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Journal of Agricultural Research voi. xxvii, No. w 
production. These observations on 5 . graminicola corroborate the writ¬ 
er’s earlier decision (32), that persistent dew or other moisture on the 
infected leaves is of primary importance. More precise physiologic 
study probably will show, however, that the relationship is more subtle 
and complex than this would imply. 
In any case, now that we know that Sclerospora graminicola pro¬ 
duces its conidia only at night when the infected leaves are covered with 
a layer of moisture, we are in a position to study the dispersal of the 
conidia, their relation to the dissemination of the species, and the part 
the conidial stage as a whole plays in the life history of the fungus; to 
investigate intensively the physiologic and morphologic aspects of its 
parasitism, and the immunity or susceptibility shown by more or less 
related hosts; in short, to determine facts of immediate application to¬ 
ward the control of this destructive parasite. 
SUMMARY 
In the peronosporaceous genus Sclerospora, oogonial and conidial 
stages are known. Both of these develop regularly in the type species, 
S. graminicola (Sacc.) Schroet. Of the nine remaining species four are 
known as yet only in the oosporic condition. All the other five alike are 
predominant and destructive in their conidial phase—the oogonial being 
absent or very rare; all show germination of the conidia by hyphae; and 
all occur in the oriental Tropics on grasses of the tribes Maydeae and 
Andropogoneae. 
To these five conidial species, 5 . graminicola is apparently in distinct 
contrast; for, although it does, indeed, develop a conidial stage, the 
oogonial condition is the predominant and obviously destructive one; 
the conidia (zoosporangia) germinate by zoospores; and the distribution 
is world wide—the host plant usually being a grass of the tribe Paniceae 
(Setaria, etc.). 
Despite these differences, however, the writer finds that the conidial 
phase of Sclerospora graminicola shows certain of the same fundamental 
features of development which he has found recently to be always present 
in three of the five typically oriental species. The first fundamental 
feature is that 5 . graminicola produces its conidiophores only at night 
and when the surface of the infected leaves is covered with a layer of 
dew or similar moisture. 
The second fundamental feature is that this production of conidio¬ 
phores runs a well-defined course as follows: The knots of stout hyphae 
crowded in the substonfatal air chambers push out prolongations through 
the stomatal slit, and form bulbous outgrowths which elongate succes¬ 
sively to clavate stalks; and these in turn develop at their tips a more or 
less extensive branch system, and ultimately sterigmata and conidia. 
The third fundamental feature is that this process of conidiophore pro¬ 
duction follows a regular nocturnal schedule which is vitally dependent 
on the presence and persistence of dew or other moisture on the leaf 
surface. 
The writer describes and illustrates conidiophore development in 5 . 
graminicola and compares the species with 5 . philippinensis and 5 . spon¬ 
tanea in the regularity and dependence on moisture of its nocturnal 
schedule. The fact that conidiophore production is nocturnal only, 
ta t during the day there remain only remnants of the previous night’s 
