POTATO DISEASES 



1711 



Cause 



The term black-leg is to be considered 

 in a general sense and includes a group 

 of bacterial organisms, all of which have 

 been found capable of producing the dis- 

 ease described under this name. Bacillus 

 phytophthorus and Bacillus solanisaprus, 

 are two that have been studied perhaps 

 more than any others. 



These organisms are capable of causing 

 soft rot when they come in contact with 

 bruised or cut surfaces of the tubers. It 

 is probable that such conditions are es- 

 sential in order that the bacteria may 

 live over winter. They cannot withstand 

 drying, and in the East it is thought that 

 winter conditions are too severe for them 

 to live over in the soil. It has also been 

 shown that decaying, bruised or cracked 

 tubers will carry the disease over and 

 produce affected plants. 



The disease appears early in the sea- 

 son. When plants have lived long enough 

 to form a hard woody stem, this trouble 

 seldom affects them; this may, however, 

 be due to the fact that the organism be- 

 comes immediately active and so kills 

 off the plants wherever it has obtained 

 an early foothold. Since it does not 

 spread from hill to hill in the field, no 

 serious epidemic has been known. 

 Wherever loss is caused, only occasional 

 scattering plants are taken; in some 

 cases this amounts to rather a high per- 

 centage, however, and should not be over- 

 looked. 



Control 



Since this bacterial disease does not, 

 to our knowledge, live over in the soil, 

 it must be introduced on the seed. It is 

 not a difficult matter to inspect the seed 

 and discard any that are decaying, 

 bruised or discolored, and this practice 

 should be followed. In addition, use the 

 seed treatment with formalin, as given 

 under potato scab, and you should be 

 able to eliminate this disease. 



General Consideration 



This disease has not been studied to 

 any extent in the Pacific Northwest, con- 

 sequently we cannot be certain that the 

 bacteria causing it will not winter over 



in the soil under our conditions. In fact, 

 our conditions are such that we may find 

 it living for some time and causing more 

 serious trouble here than it has in the 

 East. 



Bibliography Black-Leg 



1909. Maine Experiment Station Bull- 

 etin 174. 



1911. Maine Experiment Station Bull- 

 etin 194. 



Black Scab. See Potato Wart, this 

 section. 



Blight or Downy Mildew 



This is the most serious pest of the 

 potato and often causes the loss of the 

 whole crop. There are two kinds of 

 blight, both caused by fungi. The early 



Fig. 1. This Plant Was Inoculated With the 

 Black-Leg Bacterial Organism. 



