INVESTIGATIONS OF POTATO WART. 
21 
the American variety and seedling collections also. Individual 
plants showing particularly marked symptoms were selected that 
year, and tubers were saved from these for planting a mosaic and 
leaf-roll pathologium in 1921. The immune stocks so selected were 
grown in the most heavily infested garden available, where sus- 
ceptible sorts regularly showed total infection. The results of this 
experiment appear in Table 5. 
In 1922 a further planting was made of stocks of certain English 
immune varieties imported in 1919, which h«ad since become thor- 
oughly degenerate and exhibited pronounced symptoms of crinkle, 
mosaic necrosis, streak, and leaf-drop, combined in some cases with 
extreme dwarfing. 
It is noteworthy that not a single instance of deterioration of im- 
munity was observed, although all plants and tubers were examined 
minutely, and susceptible sorts, interplanted, were generally infected. 
Table 5. — Tests of the stability of wart immunity in degenerate stocks of potato varieties 
at Freehand, Pa., in 1919, 1920, and 1921. 
Variety. 
1919 
1920 
1921 
Foliage. 
Wart. 
Foliage. 
Wart. 
Foliage. 
Wart. 
Abundance 
Healthy 
do 



Leaf-roll 















Leaf-roll and mosaic. 
.....do 

do 

Dargill Early 
Mosaic 
Healthy 
do 
o 
Edzell Blue 


Leaf-roll 
Leaf-roll and Mosaic. 
do 
o 
Majestic 
May Queen 
do 

Mosaic 
Mosaic 

Early Petoskey 

Healthy 
do 
do 
do 
Curly dwarf. . . 
Mosaic 
do 
do 
do 
Healthy 




°o 




Leaf-roll 
Leaf-roll and mosaic. 

Extra Early Sunlight . . . 
Green Mountain Junior. 
Curly dwarf 
o 
ao 
o 
Irish Cobbler 
Keeper 
Leaf-roll and mosaic. 
Curly dwarf 
Leaf-roll and mosaic. 
Curly dwarf 


Round Pinkeye 
Seedling No. 3615 
Seedling No. 37662 
Seedling No. 38899 
Leaf-roll and mosaic. 
Leaf-roll and mosaic. 
do 

o 
o 
do 
do 

Seedling No. 39304 
Richter l 
do 
Healthy... 

(2) 
do 

( s ) 
Rural New Yorker 
do... 
1 This, a slightly susceptible variety, was introduced into the test to 
diminished in degenerating stocks. 
2 One warted tuber. 
3 One warted stolon. 
* Wart 50 per cent. 
if its tendency to resistance 
During an experience covering three years in the culture of the 
immune varieties, Edzell Blue, Spaulding Rose, Irish Cobbler, and 
Green Mountain, in the quarantined area of eastern Pennsylvania, 
many cases of mosaic and leaf -roll infection have been observed, but 
in no case has immunity to wart been affected. Particularly during 
1921 were the potatoes grown in this district subjected to very 
adverse conditions brought about by drought, warm weather, severe 
aphid and flea-beetle injury, and competition with weeds. Although 
plantings of susceptible potatoes continued to develop wart under 
even these conditions, no immune variety has yet broken down. 
Similarly, injuries to the tuber caused by common scab or by milli- 
pedes and burrowing insect larvae have never been observed to pre- 
dispose the affected part to wart infection, although such injuries 
must expose the cortical and meristematic tissues beneath the pro- 
tective periderm. 
