NOCTURNAL PRODUCTION OF CONIDIA BY 
SCLEROSPORA GRAMINICOLA 1 
By Wieuam H. Weston, Jr. 
Formerly Pathologist in Charge of Downy Mildew Investigations , Office of Cereal Inves¬ 
tigations, Bureau of Plant Industry, United States Department of Agriculture 
INTRODUCTION 
While studying the two conidial Sclerosporas (S. philippinensis Weston 
and 5 . spontanea Weston) so destructive to maize in the Philippines, the 
writer found that in these species conidia are produced only at night 
when the host surface is covered with dew or other moisture. This 
naturally raised the question whether -S’, graminicola (Sacc.) Schroet., 
the type species of the genus, exhibits the same peculiarity. The fact 
that it does and the bearing .of this fact on our knowledge of the im¬ 
portance and relationship of the conidial stage of the species are dis¬ 
cussed in the present paper. 
The genus Sclerospora was established by Schroeter (22) 2 in 1879 
on 5 . graminicola, a species which since then has been found quite 
widely distributed throughout temperate and tropical parts of the 
world, principally on species of Setaria, and rarely on other Gramineae. 
In the course of its life history 5 . graminicola , like most other Perono- 
sporales, passes through two phases of development: The one, charac¬ 
terized by production of immediately germinating conidia, achieving 
rapid spread; the other, characterized by formation of resistant oospores, 
serving to insure survival through such unfavorable conditions as winter 
and drought. In 5 . graminicola the conidial stage, which usually devel¬ 
ops first, appearing as a whitish downy growth on the surface of chlo¬ 
rotic areas of the host, generally seems to be of short duration, rela¬ 
tively inconspicuous, rather rare, and involves but little apparent in¬ 
jury to the host. As a result, this stage has not been commonly or 
abundantly collected and is represented by scanty and unsatisfactory 
herbarium material. Moreover, it has not been studied in detail. In 
species of such related genera as Plasmopara (10, 28) and Peronospora 
(6, 7, 8)> the conidial condition has been investigated intensively, quan¬ 
titative measurements have been made, restrictions of parasitism have 
been tested, and morphological aspects of all stages of development 
have been worked out minutely and illustrated fully. In 5 . graminicola, 
however, the conidial stage, save in such publications as those of Butler 
(2), Kulkarni ( 12 ), Shirai (24), et al., has been dismissed summarily with 
brief diagnostic or morphological descriptions, few measurements, and 
inadequate'lllustrations. 
On the contrary, the oosporic phase of Sclerospora graminicola which 
follows the conidial with a marked distortion and shredding of the leaves 
and floral parts of the host, is persistent, conspicuous, abundant, and 
obviously severely destructive to the host. As a result, it has been 
collected frequently and in abundance, and is well represented in most 
1 Accepted for publication Dec. 14, 1923. 
a Reference is made by number (italic) to “literature cited/’ 783-784. 
(771) 
Journal of Agricultural Research, 
'Washington, D. C. 
Vol. XXVII, No. 10 
Mar. 8, 1924 
Key No. G-353 
