71 



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. 



The fungus causing the disease, like the Plasmodio- 

 phora, lives in the cells of the potato buds as minute 

 specks 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 

 invaded 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 w^th 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 Agricuhure leaflet 105 and Wart Disease 

 Order, 1Q14, to be obtained free from 4, Whitehall Place. 

 London, S.W. 



