POTATO WILT, LEAF-EOLL, AND BELATED DISEASES. 31 



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 hOls. 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 hiUs originating from one tuber 

 behaved ahke. 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 hiUs normal, the second and third leaf-rolled, the 

 fom'th and fifth pairs normal, and all the remainder leaf-roUed. 

 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 pecufiar 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 



