July 14, 1923 
Degeneration Diseases of Irish Potatoes 
109 
SELECTION AND ISOLATION OF HEALTHY STOCKS 
It is possible to find fields that are healthy, either absolutely or com¬ 
paratively. Then it becomes important to know what to expect of 
such fields under known conditions of isolation from diseased stocks, 
and what to do with stock from such fields to maintain the existing 
state of health. Usually when healthy stocks are secured by potato 
growers they are planted in such a way in relation to diseased stocks 
that there is a chance to study the effects of proximity rather than of 
isolation, so that proximity will be considered here also. 
The results of planting rows of healthy stock alternating with mosaic 
stock have been described on page 102. In 1918 and 1919 rows of Irish 
Cobbler plants contracted more leaf roll next to leaf-roll rows than when 
farther removed (41 , p. 76-77). As in the mosaic experiments, the 
effects of the various factors, such as contact, soil, and virulent insects, 
can not be distinguished. Murphy has disclosed the spreading of mosaic 
and leaf roll across several rows in various parts of Canada (29, p. 47-51 , 
64-65 ). Krantz and Bisby state that mosaic dwarf spreads rapidly in 
Minnesota ( 21 , p. ig-22). 
In 1920, two Green Mountain fields with less than 1 per cent mosaic 
were observed. One was small, consisting of four rows, and was planted 
about 30 meters distant from four rows of diseased Irish Cobblers that 
came from a farm where all degeneration diseases are present. In 1921, 
about 10 per cent of the hills were diseased, with leaf roll, mild mosaic, 
rugose mosaic, leaf-rolling mosaic, and spindling-tuber disease. Some 
disease appeared even in volunteers from tubers left in the field before 
these were destroyed by chewing insects. The other field was large, and 
was planted about 50 meters distant from an American Giant field with 
24 per cent of the hills mosaic. Samples and bulk stock from this field 
contained less than 1 per cent again in 1921. 
Two growers in southern Maine isolated very healthy Green Mountains 
sufficiently in 1921 for the stock in 1922 (seen by the writers) to have less 
than 1 per cent disease, one grower giving the distance of isolation as 
about one-fourth mile. 
In 1921, the writers selected samples in several fields planted 
with healthy stock and located at different distances from fields 
planted with diseased stock. During the growing season observations 
on the percentage of diseased plants as well as on the aphid infestation 
were made. Peck samples were harvested from different parts of each 
field. These were planted in 1922 and readings on the percentage of 
mosaic were taken. Further data on these observations are presented in 
Table XXVIII. 
The results in Table XXVIII disclose that the mosaic percentage in¬ 
creased in each of the stocks and that the greatest increase obtained in 
field 38, where no hill was farther than 6 meters from Bliss Triumphs 
having 40 per cent mosaic and where there was a heavy aphid infestation. 
The stock in field 42 happened to be the one described previously as 
being 50 meters distant from mosaic American Giants in 1920 and as 
showing less than 1 per cent mosaic in 1921. Apparently, the location 
and other conditions of this stock were more favorable for infection with 
mosaic in 1921 than they had been in 1920, for one sample gave progeny 
with 66 per cent, and the bulk stock of this field showed 15 per cent 
mosaic in 1922. The results in this table show that mosaic sometimes 
spreads very readily from diseased to adjacent healthy lots, that prox- 
