Diseases of the Potato 193 



inches before or at the time the vines begin to blight will 

 prevent a large amount of tuber infection. This method 

 of control is a makeshift at best, and the practice in some 

 localities is known to have reduced the yield. Spraying 

 the soil with bordeaux mixture will aid in preventing 

 infection of tubers, but usually is not practical. 



As the fungus gtows best in the tubers under mild or 

 moderately cool moist conditions, it naturally follows that 

 infected tubers will rot but little or not at all when stored 

 in a dry place and held at a low temperature (40° F. or 

 below). The application of lime or formaldehyde to the 

 tubers in storage is of no value in preventing rot. 



The disease, then, is caused by a fungus carried to the 

 field with infected tubers. The fungus spores produced 

 on infected vines spread readily during moist weather 

 to the other vines in the field and to neighboring fields, 

 producing blight whenever infection occurs. Many 

 spores fall from the vines to the soil below and are washed 

 to the tubers, which upon infection sooner or later show 

 the dry rot. When the soil remains wet, such tubers 

 become invaded by other soil fungi and bacteria which 

 bring about a soft malodorous rot. 



As a result of many experiments it has been found that 

 spraying the vines with bordeaux mixture is the best 

 practical way to control the disease. Thorough appli- 

 cations made frequently during the summer to keep the 

 new growth covered, especially before rainy periods, and 

 in the late summer and the early fall, will prevent infec- 

 tion of the vines. If the vines do not blight, the tubers 

 will not rot. More complete directions regarding spray- 

 ing practices will be found under the chapter on Control 

 Measures. 



