Jan. 15,1917 
Blackleg Disease of Potato 
89 
It is believed that much of the rot of potato tubers, which in some in¬ 
stances has been attributed to the same organism which produces the black¬ 
leg of the stem, is due to entirely different causes. In the Northeastern 
States epidemics of potato softrot in the field and in storage are quite 
common, but these invariably follow and are associated with outbreaks 
of lateblight on the foliage caused by Phyiophthora infestans De Bary. 
The decay caused by this fungus, as is well known, is of a dry nature. 
However, if a tuber lying in a moderately damp soil becomes infected 
with the fungus, various soil bacteria at once enter into the lesions thus 
made and a rapid, soft, foul-smelling rot results. Where such decay 
starts in the field, it is very likely to follow and be very destructive in 
storage, particularly if the conditions of storage are damp. 
Repeated experiments by Jones and his associates in Vermont, Stewart 
in New York, and Woods and the writer in Maine have demonstrated 
conclusively that epidemics of such bacterial softrot can be prevented 
entirely if the parts above ground are kept free from Phytophthora in¬ 
festans, by proper spraying with Bordeaux mixture, until they are ripe 
or are killed by the frost. Moreover, epidemics of softrot of potato 
tubers following lateblight were common in New England long before 
blackleg made its appearance and now occur in sections which are en¬ 
tirely free from blackleg. As has been stated previously, the latter 
disease has been under observation by the writer to a greater or less 
extent in Maine each summer since 1907. Two severe epidemics of 
tuber decay, and other less destructive ones, have been experienced dur¬ 
ing the same period, but this was entirely controlled where thorough 
spraying with Bordeaux mixture was practiced. On the other hand, no 
severe outbreaks of tuber softrot have been observed which were not 
preceded by lateblight on the foliage, regardless of the amount of black¬ 
leg which occurred on the plants in the fields. 
It must be conceded, therefore, that ordinarily there is no serious loss 
from tuber decay from the blackleg organism in Maine. However, it 
must do some damage of this nature, for there is ample evidence that the 
infected seed tubers are responsible for the propagation of the disease 
from year to year. Also it is a natural supposition that conditions 
which enable the various soil organisms to complete the destruction of 
the tubers, begun by the lateblight fungus, would also favor decay by the 
blackleg bacteria. If decay is once started in storage or the tubers are 
kept under too warm, moist conditions and the latter organisms are 
present, they certainly would be an important factor in causing the rot. 
However, the writer has made many attempts to isolate organisms from 
softrot of potatoes following attacks of lateblight on the foliage; but in no 
case has he been able to secure a bacterial organism capable of causing 
the decay independently and alone. 
67906°—*17-3. 
