12 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



[Vol. 10 



fection of the eye or bud end is less commonly found. A stem- 

 end rotting of the Russet Burbank (Netted Gem) variety, due 

 to Bacillus atrosepticus, is well known in many of the irrigated 

 sections of the Northwest. It is most frequently seen at digging 

 tune, though it may be found earlier, especially if the latter 

 part of the growing season is cool and rainy. Affected tubers 

 are often found having pointed ends, a characteristic of some 

 value, even though not proof-positive, since signs of the rot are 

 more or less masked in freshly dug potatoes. Shrinkage of the 

 affected tissues follows exposure of the tubers to drier surround- 

 ings and a brown to black discoloration shows through the skin. 

 The presence of the rot is often exposed through breaking of the 

 skin m the process of digging. Upon closer examination of af- 

 fected tubers it will be found that the infection spreads more or 

 less irregularly. In freshly dug tubers the rot may be described as 

 a "soft rot" and is accompanied by a more or less putrid odor, but 

 the blackleg as such is not to be confounded with this phase. 

 Mixed infections, including the blackleg organism along with 

 saprophytes and probably Bacillus carotovorus, are responsible 



for such conditions 



Dissection of tubers affected by the blackleg rot reveals the 

 ct that the tissue may be involved to a considerable depth 



often reachin 



of 



rosed tissues ranges from nearly normal to brown and black, 

 but upon exposure to air these turn dark brown to black very 

 rapidly (pi. 2, figs. 2 and 4). Advance of the rot is usually 

 most rapid just beneath the epidermis (pi. 2, fig. 2), but in gen- 

 eral spread in the tubers is not confined to any single region 

 or tissue. Infected round tubers are often found to have a dark- 

 colored hollow center. 



A soft, choosy rot caused by the blackleg bacillus is often seen 



freshly dug tubers, though by some this type of rot is 



pposed to be d 



of 



tency found in affected tubers in transit or in storage is some- 

 times confounded with rots caused by Fusarium spp. Some very 

 excellent illustrations of the tuber rot, including one colored plate, 

 are those published by Shapovalov and Edson ('21). 



Laboratory diagnosis. —To confirm the field observations 

 it was necessary to make experiments in order to isolate the 

 causal organism and prove its specific identity. Since sapro- 





