24 Potato Disease and Allied Diseases [CH. 
tubers near the surface are often more diseased; and 
(4) that the browning of the tissues extends inwards. 
It has been shown that zoospores will not pass through 
a depth of 6 inches of sandy soil. An experiment was 
made by covering potatoes with this depth of soil to 
see if the tubers were thus prevented from becoming 
diseased. Although a number of the tubers became 
diseased the percentage was much less than in the 
case of tubers covered with a lesser depth of soil. It is 
important to know how the young tubers are infected, 
because if it is entirely by means of the zoospores we 
should hope to aim at preventing their infection by 
trying to keep down the formation of conidia or to 
prevent the zoospores from reaching the young tubers. 
It is possible that the tubers are infected directly by 
the growth of mycelium through the stem as well as 
indirectly by means of the zoospores. 
By killing the leaves of the potato plant the disease 
stops their functions as feeding and breathing organs, 
and as the plant is now capable of taking only very 
little food from the air the growth of the tubers prac- 
tically ceases. Having lessened the crop and reduced 
the value of the tubers lifted the disease has by no 
means finished its career. When slightly diseased 
tubers are stored in clamps, the disease spreads, 
especially when there is a sufficient supply of moisture 
and a suitable temperature. Diseased tubers also 
allow foreign organisms to enter thereby setting up the 
well-known “wet rot.” 
Take some diseased tubers, wash them, cut slices 
across the diseased areas with a sterilized knife and 
place them in a large sterilized Petri dish lined with 
moistened filter papers. 
