LESSONS FROM POTATO CONFERENCE. 
117 
This fact was confirmed by Jensen in 1883, and has since been further 
confirmed by the present writer under greenhouse and garden condi- 
tions, and by Melhers in America under field conditions, and there can 
be little doubt but that this is the source from which infection of the 
crop commonly occurs." What, a lesson is there on the absolute 
necessity for planting clean tubers ! Dr. Pethybridge proceeds to 
deal fully with spraying as a preventive of blight. It is now looked 
upon in many countries, and rightly so, he says, as just as necessary 
an operation in growing potatos as are proper cultivation and manur- 
ing. He discusses the possibility of raising varieties which will be 
immune to blight. So far some late ripening varieties are almost, 
if not quite, resistant, but such resistance seems to diminish with age. 
I must leave this interesting paper with this quotation : " Wherein 
the power of resistance to blight lies is not definitely known, but from 
the small amount of experimental work which has been done on the 
matter up to the present it would appear that it is not of a mechanical 
nature, such as thickness of skin, impenetrability of the cell walls 
and the like, but is due rather to the presence of some substance in the 
cells which inhibits the development of the fungus mycelium within 
the tissues. The subject is one of extraordinary interest and demands 
further investigation. Whether varieties exist which are absolutely 
immune to the blight is not known. The search for them or the raising 
of them by breeding are problems for the future." 
The last group of papers, those on Leaf -roll (Curl), Mosaic and 
allied diseases is most suggestive. The writers are Mr. Paul A. 
Murphy of the Irish Department of Agriculture, Dr. H. M. Quanjer 
of Holland, and Mr. A. D. Cotton of the Ministry of Agriculture. 
We are asked by Dr. Quanjer to drop the old name of " Curl " and 
adopt the term " Leaf -roll " instead for the specific disease, as the term 
"Curl" has been hitherto used in a general sense to cover different 
types of disease. There seems reason in this request — " Curl " has been 
with us for 150 years. It is time we knew the different varieties of it ! 
All the writers agree that leaf-roll is a serious, Quanjer calls it a 
dominant, disease in the South of England, the Southern States of 
America, and the Continent of Europe — at least, he adds, in certain 
varieties. All agree that the disease is carried in the tubers, and 
therefore that seed from infected crops spell failure up to 50 per cent, 
in the areas just mentioned. Cotton proved that in twelve centres 
in England last year. In his peregrinations in Scotland in 1920 he 
discovered a field of ' Arran Comrade 1 which was affected with leaf- 
roll. He secured 12 cwt. of that crop and 12 cwt. from a perfectly 
healthy crop growing some miles away in an adjoining county. He 
had 1 cwt. of each sent to twelve stations in England and grown, and 
found that the clean healthy stock yielded about double the crop of 
the other. When a gentleman in Mr. Cotton's position gets hold of 
a series of figures like these the matter is not going to rest there, and I 
quite agree it shouldn't. I got in touch with the grower of the 12 cwt. 
