J01KAL OF AGMCOLTPRAL KESEARCH 
Vol. VIII Washington, D. C., February 12, 1917 No. 7 
STUDIES OF THE GENUS PHYTOPHTHORA 
By J. Rosenbaum, 1 
Mycologist , Cotton, Truck , and Forage Crop Disease Investigations t Bureau of Plant 
Industry t United States Department of Agriculture 
INTRODUCTION 
From a study of one of the species of Phytophthora it appeared that 
there was a great need of a comparative study under the same conditions 
of all the described species of the genus. Hitherto, the various species 
had never been gathered and grown by any one investigator; nor had 
comparisons been made when grown under similar conditions; nor had 
any general survey of the genus been made since it was established, 
although species had been added from time to time. The cytology of the 
genus has likewise been almost entirely neglected. As a result of this, 
together with the imperfections in the descriptions in some cases, it is 
almost impossible to identify a species with any degree of accuracy. 
Moreover, because of the scarcity of spore forms on some of the original 
hosts, as well as their variation when present, herbarium specimens are 
practically useless for purposes of identification. Another reason which 
has doubtless tended to check work with this genus is the difficulty 
encountered in growing some of the species of Phytophthora in culture, 
even after they have been obtained pure on artificial substrata. 
In this group of fungi it is often not sufficient in making transfers to 
remove a bit of mycelium. The coenocytic nature of the mycelium 
necessitates the transfer of an especially long strand so that the cells 
may remain unbroken. They are so difficult to grow that some of the 
species, such as P. thalictri and P, agaves , the latter of which has not yet 
been described, have not so far been reported in culture. 
The object of this paper is to present the results of the writer’s studies 
of the genus from a systematic and a biometrical standpoint. A tenta- 
1 The writer began this work in the Department of Plant Pathology of Cornell University, and com¬ 
pleted the major portion of it as a member of the Office of Cotton, Truck, and Forage Crop Disease Investi¬ 
gations. He wishes gratefully to acknowledge the many courtesies shown him by Prof. H, H. Whetzel 
and Dr. Donald Reddick, in whose laboratories the work at Cornell University was carried on; the valuable 
suggestions received from Dr. Raymond Pearl, of the Maine Agricultural Experiment Station, and Dr. 
H. H. Love, of Cornell University, in connection with the biometrical portion of the paper; and the aid 
given by Mrs. N. E. Fealy, of the Department of Agriculture, in editing the manuscript. 
Journal of Agricultural Research, 
Washington, D. C. 
hd 
(233) 
Vol. VIII, No. 7 
Feb. 12, 1917 
Key No. G—106 
