POTATO ROT. 159 



warm and moist the disease usually spreads with great rapidity, 

 a whole field assuming a brownish or blackened appearance 

 within a few days from the first evidence of the presence of the 

 malady; or if the weather is cool or dry, it may be difficult to 

 distinguish the effect of the fungus from the natural maturing 

 and dying of the foliage. 



Early potatoes are seldom if ever injured, as the disease does 

 not make its appearance until July or August. 



Just how the tubers become infected is not known. The 

 spores form on the under side of the leaves and fall to the 

 ground. It is probable that these spores are washed by rains 

 into the soil where they come in contact with the tubers and 

 gaining entrance cause the well-known dry rot. It is possible 

 that the mycelium of the fungus descends the stems and enters 

 the tubers in that way, though the former manner of contam- 

 ination is considered the more probable. 



The injury due to the effects of the fungus may result from 

 two causes. If the tops of the late potatoes are killed in July or 

 August, the tubers will remain very small, even though they do 

 not decay; but this latter condition almost invariably accom- 

 panies the dying of the tops, (if the dying be due to this fungus,) 

 and when such is the case, the loss due to decay is usually much 

 greater than from the former cause. 



Although the direct effect of the disease on the tubers is to 

 produce a dry rot, yet this unhealthy condition of the potato may 

 often induce a "wet rot" which is especially noticeable at the 

 time of digging. 



If the weather is such that the fungus is developing rapidly, 

 a very disagreeable and characteristic odor can usually be 

 detected. 



This subject has received much attention from experimenters 

 during the past few years and some striking results have been 

 obtained, proving almost beyond a doubt that the disease can be 

 held in check if not absolutely prevented. Bordeaux mixture 

 has invariably given the best results as a preventive of the 

 malady. 



By the use of Bordeaux mixture, at the Vermont Experiment 

 Station, the total product in 1892 was increased from the rate of 

 169 bushels per acre from an unsprayed plat, to 400 bushels from 



