94 POTATO DISEASE. 



half to one line. The change proceeds from the edges of the cut 

 surface under the skin of the tuber, until the whole mass is envel- 

 oped in a brown coating. 



The disease penetrates deeper and deeper into the tuber, until 

 the latter is completely infected. If much moisture be present, 

 the mass dissolves to a dark, foul liquid ; otherwise it dries away 

 and shrivels together, as happens in the diseased potatoes in a dry 

 cellar. 



On the section of the inoculated half, patches of mold appear as 

 the discoloration commences. These extend rapidly, and when 

 magnified are seen to be the fruit bearing branches of the fungus. 

 They break out also through the skin after the parts underlying 

 have become brown by the ravages of the mycelium. 



With the other half of the potato, matters have gone on very 

 differently. A discoloration is indeed noticeable at first ; but it is 

 slight, and is due to the formation of a new skin. In a short time the 

 wound heals over, and thenceforth no further change happens, 

 though months elapse, except such as would occur with sound 

 uncut potatoes under the same circumstances. The same result 

 follows when a potato is sown v/ith spores, and buried in moist 

 earth. It is not needful that the spores be applied to a cut surface. 

 The fungus when it begins to grow, will penetrate the potato skin 

 without diflSculty. About a week is required for the disease to 

 become evident. 



These facts, which Speerschnieder and De Bary have repeatedly 

 verified, and which any one may observe without difficulty, illus- 

 trate the manner in which the rot in the tuber is a consequence of 

 the blight of the leaf. The spores which fall from the mature fun- 

 gus that is on the leaves, are carried by rains down into the ground, 

 and reach the tubers, provided the latter are not too deep-lying, and 

 thus infect them. If the soil of a field that is brown from blight, be 

 examined microscopically, there is no difficulty in finding spores 

 among the particles of earth. 



A simple experiment furnishes proof that this is the actual pro- 

 cess. De Bary buried potatoes in sand from one-half to three 

 inches deep, laid blighted potato tops on the surface of the sand, 

 and sprinkled the whole moderately with water : in all cases the 

 potatoes thus treated became diseased within eleven days. It is 

 hardly necessary to state that to make these experiments conclu- 

 sive, other potatoes were treated similarly in all respects, save that 



