Family Solanaceae 317 



growing upward, forming a compact top. In severe 

 cases, afiected plants turn yellowish, topple over, and 

 die. On pulling out a diseased hiU, we find that the 

 stem end near the seed potato is black (fig. 59 f). 

 This blackness may extend even one or two inches on 

 the stem above the ground. The seed pieces in this 

 case soft rot. Occasionally the newly formed tubers 

 become infected in the soil and rapidly soft rot. 

 The disease is carried in the interior of the infected 

 seed tubers, and in this way is distributed from one 

 locality to another. Blackleg is now prevalent in 

 Maine, South Carolina, Virginia, Maryland, Dela- 

 ware, New Jersey, New York, Ohio, Oregon, and possi- 

 bly also in Florida, Georgia, New Hampshire, North 

 Carolina, Rhode Island, Vermont, and Wisconsin. 



The Organism. Bacillus phytopthorus is a rod- 

 shaped organism, motile by means of peritrichiate 

 flagella. It is an aerobe, non-sporiferous, liquefying 

 gelatin slowly and producing no gas. On agar, the 

 colonies are grayish white, round, and smooth. 



Control. Careful selection of the seed tubers is 

 essential. Those which show evidence of internal 

 discoloration or rot should be disinfected with 

 formaldehyde, see p. 336. 



Southern Wilt, see To3viato, p. 342. 



Common Scab 



Caused by Actinomyces chromogenus Gasp. 



Common scab is a disease which is generally skin 

 deep. The kind of injury and the severity of infec- 



