LESSONS FROM POTATO CONFERENCE. 
119 
the latitude and altitude are." This is very comforting to Scotsmen, 
but they will require to do their part by seeing that their stocks are 
clean, and they must be prepared to go to any amount of trouble to 
keep them so. For several years Dr. Quanjer has worked in conjunc- 
tion with Dr. Oortwyn Botjes. Dr. Botjes isolated the healthy 
strains of the different varieties on his farm of 100 acres, and Dr. 
Quanjer isolated the different diseases. Their work reads like the 
remarkable work done in the medical research world in connexion with 
malaria and yellow fever. It was Botjes' efforts to try and eliminate 
leaf -roll by careful selection of healthy plants that first called 
Quanjer' s attention to the injurious influence diseased plants had 
on their healthy neighbours. Let me quote in Quanjer's words what 
followed : 
" The aphid experiments were undertaken during the last three 
years in insect-proof cages by Dr. Botjes for leaf-roll and by myself 
for mosaic. Selected and numbered tubers from healthy grown plants 
were cut into four pieces; one series of fourth parts was, during the 
latter part of the winter 1918-19, grown in a hot-house to control their 
state of health ; another series of fourth parts of the tubers, which 
proved healthy, was afterwards grown for being infected by green- 
fly on leaf -rolled plants ; another series for the mosaic-infected green- 
fly ; and the last series for green-fly fed on disease-free plants. All 
other insects were excluded from the experimental plants. We never 
came to a decision the same year whether the transmission of the 
disease took place, since the symptoms of primary disease are not 
always to be trusted. But we always cultivated the progeny of each 
plant treated and waited for the symptoms of secondary disease the 
following season. It was a slow method of experimenting, slower than 
that followed by our American colleagues, but the evidence obtained 
was conclusive. Every plant on which leaf-roll aphides were fed got 
leaf-roll progeny ; every plant on which mosaic aphides were fed got 
mosaic progeny ; and every plant on which healthy aphides were fed 
gave healthy progeny." 
It is time I proceeded to say a little about preventive measures 
suggested by the writers of these papers. What we growers have been 
often inclined to dismiss simply as degeneration in a variety has been, 
I fear, in many instances an undermining of the constitution brought 
about by disease. Is the road out of all the trouble the raising of new 
varieties from seed ? Quanjer tells us that " the mosaic diseases 
of tobacco, mangolds, sugar, beet and cucumber cannot be transmitted 
by the grain (true seed), but this has not yet been conclusively proved 
in the potato." There is a point here for breeders — work on parents 
that are known to be absolutely free of mosaic and leaf-roll. But, 
what lines are growers for seed to take with their general crops ? Get 
stock seed from disease-free stocks, get it from the north, and if any- 
diseased plants appear get them rogued out as soon as observed. In 
Holland a system of inspection has been in operation since 1903— 
a co-operative method arranged by the farmers themselves. In 
