166 



SCIENTIFIC AGRICULTURE. 



January. 1922. 



GROrP :j. 

 DISEASES CAUSED BY BACTERIA. 



Black leg or Black stem-rot 



{B. atrosepticus v. Hall). 

 The most important disease of potatoes 

 caused by bacteria is that known common- 

 ly as "Black-leg''. Other names are: — 

 "Basal stem rot", "Bacterial black rot", 

 "Black shank disease", "Black stem-rot". 

 It is obvious that "Black leg" is an un- 

 fortunate name and that "Black stem-rot" 

 or "Bacterial black rot" would be prefer- 

 able. Throughout the course of this dis- 

 cussion, the name used will be "Black 

 stem-rot ' '. 



Occurrence. 

 Black stem-rot of potatoes occurs in Nor- 

 thern and Central Europe, Great Britain. 

 Ireland, Canada and the I'nitcd States. 

 Its economic importance varies with the 

 season, being pronounced, in moist, cool 

 seasons. Murphy states that in 1915, 7 



Figure 9. — Rot of tubers caused by the black- 

 stem organism. Note at A the canals with 

 creamy bacterial slime. Note at B the stem- 

 end rot of the tuber. 



p. c. of the crop in New Brunswick and 

 10 p. c. in Nova Scotia and Prince Ed- 

 ward Island were destroyed by this dis- 

 ease. In Ontario still greater losses occur 

 in years suitable meteorologically. A 

 black stem-rot of potatoes was described 

 first in America by Harrison in 1906. 

 Symptoms. 

 Infected plants usually begin to show 

 late in June or early in July and are quite 

 prominent by the middle to the end of 

 July. Such plants are somewhat dwarfed, 

 although not neces.sarily 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 fa/'t tliat tlie leaflets 



are not rigid and brittle. If the infection 

 happens to occur after the plant has grown 

 considerably, and if the infection also hap- 

 pens to progress rapidly because of suit- 

 able humidity and temperature, the growth 

 of the upper part of the plant will be sud- 

 denly checked. Thus internodes are sliort- 

 ened and leaves considerably dwarfed 

 giving rise to a "rosette" top. If the at- 

 tack is still more sudden and severe the 

 stem ma,y Avilt suddenly and fall over with- 

 out previous symptoms other than the flac- 

 cid leaves. 



On pulling gently at such stems they eas- 

 ily come away at the level of the gi'ound 

 because there the tissues are rotted through. 

 (Fig. 8.) 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 im- 

 portant 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 ac- 

 counts for the typical soft rot which occurs. 

 It does not specifically attack vascular 

 tissues but by rapid multiplication the 

 l)acteria plug up the xylem thus preventing 

 the upward passage of water and account- 

 ing for the rolling of leaves, rosette top and 

 wilting. 



As to the organism causing the disease 

 «it is highly probable that more than one 

 may give rise to similar sjTnptoms. Morse 

 inclines to the view that in Maine it is 

 B. atrosepticus van Hall. This typically 

 attacks stems but does not cause excessive 

 tuber rot. Harrison, from Ontario mater- 

 ial, described B. solanisaprus which caused 

 severe tuber rot in addition to black stem- 

 rot. This was the first description of such 

 a disease in America. It lias since been 

 found in Michigan (Fig. 9) but is ascribed 

 by Coons to B. atrosepticus \ an Hall. In 

 the "Western and irrigated parts of the 

 Cnited States an organism causing a .sim- 

 ilar tuber rot and stem-rot is ascribed by 

 Shajiovalov and Edson to B. phjftoph- 

 thorus Appel. 



