POTATO WILT, LEAE-ROLL, AND RELATED DISEASES. 29 
source of this seed was thought to be Maine. Dr. I. E. Melhus found 
another field in northern Maine with 100 per cent leaf-roll, a note- 
worthy occurrence when the extremely vigorous and healthy condi- 
tion of the potatoes in that district is considered. 
As stress will be laid, later in this bulletin, on the development of 
leaf-roll in seedling varieties, in connection with the problem of 
controlling this disease, it will be well to describe the collection which 
formed the basis of the writer's studies. 
HISTORY OF THE SEEDLING COLLECTION. 
In 1904 Prof. L. R. Jones, then botanist of the Vermont experi- 
ment station, was sent to Europe by the Bureau of Plant Industry 
to search for potato varieties resistant to late-blight, Pliytoflitkora 
infestans. He brought back about 100 varieties, which were placed 
on trial at several points, including the Vermont experiment station 
at Burlington, where Prof. Stuart began crossing the European sorts 
with each other and with American varieties. Notable success was 
achieved in 1909 in securing seed from a large number of crosses; 
about 25,000 seedlings were raised the following season, propagated 
that year in Washington, the following year in New York, and in 1912 
in both New York and Maine. 
It had been observed by Prof. Stuart that some of his seedling 
varieties from earlier crosses exhibited sudden loss of vigor. Occa- 
sional numbers which had in the beginning showed promise would 
produce only weak or abnormal progeny. 
DISEASE PHENOMENA IN THE SEEDLINGS. 
Such was the condition found by the writer in the breeding fields 
in Maine and New York in 1912. These fields consisted of 10 and 16 
acres, respectively, and contained over 10,000 seedlings of known 
parentage, 5 hills of each sort. 
As might be expected, these seedling potatoes showed every degree 
of variation in plant characters, color and size of leaves, habit of 
growth, etc., but in addition many- showed distinct evidence of a 
diseased condition, and indeed of quite distinct types of disease,, 
which are herein described as leaf-roll, curly-dwarf, and "streak." 
It is noteworthy that in neither field was there any trace of Fusa- 
rium wilt, nor of Verticillium wilt, blackleg, or mosaic disease, 
although the latter three were common in adjoining fields. This is a 
very important fact, since it strongly supports the argument that 
these are distinct diseases. The reason for the nonoccurrence of these 
troubles is that the seedling varieties, since their origin from seed, 
had been grown from selected tubers, and no stronger proof is needed 
that such diseases may be controlled in commercial seed growing by 
the tuber-unit selection method, applied, if need be, to a seed plat of 
Mini ted area, from which the main crop is propagated. 
