Jan. is, 1917 
Blackleg Disease of Potato 
85 
healthy plants, which have reached practically their full development, 
obscure the vacant areas, giving the appearance of almost a perfect stand 
over the entire field. 
Soil conditions also are factors which influence outbreaks of blackleg. 
All other things being equal, the disease is more likely to occur in wet 
than in dry soil, and is more prevalent when the early part of the growing 
season is characterized by abundant rainfall. 
Different varieties of potatoes show a marked difference in their sus¬ 
ceptibility to the disease. This is well shown in the case of the Irish 
Cobbler and the Green Mountain, which represent by far the greater 
proportion of the potatoes grown in Maine at the present time. The 
former is an early variety, largely grown for seed purposes for southern 
planting, and is highly susceptible. The latter is a late variety which is 
primarily raised for table use and is seldom severely affected with blackleg. 
In fact, as long as Maine potato growers planted this variety almost 
exclusively, blackleg was of minor consequence. Harrison (17) has shown 
by means of inoculation tests that these differences in ability to resist 
the attacks of blackleg are exhibited by quite a number of varieties. 
It should be understood that the above characterization of the dis¬ 
ease is made without reference to the published descriptions of this and 
similar types of potato-stem disease as they occur elsewhere and is based 
entirely on personal observations which have been made largely in the 
State of Maine. 
There is a somewhat similar form of potato-stem disease which the 
writer has seen with some frequency in certain Western States, but which 
is not common in Maine. It differs from the ordinary type of blackleg 
in that while the stem is blackened and discolored above ground there is 
little or no external evidence of disease below ground. In all the cases 
observed, however, a closer examination has shown that the pith has 
been destroyed between the base of the stem and the externally diseased 
area above ground. 
A few typical cases of this trouble, which some have been inclined to 
consider as not identical with blackleg, were seen in Presque Isle, Me., 
in the summer of 1916. In some instances the disease had spread through 
the stolons to the young tubers. From one of these tubers Mr. G. B. 
Ramsey isolated a bacterium which produced typical blackleg when 
inoculated into young, growing stems. 
GEOGRAPHICAL DISTRIBUTION OF THE DISEASE 
The fact that blackleg or a very similar appearing disease of the potato 
stem and tuber has been observed in Germany, France, Belgium, Holland, 
England, Ireland, Canada, and the United States has been pointed out 
in the previous section. 
From the accounts of Frank (13-14) and Appel (1-7) it is evident 
that the disease is common, widespread, and often destructive in Ger- 
