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rapidly increase in size, and several often run together 
till the potato becomes a mass of excrescences. Plants 
which are attacked often grow larger and bear green. 
leaves for a longer time than healthy plants. ie 
The fungus causing the disease, like the Plasmodio- 
phora, lives in the cells of the potato buds as minute. 
spécks of naked protoplasm. In this case the presence of: 
the fungus causes the invaded cells to enlarge, but renders 
them incapable of further division. On the other hand 
the healthy cells around are stimulated to such active 
growth and division that the warts are soon produced. 
Infection always occurs by motile spores from the soil, 
which can only penetrate the healthy cells of a potato 
in. the young condition. Later the potato forms a skin 
of corky cells through which the fungus cannot 
penetrate. After growing at the expense of the 
mvaded cells the fungus ultimately occupies the whole 
of the cell cavity, and then taking on a thick, very re- 
sistant wall, forms a resting spore. With the decay of 
the warty tubers these resting spores find their way 
into the soil and may remain there as a source of infection 
for many years. When they germinate after an 
exceptionally long period of rest the thick wall bursts and 
liberates large numbers of actively moving spores, each 
possessed of a single whip of protoplasm to propel it. 
These are the spores which infect new potato tubers. So 
far no method of successfully treating this disease by 
adding chemicals to the soil has been devised, but certain 
varieties of potato are much less susceptible to the disease 
than others, and it is advisable to grow these if the 
disease is present. Indeed, it is now compulsory for every 
person growing potatoes to do this and to follow certain 
other stringent regulations in areas where the Wart 
Disease is prevalent.* 
In addition to the Wart Disease there are several other 
scab diseases of potato tubers which, owing to a certain 
degree of similarity, may at times be confused with the 
Wart Disease. The Black Speck, or Violet Rhizoctonia 
Disease, caused by Rhizoctonia Violacea, can be detected 
by the minute size of the blackish warts which can be 
* See Board of Agriculture leaflet 105 and Wart Disease 
Order, 1914, to be obtained free from 4, Whitehall Place, 
London, S.W. 
