POTATO WILT, LEAF-KOLL, AND RELATED DISEASES. 43 



In 1912, it was exceedingly prevalent in some fields of Green Moun- 

 tain in Aroostook County, Me. The number of plants affected 

 varied from 1 per cent up to practically 100 per cent. Some fields 

 of several acres were seen where hardly a normal plant could be 

 found. The disease was present again in 1913 in the same district, 

 always on the Green Mountain variety. Mosaic has not been found 

 in the potato districts of Wisconsin, Minnesota, Colorado, or other 

 Western States, though an extended survey of these States was 

 made in 1912 and 1913. 



There is evidently much difference in varietal susceptibility. 

 Hundreds of fields were examined in Maine where the Green Mountain 

 variety was growing side by side with Irish Cobbler, but practically 

 no mosaic was observed in the latter, whereas it was very common in 

 the former. There appears to exist also a corresponding difference 

 in the tendency of strains or stocks of the same variety toward mosaic. 

 Different fields of the Green Mountain variety showed from none to 

 100 per cent of diseased plants. An experiment in the Arlington 

 greenhouses further demonstrated this point, though undertaken for 

 another purpose — the control of silver scurf. Two greenhouse beds 

 were planted with the variety Eureka, using seed from two sources. 

 One lot showed 46 mosaic and 31 healthy plants, eliminating doubtful 

 cases, or 59.7 per cent diseased. The second lot had 100 per cent 

 free from mosaic. Portions of the first lot had been treated with 

 formahn, corrosive sublimate, and heat, with control lots untreated. 

 These treatments did not appreciably affect the proportions of 

 mosaic which developed. 



That mosaic is transmitted through the tubers is thought to be not 

 improbable. An experiment to test this was carried out in Maine 

 during the past season with somewhat inconclusive results. Tubers 

 from mosaic hills marked in 1912 were planted in hiU-unit rows, with 

 controls. The progeny were in part mosaic and in part of a doubtful 

 character, smaller and less vigorous than the controls, but with less 

 clearly marked mosaic than the parent hills. On account of some 

 confusion of the labels, it is thought best to repeat the test before 

 drawing conclusions. 



The cause of potato mosaic is unknown, nor have experiments been 

 made to determine whether, like the mosaic of tobacco, it is commu- 

 nicable from plant to plant. Allard (1912) has shown that the tobacco 

 mosaic can not be transferred from tobacco to potato by inoculation. 

 The exact nature and relationship of potato mosaic to other similar 

 troubles remains to be worked out. In this article, which is primarily 

 diagnostic, it is aimed to point out that such a disease exists and 

 that it may become a factor in the problem of varietal deterioration 

 of such importance as to require consideration when selecting or 

 inspecting seed stocks for certification or purchase. 



