176 THE POTATO 



or bruises from hail, seems to furnish the condi- 

 tion for the entrance of the fungus into the leaf. 

 Likewise any decline in the vigor of the plant 

 seems to invite attack. Drought, poor soil, de- 

 layed development due to cold weather, exces- 

 sive heat tending toward wilting or sun-scald, all 

 make the plants less able to withstand the at- 

 tacks of this blight. In other words, the more 

 nearly perfect the plant and the more vigorous its 

 growth the less likely it is to suffer from this para- 

 site. 



"But little is known concerning the source of the 

 disease. The tubers seem to be wholly free from 

 attack, and there is therefore no reason for sus- 

 pecting that the seed potatoes carry the disease 

 over from one year to the next. Certain it is 

 that somewhere the several-celled black spores 

 winter over and start the disease again the fol- 

 lowing season. That this might happen wnere 

 the dead tops are not destroyed, but are scattered 

 about over the field and farm, is easily under- 

 stood. 



"Satisfactory treatment for this disease has not 

 yet been found. Many experiments, however, 

 have shown that the effects of the disease may be 

 greatly reduced by two or three thorough sprayings 

 with Bordeaux mixture. The spraying must be 

 thoroughly done and the first application must be 

 made previous to the appearance of the blight. 

 After the leaves have become filled with the my- 

 celium and the spots are beginning to show, it 

 is too late. Prevention must be the aim, and 

 this is accomplished by putting the leaves in 

 such a condition by the application of the Bor- 

 deaux that the spores cannot germinate upon the 

 leaf surface." 



