POTATO WILT, LEAF-ROLL, AND RELATED DISEASES. 51 
good type of curly-dwarf in another cross between the same parents, 
No cases are recorded where both leaf-roll and curly-dwarf were 
found in the same seedling number, but there are several instances 
where diseased and healthy plants occur in the same row. The 
results of 1913 are more striking in their proof of the hereditary nature 
of leaf-roll and curly-dwarf. This field contained 20 hills of each 
variety, planted with 10 tubers, each cut in half, and the two halves 
of each seed potato dropped in adjoining hills. As a general rule, 
all the 20 hills were uniformly diseased, as shown in Plate VI, 
illustrating No. 2171, one of the best types of leaf -roll in the collec- 
tion. Two plants from this row are shown in a closer view in Plate V. 
Compare also Plate XII, showing the uniform affection by curly- 
dwarf of Nos. 821 and 822, which are hybrids between Sophie and 
Keeper. 
In several cases in 1913 only a portion of a variety was affected, 
but with few exceptions the two hills originating from one tuber 
behaved alike. Row No. 1763, for instance, had two hills with 
leaf-roll, then four normal, then two leaf-rolled. Row No. 1613 
had the first pair of hills normal, the second and third leaf-rolled, the 
fourth and fifth pairs normal, and all the remainder leaf -rolled. 
Other examples of similar inheritance of curly-dwarf are cited on 
page 38. 
WESTERN OUTBREAK OF LEAF-ROLL. 
For many years there has been an important center for potato 
production in Weld County in northern Colorado, known as the 
Greeley district. More recently a considerable acreage of potatoes 
has been grown on the North Platte River in western Nebraska. 
Since the average rainfall at Greeley is not large, all potatoes 
must be grown under irrigation. The potatoes generally receive 
sufficient rain in the spring to keep them growing until July, when 
irrigation is begun and repeated as needed. Rotations of crops have 
been generally practiced. A common one is, alfalfa two or three 
years, potatoes, beets, and grain. The methods of culture have been 
considered good, and large yields were secured for years. It has 
been estimated that 35,000 to 40,000 acres are annually planted to 
potatoes in the Greeley district. The total yield per year was stated 
by Bennett (1907) to be 9,000 to 14,000 carloads, or 4,000,000 to 
6,000,000 bushels. This crop has been the greatest factor in pro- 
moting the prosperity of this section. The leading varieties have 
been Pearl, Rural New Yorker, and Early Ohio. 
Some difficulties had been experienced from diseases of potatoes 
previous to 1910. The greatest stress had been laid on the Rhizoc- 
tonia stem-blight, a trouble which assumes a peculiar form in this 
western country. (See under "Rosette," p. 40.) Potato culture has 
been, in fact, restricted to the lighter soils, the physical condition of 
