62 



THE DEPARTMENT OF AGRICULTURE. 



stances causes the production of small potatoes elsewhere. They may cluster round 

 the base of the stem or be found 1 above ground in the axils of the leaves; i.e., 

 between the leaves and the stem (Fig. 61). Another stage of the fungus (Corticum) 

 often appears on the potato-stems as a grey mould hi the latter part of the season. 

 Control is difficult, since it is almost impossible to get a disinfectant strong 

 enough to kill the fungus-masses right through without injuring the potato. Tubers 



Fig. 62. Potatoes to the left the product of ten wilted hills; those on the right the 

 product of ten normal hills (after Orton). 



with large RMzoctonia, masses should be discarded for seed and the remainder 

 disinfected. Corrosive sublimate is effective as a disinfectant for Rhizoctonia, 

 while formaldehyde is not. Potatoes that are vigorous and well cared for often 

 seem to be able to carry a surprising amount of infection without much apparent 

 injury. 



FUSARIUM WILTS AND ROTS (Fig. 62). 



These are due to fungi of the genus Fuaarium, which, affect both the growing 

 plant and the tubers. The commonest form of the disease is the so-called " brown- 

 ring" (Fig. 63), which shows as a ring of brown dots in the flesh when a thin 



Fig. 63. Stem-end rot or brown-ring (original). 



slice is taken off the stem-end of the tuber. On cutting deeper into the potato the 

 ring becomes fainter and finally disappears. Often it is only visible just below the 

 point of attachment of the stolon. The brown discoloration is due to invasion of 

 the sap-vessels by the mycelium of tlie fungus. (Such tubers, if not too badly 

 infected and kept under good storage conditions, may keep quite well until spring. 

 Many, however, undergo a process of dry-rot. Under moist conditions a " soft " rot 

 occurs, which is often helped by various bacteria. In certain seasons where fall 



