CONTROL, OP' bi,ack]:.e;g disease of the potato. 205 



fectants and germicides. It was found that they were readily 

 killed by drying, but broth cultures retained their vitality until 

 all of the moisture ■ was evaporated — ten months or more, or 

 longer than the organisms would be required to remain alive 

 in infected tubers. Exposure to sunlight for 60 minutes in 

 petri-dish cultures killed all the organisms even in the less in- 

 tense light of November and December. They were also readily 

 killed when transferred to very weak formaldehyde solutions. 

 At no time have any of the dififerent strains studied been ob- 

 served to form spores. 



Spraying sound tubers with fresh, virulent cultu;es in 1909 

 and allowing them to dry and remain in an open shed away 

 from direct sunlight for a week before planting failed to convey 

 the disease to the resulting plants. This gave additional evi- 

 dence that the organisms cannot remain alive for any length 

 of time on the dry unbroken skin of healthy tubers. Wetting 

 healthy seed pieces, after cutting, with cultures and watery ex- 

 tracts of diseased stems followed by keeping the soil constantly 

 moist led to the production of diseased plants. Evidence was 

 also constantly accumulating to show that seed tubers were the 

 chief, if not the only source of infection. Therefore, it seemed 

 conclusive that it was only those tubers which were partly 

 decayed, cracked or imperfect which provided the proper con- 

 ditions for harboring the germs alive and in condition to com- 

 municate the disease to the growing plants. As will be pointed 

 out later there is. every reason to believe that the germs are 

 spread from diseased to healthy seed pieces by means of the 

 knives used in cutting and by the freshly cut surfaces of the 

 seed pieces coming in contact in the storage barrels and in the 

 planters. If the cut seed tubers are allowed to "heat" a little 

 before planting, on account of being barreled up too long, this 

 danger of transference of the disease to healthy seed pieces is 

 greatly increased. 



In the fall of 1909 three barrels * of seed potatoes were ob- 

 tained in Presque Isle from a field where a large percent of 

 the plants had been killed by blackleg the summer before. Un- 

 fortunately late blight was very prevalent on this field during 



■' A barrel of potatoes as the term is here used is approximately equal 

 to 2 3-4 bushels or 165 pounds. 



