REPORT OF THE SOCIETY 



Occurrence 



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Black stem-rot of potatoes occurs in Northern and Central Europe, Great 

 Britain, Ireland, Canada and the United States. Its economic importance 

 varies with the season, being pronounced in moist, cool seasons. Murphy- 

 states that, in 1915, 7 p.c. of the crop 

 in New Brunswick and 10 p. c. in 

 Nova Scotia and Prince Edward 

 Island were destroyed by this dis- 

 ease. In Ontario still greater losses 

 occur in years suitable meteorolo- 

 gically. A black stem-rot of pota- 

 toes was described first in America 

 by Harrison in 1906. 



Symptoms 



Fig. 7 — Rot of tubers caused by the blackstem 

 organism. Note at A the canals with creamy bac- 

 the terial slime. Note at B the stemend rot of 

 tuber. 



Infected plants usually begin to 

 show late in June or early in July 

 and are quite prominent by the mid 

 die to the end of July. Such plants 

 are somewhat dwarfed, although not 

 necessarily markedly so, and their leaves are pale or yellowish. Rolling of the 

 leaflets along the midrib, somewhat as in true Leafroll, occurs but it can be 

 distinguished by the fact that the leaflets are not rigid and brittle. If the infec- 

 tion happens to occur after the plant has grown considerably, and if the infec- 

 tion also happens to progress rapidly because of suitable humidity and tempera- 

 ture, the growth of the upper part of the plant will be suddenly checked. Thus 

 internodes are shortened and leaves considerably dwarfed giving rise to a ''ro- 

 sette" top. If the attack is still more sudden and severe the stem may wilt sud- 

 denlj^ and fall over without previous symptoms other than the flaccid leave s. 

 On pulling gently at such stems they easily come away at the level of the 

 ground because there the tissues are rotted through. (Fig. 6.) From the 

 rotted stem tissues the bacteria may travel along the stolons to the tubers. In 

 these, if conditions are suitable for the organism, a total rot may occur, but if 

 conditions happen to be less suitable only stem-end infection may occur. This 

 may not be easily visible to the naked or unobservant eye and yet it is the impor- 

 tant factor in overwintering. 



Life History 



The organism overwinters in the stem-end of slightly diseased tubers where 

 a low temperature, keeps it dormant. It is found that growth is slow at 46 deg. 

 F. and it ceases at 39 deg. F. 



The organism attacks parenchyma cells dissolving the middle lamellae and 

 this accounts for the typical soft rot which occurs. It does not specifically 

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