Il8 JOURNAL* OF THE ROYAL HORTICULTURAL SOCIETY. 
infected stock. In 1919 he grew half a ton of the original stock 
of ' Arran Comrade ' and it behaved well but was attacked with green- 
fly. Growing alongside was 'Edzell Blue,' among which some plants 
were affected with leaf-roll. The 1921 crop which Mr. Cotton saw 
was grown from the produce of that half-ton — " leaf-curl or no leaf- 
curl, it did me very well," the grower told me. It was all sold for ware 
(except the 12 cwt. Mr. Cotton got), and it yielded 15 tons per acre ! 
It was a good thing it was all sold for ware ; had it gone to the South 
of England for seed, it would have been a serious matter for the man 
who planted it and for the reputation of the variety. 
Quanjer in his visit to Scotland declares he saw the process of 
infection going on ; ' Crusader ' which was growing alongside a variety 
badly infected with leaf -roll had contracted the disease where it was ad- 
jacent to the other variety, and was clean where it was not in proximity. 
At Kew Mr. Cotton in 1920 grew small lots of ' King Edward,' 
' Lochar,' and ' President,' selected from stocks which were perfectly 
free from disease. Alongside he grew an infected stock of ' Kerr's 
Pink. ' By the end of July all were infected with leaf-roll. A duplicate 
set of the clean stocks was planted half a mile away in an isolated 
position. ' King Edward ' and ' Lochar ' remained healthy right to 
the end, but two plants of ' President ' went wrong. It came from 
a supposed clean stock in Holland. The ' King Edwards ' and 
' Lochar ' were from Scotland. The crop in 1921 from tubers saved 
from these experiments proved that leaf-roll or mosaic developed 
from the infected plants in every case, and the healthy stocks repro- 
duced healthy stocks with the exception of two plants. 
Mr. Murphy grew in Ireland in 1921 duplicate sets of Mr. Cotton's 
Kew-grown tubers with the same results— tubers from plants exposed 
to infection all gave produce affected with leaf -roll, and from those 
grown in isolation healthy plants resulted. 
Mr. Cotton says his experiments " clearly explain the abundance 
of leaf -roll and mosaic in small gardens. Seed of all kinds is in use 
and there is always an abundance of infective material. ... A certain 
amount of clean northern seed is introduced by the best allotment 
holders each year, but the crop is promptly infected, the extent varying 
with the susceptibility of the variety and the nature of the season. 
Consequently, allotment seed usually deteriorates sooner than any 
other." 
All the three writers agree that the diseases we are dealing with 
now are highly infectious, and that the carriers of the disease are 
aphides or green fly. In this connexion it must be pointed out that 
these diseases are and can only be serious and virulent in districts 
where green-fly abounds, and that " the further north the less disease, 
until one reaches the extreme north of Scotland, where there is reason 
to believe that leaf-roll and probably mosaic also do not spread, and, 
consequently, that under entirely natural conditions they would not 
be found to occur " (Cotton). Quanjer confirms that. His words 
are : " In Great Britain we see that potato-seed is better the greater 
