Jan, is, 1917 
Blackleg Disease of Potato 
83 
instead of spreading out normally, tend to grow upward, forming a more 
or less compact top, frequently with the young leaves folded and curled 
up along the midrib. Later they become lighter green and even yellow, 
and the whole plant gradually dies. If the disease progresses rapidly, 
quite a different picture is presented, and the plant may fall over sud¬ 
denly and wilt with very little previous signs of disease. At first sight 
the general aspect of the affected plants does not differ from that pro¬ 
duced from several other causes which injure or kill the parts at or below 
the surface of the ground, such as the attacks of fungi or insects, or even 
mechanical injury at or near the base of the stem. Occasionally, and 
more frequently some seasons than others, when-the plants are attacked 
before setting tubers and when the progress of the disease is slow, numer¬ 
ous aerial tubers are formed on the stalk at the. surf ace of the ground or 
in the axils of the leaves above. As is well known, this condition may 
arise from certain other injuries of the stem as well. 
The diagnosis of suspected cases is easily confirmed by pulling up the 
diseased plants. Stems attacked by blackleg show an inky-black dis¬ 
coloration extending from the base of the stem, where it is attached to 
the seed piece, up to the surface of the soil, except in the early stages, 
and very frequently 1, 2, or even 3 inches above the ground. Some 
seasons, under favorable weather conditions, the disease may with con¬ 
siderable frequency follow up the stem for several inches or even out on 
the larger branches, destroying the entire stem with great rapidity. 
This is more likely to occur when th,e plants are growing in a naturally 
moist soil and during periods of moist, cloudy weather. 
If two or more stalks arise from one seed piece and one develops the 
disease, the remainder will invariably, sooner or later, succumb to it also. 
Very frequently during the last of July, or during August in Maine, when a 
diseased stalk is observed, a careful search will reveal close beside it the 
dried remains of another which died some time earlier. It is these stalks 
that are not visibly affected till later in the season which usually show 
the rapid and complete invasion of the above-ground parts of the stem 
by the bacteria. However, there is no evidence to indicate that the 
disease is ever communicated from one stalk to another by traveling 
down one and up the other. In a large number of inoculations of living 
potato plants in flowerpots in the greenhouse and in boxes of soil in the 
field, including all regions of the plant, from the petioles of the leaves to 
parts of the stem below the surface of the soil, the writer has seen little 
evidence, except in the case of certain inoculations on below-ground parts 
made by Mr. G. B. Ramsey, the writer's associate, in the summer of 
1916, of progress of the disease downward on the stem. It is always 
upward. 
With one exception, the writer has not observed blackleg occurring in 
patches or localized areas. The attacked plants are scattered promiscu¬ 
ously over the field. The pieces of seed tubers from which such diseased 
