July 14,1923 
Degeneration Diseases of Irish Potatoes 
95 
Mosaic is well distributed over the United States (45, p. 248 ; 14, p. 
158), having been reported in 1917 and 1918, from 21 States, including 
all those in which potatoes are an important crop. The existence of 
several types and the modification of symptoms by varietal and environ¬ 
mental factors make it probable that the true extent and meaning of 
the geographical distribution of mosaic is merely beginning to be fully 
realized. Sixty-eight strains of Green Mountains from seed grown in 
Canada, Maine, New York, Vermont, and Wisconsin, were tested in 
one place and found to have an average of 33 per cent mosaic (1, p. 11) 
and as high as 48 per cent (55, p. 8). Mosaic was common in seed- 
potato fields of Maine and northern New York in 1915 ( 46, p. 357). 
The average mosaic percentage for Green Mountains in New York and 
Aroostook County, Me., is given by Barrus (6, p . 13) as 50 per cent or 
more, with 20 per cent or less as rare. During 1919, the writers made a 
careful estimate of the amount of mosaic in 40 Green Mountain fields 
and the same number of Bliss Triumph fields in northeastern Maine. 
Many of these were supposed to be above the average in freedom from 
the disease. Mosaic varied from one-half per cent to 100 per cent, 
averaging 28 for the Mountains and 46 for the Bliss. Similar results 
followed the examination of fewer fields of these varieties in 1920, and 
20 supposedly choice American Giant fields were found to contain 
from 3 to 46 per cent, averaging 13. In Green Mountains in 1921, 26 
fields in northeastern Maine averaged 32 per cent, and 58 fields in New 
Brunswick, mostly in the northern part, averaged 13 per cent mosaic. 
Most of the mosaic in commercial fields in northeastern Maine and New 
Brunswick is of the mild type. Variation in Louisiana from 3 to 90 per 
cent in Bliss Triumps is reported (12, p. 8), with losses as high as total 
failure (12, p . 3). 
The reduction of yield rate by leaf roll has not been determined in 
plots. Leaf roll does not extend so far north as a prevalent disease as 
does mosaic. Both are prevalent in western New York and southern 
Ontario (29, p. 36), while in northern Ontario mosaic is troublesome but 
leaf roll is not (28, p. 5). The writers have found leaf roll much less 
common than mosaic in northeastern Maine in varieties badly affected 
elsewhere with both (41, p. 75). When leaf roll causes net necrosis in 
the tubers, the quality of the crop is affected as well as the yield. 
Leaf-rolling mosaic alone appears to affect the plants about as mild 
mosaic does, but in combination with spindling-tuber disease (giving 
a form of curly dwarf in Green Mountains as least) the yield rate is con¬ 
siderably reduced (46). 
The spindling-tuber disease seems to reduce the yield rate less by the 
first-year symptoms (in plants growing from normal shaped tubers 
produced by plants infected late in the season) than by the second- 
year symptoms (in plants growing from spindling tubers) (PI. 14, B). 
It is present in all percentages of incidence in several varieties in 
northeastern Maine. Its presence in at least 10 widely separated States 
is known from personal observations by the writers and from personal 
and written reports by pathologists. Its production of abnormally 
shaped tubers injures the quality of the crop for sale as seed and table 
stock. 
Rugose mosaic, streak, and unmottled curly dwarf are each more 
injurious to the yield rate than the four diseases just mentioned. They, 
and also leaf-rolling mosaic and spindling-tuber disease, are probably 
