8 4 
Journal of Agricultural Research 
Vd. VIII, No. 3 
plants have grown are invariably found to have been attacked by a soft 
wetrot, or may have been destroyed entirely before the examination is 
made. At this time and for some weeks thereafter the seed pieces from 
which surrounding, healthy plants have sprung are entirely sound. If 
young tubers have formed before the plants are destroyed, the disease 
frequently passes along the stolons upon which they are produced and 
infects these also, producing a rapid softrot. This is by no means the 
invariable rule, as might be inferred from the statements of some writers. 
However, as will be shown later, it is apparently such cases of infection 
of the growing tubers which are responsible for the propagation and 
spread of the disease, either directly or indirectly. 
The exception noted above, suggestive of the spread of the disease in 
the field, occurred in Dover, Me., in 1908, on the same field from which 
pure cultures of the causal organism were first obtained. There was 
very little blackleg in the entire field of 20 acres, except on one spot of a 
few square rods where all of the plants were diseased. • It was first noted 
by the owner near the center of the affected area, from which it gradu¬ 
ally spread outward. The season had been excessively wet, and this 
area coincided with a low, undrained pocket or depression in the field, 
where water would stand for a few hours after each heavy rainfall. 
Sometimes the attacked plants attain a height of only 2 or 3 inches 
above the surface of the soil, and the fact that many hills are entirely 
missing on fields which show a high percentage of diseased stems indi¬ 
cates that in such cases either the seed pieces decay before sprouting or 
the sprouts are killed before they reach the surface. As a rule, however, 
the plants first begin to show signs of disease when they are 6 or 8 inches 
high and are growing rapidly. In northern Maine this is about the first 
of July, although the disease frequently makes its first appearance some 
time in June. Again, the plants may attain nearly full size before many 
of them on a field appear to be attacked. 
The progress of the disease is'markedly influenced by weather condi¬ 
tions. Very moist, cloudy weather during the first few weeks of growth 
may tend to favor rapid progress, resulting in the early death of the young 
plants, so that only the dead stalks remain scattered among the healthy 
plants within a month or six weeks or even a less time after its first 
appearance. A period of dry weather coming on after the disease is well 
established below the ground may check its progress but cause the death 
of the plant at an equally early period on account of its inability to with¬ 
stand the lessened water supply. Again, conditions between these ex¬ 
tremes may prolong the attack well into August. More blackleg is 
observed in wet than in dry seasons. 
Fields which showed quite a percentage of blackleg early in July may 
give a decidedly different impression to the observer by the middle or 
last of August. At that time the diseased plants may have entirely dis¬ 
appeared; and on account of the scattered occurrence of the former, the 
