DI.— THE STABILITY OF WART IMMUNITY. 
By Freeman Weiss. 
m Since the use of immune varieties continues to be the most effec- 
tive means known of dealing with potato wart, both as to avoiding 
the loss of the potato crop in infested districts and as to restricting 
the extension of the pathogen beyond its present range, the question 
of the permanence of immunity is of great concern to growers and 
quarantine officials alike. Fortunately, the information which has 
been adduced in England and in Germanv from immunity tests, 
which have extended over a much longer period than the similar line 
of investigation in the United States, furnishes a satisfactory answer 
to this question. In Leaflet 105, 20 issued in 1918 by the British Board 
of Agriculture and Fisheries, the following statement appears: 
The trials carried out at Ormskirk . . . demonstrate beyond all doubt the 
absolute immunity ... of certain varieties. Disappointment in the past as 
to resistant varieties has been due either (a) to wrongly named seed, or (6) to the 
presence of "rogues," or (c) to the use of varieties which, though formerly supposed 
to be immune, had not been properly tested on badly and uniformly infected soil. 
As far as known the immunity of no variety which has been thoroughly tested has as 
yet broken down. 
Subsequent evidence has not yet appeared from the English tests 
which in any way requires the modification of this conclusion, and the 
experience of three years with immune varieties in the United States 
is wholly confirmatory. It is significant that the list of immune 
varieties which was published 21 as a result of the first year's trials has 
not required amendment except to add new varieties. 
SchafTnit, 22 however, holds that immunity is not absolute, but 
only relative; that varieties showing immunity under normal con- 
ditions may lose this faculty as soon as the plant no longer exhibits 
its normal form of development or is weakened as a result of tuber- 
transmissible pathological conditions expressed in leaf-roll and mosaic 
mottling. The opinion has been expressed by Gough, 23 however, 
that the loss of immunity reported by Schaffnit is probably the result 
of using originally impure seed stocks or the occurrence of mixtures 
during the test. 
Some data in this connection have been derived from the tests at 
Freeland, Pa. The immune varieties which were imported from 
England in 1919 exhibited in their first year a considerable propor- 
tion of leaf-roll and mosaic mottling. The seed for 1920 was saved 
from these same stocks and increased percentages of leaf-roll and 
mosaic, in some cases affecting the entire crop, were noted in the 
1920 plats. Both diseases appeared to a greater or less extent among 
20 Anonymous. Wart disease (black scab N i of potatoes (Svnchvtrium endobioticum Percival). Leaf- 
let Bd. Agr. and Fish. [Gt. Brit.], No. 10.5, 6 p., 4 fig. (Issued 1904, rev. 1918). 
11 Lyman, G. R.. and others. Op. cit. 
22 Schaffnit, E. Versuehe zur Eekiimpfung des Kartoffelkrebses in Jahr 191^-19. In Ztschr. Pfianzen- 
krank. Bd. 30, p. 59-67. 1920. 
43 Gough, G. C. [Discussion on immunity from wart disease.] 7» Rpt. Internat. Potato Conf.,1921, 
p. 103. [1922?] 
20 
