96 POTATO DISEASE. 



there are two methods by which the fungus that is contained in the 

 seed potato may propagate after the Latter is planted, and thus 

 from a single infected tuber may devastate a whole field or neigh- 

 borhood : 



1st. As has been described in the previous article the potato 

 containing the mycelium (fungus without seed or spores) in its in- 

 terior, if cut or bitten so that the cuticle is injured over the dis- 

 eased part, shortly produces, under favorable conditions of mois- 

 ture and warmth, spore-bearing branches, which multiply and 

 produce new fungi. In a stiff soil, at a considerable depth, and in 

 case of uncut or unwounded tubers, this kind of propagation by 

 spores does not take place. 



2d. The m.ycelium, which has lain dormant in the tuber during 

 the winter, and has perhaps developed in it to so slight a degree 

 as to escape ordinary observation entirely, grows in the planted 

 tuber, follows the young shoots in their extension, and with them 

 passes out of the soil. When the mycelium enters a young shoot 

 in large quantity, the latter shortly becomes black and dead Such 

 shoots may often be observed when diseased tubers are allowed 

 to sprout. On the contrai-y, if the mycelium is not abundant, 

 the shoot preserves its beautiful appearance externally, and grows 

 without any perceptible drawback, although on microscopic exam- 

 ination the mycelium may be found, as well as the discolored track 

 of disorganized tissue through which it has made its way. 



Considering the facts stated in our previous article, it is evident 

 that proof being given that the mycelium may survive winter in the 

 tubers, from them penetrate the shoots, and thus get above the 

 soil, it must be admitted as a consequence, that a few diseased 

 seed potatoes may infect a whole field more or less widely, accord- 

 ing as the conditions of increase or distribution are favorable or 

 otherwise. 



When a young shoot containing the fungus in large quantity has 

 grown a few inches above the soil, it will as experiments demon- 

 strate, shortly suffer discoloration and afterwards perish. These 

 instances of potato disease in the early summer attract little or no 

 attention, because they are not numerous, and because the dis- 

 eased shoots are surrounded and hidden by healthy ones, which 

 may have issued from the same tuber. If now moist and warm 

 weather ensues from the surface of the shoot which is blackened by 

 the ravages of the mycelium, there arises a forest of fertile fungi, 



