REFLECTIONS ON DISEASE TREATMENT 173 
soil and the other with virgin soil, and six tomato plants 
were planted at equal distances apart in each trough. 
Five pounds of soil copiously infected with Verticillium 
albo-airum was introduced at one end of each box, and 
the temperature maintained as near as possible to 65° F. 
The tomato plant nearest the point of inoculation in the 
sterilized soil was the first to show signs of wilt, which 
appeared in twenty days. In six weeks all plants in the 
sterilized soil, but only two in the virgin soil, were 
infected. This indicates clearly that the fungus in 
question spreads more rapidly in sterilized soil than in 
unsterilized soil, and provides a reason for the fact that 
diseased soil may be more heavily infected two or three 
years after sterilization than it was before treatment. 
In sterilizing soil, therefore, every possible means to 
avoid reinfection should be taken. It is a good plan to 
sterilize with formaldehyde all brickwork under the 
ground and a foot above it, and any soil that cannot be 
treated by steam. 
Water Sterilization 
It has been shown previously (3) that the water supply 
of modern nurseries may be so highly contaminated with 
disease organisms as to form a continuous and important 
source of infection to glasshouse plants. In considering 
the elimination of this form of infection the grower is 
faced with two alternatives: he may either discard the 
contaminated well and construct a new one upon such 
lines as to ensure its freedom from pollution, or he may 
endeavour to cleanse the 6ld one. The former plan will 
appeal to growers as being the most satisfactory in the 
end. ‘The best wells are made by boring into the subsoil 
to a depth of 100 feet or more, and should be well bricked 
in for the top 10 to 20 feet, to prevent the entrance of 
surface drainage, by which means contamination enters. 
All deep artesian wells have proved practically free from 
fungi and should be used wherever possible. 
