Oct. 8, 1921 Blackleg Potato Tuber-Rot und^r Irrigation 89 



lower and the soil moisture was more abundant, greatly facilitated the 

 development of blackleg. 



FIELD OBSERVATIONS 



The typical case of blackleg-rot on round varieties in the East has been 

 figured in publications and charts issued by several agricultural institu- 

 tions. As a rule, decay begins at the stolon end of the tuber with a 

 comparatively small amount of rot visible on the outside or often only a 

 small, black, circular opening. This opening leads to the interior of 

 the tuber, where a progressive decay develops in the form of an irregular 

 black, soft, or slimy hollow until nearly all of the tuber is consumed 

 (PI. 15, D, B). However, the development of the disease may deviate 

 from this type even in eastern and northern sections of the United 

 States, when conditions are abnormal and favorable to the disease, such 

 as those in moist places or in wet seasons. Morse stated (4) with refer- 

 ence to blackleg in Maine that — 



When this disease occxirs on a field it doubtless is responsible for much of the soft 

 rot of the tubers observed in wet seasons. 



It appears from certain observations made by the writers that under 

 conditions of excessive soil moisture the bacteria in stems or seed tubers 

 may be carried at least to the adjoining tubers of the same hill. The 

 latter then become infected from the outside, or, if they are already 

 infected through the stolons, the infection spreads in moist surround- 

 ings more rapidly on the outside over the surface of the tuber, or evenly 

 throughout the flesh. Specimens of this sort were observed on the 

 Eastern Shore of Maryland and Virginia (PI. 15, G) and in Wisconsin, 

 Minnesota, and Washington (PI. 15, F) on various round types of pota- 

 toes. More accentuated symptoms of this order were found in irrigated 

 sections of Colorado. The most peculiar manifestations of the blackleg 

 tuber-rot were seen in the Snake River Valley of Idaho, where the 

 Netted Gem variety is grown on a large scale. The following forms were 

 observed there dm-ing a field survey arranged by the Office of Cotton, 

 Truck, and Forage Crop Disease Investigations in 1920. 



I. The stem-end rot of pointed-end Netted Gems. The external 

 appearance of this form is extremely misleading (PI. A, 1-4). It becomes 

 prevalent in southeastern and eastern Idaho during the latter part of 

 the season, shortly before the harvest. The relatively low temperature 

 prevailing at this time of year is, no doubt, an important factor in the 

 rapid progress of the disease. If the soil has plenty of moisture, freshly 

 dug affected tubers show no shrinkage and preserve their natural shape 

 (PL 15, A-C). In the course of two weeks the shrinkage is evident and 

 the decay takes on an inward trend (PI. A, 4). By another two weeks, 

 drying and folding of the decayed tissues become very pronounced, and 

 the external appearance at this stage of the decay may well pass as an 



