Jan. 15, 1917 
Blackleg Disease of Potato 
9 i 
amount of the previous crop had been lost by decay, whether of the 
nature of a softrot or not, is not necessarily an indication of the presence 
of the blackleg disease. 
Out of a large number a single case will be cited here as evidence that 
the disease is first introduced by means of infected seed tubers. In 1907 
a 4-acre field on the University farm at Orono, Me., where blackleg had 
not previously appeared, was planted with seed tubers from several 
different sources. Along one side three barrels of potatoes, each from a 
different source, were planted. Quite a percentage of diseased plants 
were found where one of these barrel lots was used; but a careful search 
several times during the summer failed to reveal any such on the remainder 
of the field. 
Observations in Maine indicate that under the climatic conditions 
which exist there infected seed potatoes are the sole source of infection 
and distribution and that the disease does not live over the winter in the 
soil. Planting the same field with potatoes two years in succession is 
quite frequently practiced. Where the disease occurred the first year 
and sound seed tubers for the second crop were carefully selected and 
then disinfected with formaldehyde as described in the following section, 
blackleg was either entirely eliminated or was much reduced, depending 
upon the thoroughness of the treatment. 
On fields which are planted with potatoes the second time in succession 
there is usually quite a percentage of volunteer plants which spring 
from tubers which remained in the soil over the winter. These plants 
are frequently easily recognized by their irregular occurrence on the 
sides of the rows or between the hills. The writer has never seen such 
plants affected by blackleg. This observation was quite unexpected, 
for it seems not unreasonable to suppose that, if the tubers were suffi¬ 
ciently protected from frost to be able to germinate, the bacteria causing 
the disease might live over in the soil in such infected tubers as well as 
in storage 
CONTROL MEASURES 
Laboratory studies showed that the organisms associated with the 
disease did not form spores, were not resistant to drying, and were 
readily killed by germicides. . This, together with the fact that the dis¬ 
ease did not spread from hill to hill in the field and that under Maine 
climatic conditions decayed, diseased, or otherwise imperfect seed tubers 
appeared to be the ultimate source of infection, suggested the probability 
that blackleg might be easily and cheaply controlled or even eliminated 
from a given farm, field, or locality. The most feasible measures which 
presented themselves were careful sorting before and at seed cutting, and 
rigid rejection of all diseased or imperfect seed tubers, especially those 
which showed any blackened or decayed areas, supplemented as an 
added precaution by disinfection with formaldehyde or mercuric chlorid. 
