178 THE POTATO 



branch, and produce small pear-shaped bodies on 

 the tips of the branches. These latter structures, 

 known as sporangia, serve to spread the disease 

 to other parts of the field. They are very readily 

 detached from the filament upon which they are 

 grown and then fall upon the soil, or are carried 

 far by the wind. If they happen to fall upon a 

 potato leaf they will begin to grow just as soon as 

 a little moisture either from rain or dew is present. 

 This growth consists either in the formation and 

 discharge upon the surface of the leaf of several 

 free swimming spores, capable of infecting the 

 plant, or in the direct formation of a filament 

 which enters the leaf through a stoma and develops 

 again a mycelium. From this mycelium other 

 similar reproductive bodies are formed, in turn, to 

 further infect the field. 



*' During the time that the fungus is spreading its 

 mycelium through the tissues of the leaf there is 

 little to indicate its presence. When the fruiting 

 stage is reached it soon becomes evident enough by 

 the formation of brown spots, which grow grad- 

 ually larger and larger, finally turning black and a 

 little later decomposing and emitting a disagreeable 

 but characteristic odor. If one of these . infested 

 areas be examined closely it will be found to be 

 bordered by a grayish white mildew. This latter, 

 under examination with a lens, is seen to be the 

 branched fruiting hyphse bearing the sporangia 

 described in the preceding paragraph. 



'* For the development of the mycelium — that is, 

 for the growth of the fungus within the potato 

 plant — moderately cool weather seems the most 

 favorable. For this reason this disease rarely 

 proves troublesome where high temperatures pre- 

 vail for considerable periods of time. Spore pro- 



