be due to environmental conditions of the regions in which they are 

 found. Obviously to settle these important points conclusively there is 

 need of extensive cross-inoculation experiments and of comparative 

 studies, using optimum material and methods which emphasize impor- 

 tant characters quantitatively as well as qualitatively. 



The problems of the origin of these destructive Sclerosporas of maize 

 and of their geographic distribution, their appearance in the Orient 

 where maize has only been introduced since about 1496, and their absence 

 as yet from the Western Hemisphere where maize originated, are all too 

 involved for consideration at present. 



