POTATO DISEASE. 93 



green, and to all appearance healthy, to-day is black with blight, 

 except the most magical increase of this parasite. Nothing else 

 can enable us to comprehend how a part of the field — a streak 

 across it — is blighted, while the rest is undamaged. 



De Bary has produced the blight on healthy potato leaves by 

 sowing the spores and causing the "fungus" to develop on them. 

 To accomplish this it is only necessary to bring a spore in a drop- 

 let of water that is stationed on a bit of potato leaf, or to keep the 

 spore and the leaf in a sufficiently moist place for a few hours, to 

 see with the microscope the fungus develop and the leaf turn yel- 

 low and finally brown, with all the symptoms that are observed 

 when the disease is taken in the natural way. By these observations 

 and experiments it appears proved beyond all cavil, that the 

 Peronospora infestans is the cause of the leaf blight, which is the 

 invariable precursor of the rot of the tuber. 



The question next comes up : What has the fungus to do with 

 the rot itself — with the potato disease proper ? 



On this point the evidence is no less conclusive. Dr. De Bary* 

 describes the following simple experiment, which demonstrates 

 that the tuber rot is the work of the fungus. A perfectly healthy 

 potato is well washed and cut into halves. Each half is placed in 

 a separate saucer, with the cut surface uppermost, and is covered 

 with a tumbler or bell-glass, to protect it from dust and disturb- 

 ance. A little pure water is placed in each saucer to keep the 

 potato from drying away. 



Upon the cut surface of one of the pieces a number of spore-sacks 

 of Peronospora are scattered, care being taken that none shall get 

 across to the other piece. Both are now left to themselves, pro- 

 tected by the bell-glasses, and under the same conditions of tem- 

 perature, moisture, &c. In ten or more days, according as the 

 weather is warmer or cooler, the experimenter may observe that 

 the half upon which the spores were sown, begins to exhibit decided 

 symptoms of the disease, while the other half remains perfectly 

 healthy. The symptoms are precisely those which are always 

 observed in the potato rot. The surface of the tuber first turns 

 brown at the points where juhe spores were deposited ; the discol- 

 oration extends outward from these spots in all directions, and in 

 a few days the whole section is brown to the average depth of one- 



* In his work, " Die gegenwaertig herrschende Kartoffel-Krankheit ; ihre Ursache 

 und ihre Verhuetung. 



