THE POTATO ROT, 105 



tacked, it spreads over the potato, more rapidly near the surface than toward 

 the centre, so that the whole outside to the thickness of an eighth of an inch 

 may become diseased, while the interior remains for a time sound. The 

 rot often appears first on the half ripe seed end, or knobby points. In moist 

 situations the diseased portions are soft and watery, and the rot spreads 

 quite rapidly, and in this condition is often called "wet rot." In a dry 

 situation the affected parts dry and shrivel up and spread more slowly, or in 

 cool weather apparently not at all. Thus dry rot and wet rot are not 

 necessarily different diseases. 



CAUSES ASSIGNED. 



Material amounting to hundreds of volumes has been written on the cause 

 and remedy of the potato rot. The most widely assigned cause is a loss of 

 vigor, or debility of the potato, since its introduction into cultivation. 

 Various causes are assigned for this : The use of tubers instead of seed for 

 propagation, the use of small or unripe tubers, the cutting of tubers for 

 seed, excessive manuring and high cultivation, deficiency in the soil, our 

 changeable climate, the excessive production of tubers, and the slight ten- 

 dency to produce blossoms and seed, in our cultivated varieties. Notwith- 

 standing all this it is not very clearly proved that the potato has degenerated. 

 Wild sorts brought from their native mountains in Central and .South Amer- 

 ica, and grown by the side of our cultivated varieties fail to show any 

 remarkable difference in vigor, and they are certainly not exempt from rot, 

 either here or in their native countries. 



Some attribute the rot to atmospheric influences, since it has been noticed 

 to follow certain kinds of weather. A man in Washtenaw county announces 

 the cause as nitric acid deposited from the atmosphere on the plant in rain 

 or dew, and his neighbors testify. to the truth of his discovery. It is com- 

 monly believed in the State of New York that the rot this season was caused 

 by the excessive lightning which occurred during the month of August. 



Others attribute it to plant lice, or other insects, which poison the plant; 

 to over-ripe, under-ripe, too large or too small seed ; to the absence of salt or 

 some other element in the soil ; to want of ozone in the atmosphere ; to a 

 plethora or stagnation of its juices; to a Divine judgment, or the influence 

 of comets. 



THE TRUE CAUSE. 



Notwithstanding all this difference of opinion all scientific writers who 

 have examined the subject are now agreed as to the cause of the potato rot. 

 From what has been said thus far it might be inferred that the rot is a 

 single disease. But we must distinguish, as Dr. Farlow says, "between 

 potatoes affected by the rot and rotten potatoes." Any potato left in a 

 sufficiently wet place will become rotten, especially if injured by freezing or 

 in any other manner. After the attack of the true potato disease, rot from 

 other causes sometimes sets in and hastens decay. Several kinds of fungi 

 attack and injure the tops. But, from what has been said, it will not be 

 found difficult to distinguish the true potato rot from all other diseases of 

 the potato, especially after one has once recognized it. Besides, the injury 

 to the crop from all other diseases is usually much less than that caused by 

 the true potato disease. 



This disease is caused by a parasitic fungus, a minute, mildew-like plant 

 which attacks the potato plant, both tops and tubers, while living, and feeds 

 14 



