48 FIRST REPORT ON ECONOMIC BIOLOGY. 



a period of five years, as proved by careful experiments conducted at 

 Kew. This infection is brought about by the liberation of the resting- 

 spores of the fungus into the soil. When a potato becomes diseased 

 the presence of the fungus in the tissues enables other fungi and 

 bacteria to gain admission, and the tuber often rots and decays under 

 the combined influence of these various organisms before the time for 

 lifting the crop arrives. No amount of legislation can prevent the 

 rotting of such potatoes, and the consequent liberation of resting- 

 spores in the soil. It is an insult to any potato-grower to caution him 

 against using for ' seed ' potatoes obviously attacked by black scab ; 

 on the other hand, if he uses ' seed ' so slightly attacked that he 

 cannot see the disease, he is not responsible for the result. The only 

 suggestion that can be offered is that of procuring ' seed ' from a 

 district where the disease has not been notified. So far as is known, 

 the disease is confined to the potato, consequently when land is known 

 to be infected, by having produced a diseased crop, do not attempt to 

 grow a crop of potatoes again for at least six years, unless very 

 stringent measures are taken to destroy the organisms present in the 

 soil. The land must be fallowed and dressed with gas-lime, at the 

 rate of from four to five tons per acre. Slightly diseased potatoes 

 should not be given to animals raw, but should be always boiled 

 first." 



FiNQER-AND-TOE DISEASE. 



Plasmodipphora brassicae, Wor. 



This fungus was first described by Woronin in 1878, and his 

 observations have repeatedly been confirmed. The fungus in ques- 

 tion belongs to a family known as Myxomycetes or Slime Fungi, which 

 are characterized by the absence of cells or cell walls during their 

 vegetative period, but later, the protoplasm becomes divided up into 

 small portions, each of which is ultimately surrounded by a definite 

 cell wall. These are termed spores, and may retain their vitality for 

 several years. Under suitable conditions, germination takes place, 

 and part of the contents become exuded. At first the cell wall is 

 noticed to crack and a small portion of the protoplasm passes out. 

 Little by little it becomes attenuated into a long hair-like body, the 



* Largely abstracted from a lecture delivered before the Warwickshire Chamber of Agriculture, 

 May and, 1908. 



