INVESTIGATIONS OF POTATO WART. 6 
following season a systematic test of all obtainable American varieties 
was begun. In addition, investigations of a number of immune 
English varieties were made to determine their commercial value 
as well as their reaction to wart in the United States. A preliminary 
report 4 of these investigations was issued in 1920, covering the 
results of the trials of the first season. These tests established the 
fact that the wart pathogen was consistent in its behavior in Ameri- 
ca and Europe in that the immune varieties of Europe remained 
free from infection also in this country. It was furthermore shown 
that although most American varieties were susceptible, including 
some of the particularly valuable commercial types, 10 varieties, 
including the well-known Irish Cobbler, Green Mountain, and Spauld- 
ing Rose, could with reasonable assurance be classed as immune. 
As a result of these tests it became possible to modify the quarantine 
regulations in the Pennsylvania areas so as to permit the culture of 
the immune sorts, Edzell Blue and Spaulding Rose, in infested 
gardens, and the supplying of approved immune seed potatoes became 
a function of the quarantine administration. The general effect on 
potato culture of changing over from the mixed stocks of predomi- 
nantly susceptible potatoes to selected seed of immune varieties is 
described in a later section of this bulletin. A similar modification 
of quarantine procedure has been found advisable in the infested 
areas in West Virginia and Maryland, and it is now well established 
that potato culture need nowhere be abandoned on account of this 
disease so long as suitable immune varieties are available. 
The project of determining the wart reaction of all American 
potatoes and also their adaptability for commercial culture in the 
regions infested by wart and areas adjacent thereto has been con- 
tinued on a more extensive scale for three additional years. In 
general, methods of planting and culture similar to those described 
in the earlier report have been followed. The site of most of the 
trial plats has been the vicinity of Freeland, Pa., where gardens which 
are heavily infested naturally and are in favorable tilth for potato 
culture are already at hand. Since the same gardens, for the most 
part, have been employed again and again, accurate information as 
to the thoroughness and density of infestation is now available, a 
factor of great importance in analyzing the results. Additional tests 
have been carried on at Thomas, W. Va., and in the greenhouses of 
the United States Department of Agriculture at Washington, D. C. 
Single rows of a given variety or seedling, crossing the garden 
transversely and averaging 30 feet in length, have been the standard 
form of plat, but larger test plantings have been made in some cases. 
Ordinarily, only a single plat of each variety has been grown each 
year; that is, no attempt has been made to duplicate plats sys- 
tem atically, but in a number of cases certain varieties have been 
repeated in other gardens either in the same or a different region. 
At regular intervals of nine rows a susceptible variety has been 
planted as an indicator of the presence of the pathogen in a virulent 
state. As check varieties, Triumph and various stocks of Rural 
New Yorker and Up-to-Date have been employed; the last two 
named are very readily susceptible and react to infection by the 
* Lyman, G. R., Kunkol, L. O., and Orton, C R. Potato wan. U. S. Dept. A.gr.. Dept. Clrc. Ill, L9 
p., 4f]g. 1920. 
