POTATO WILT, LEAF-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-bhght, Phytoplithora 

 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 seedlmgs were raised the following season, propagated 

 that year in Washington, the foUoAving 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 

 the^^.e 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 selocted 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 

 limited area, from which the main crop is propagated. 



