90 
Journal of Agricultural Research 
Vol. VIII, No. 3 
SOURCES OF INFECTION AND MEANS OF DISTRIBUTION 
Various other agencies have been suggested as being responsible for 
the spread of the disease; but a long series of observations have convinced 
the writer that in Maine the ultimate source of the trouble is, either 
lirectly or indirectly, infected seed tubers. A large amount of evidence 
bearing on this point has been collected during the last nine years, both 
by observation in Maine and by correspondence with others where 
‘Maine potatoes are used for seed. As has already been mentioned, only 
me case has been observed where the disease appeared to spread in the 
field; and this is easily explained by peculiar local conditions. Since 
the organisms, as will be shown later, are killed fairly readily by drying, 
they are probably incapable of existing for any length of time in a living 
state on the dry surfaces of potatoes. It is the writer’s opinion that they 
are carried over the winter in decaying, bruised, cracked, or otherwise 
imperfect seed tubers. 
In Maine, potatoes are not stored in covered pits in the ground, as is 
the case in Ireland and in certain other parts of Europe. They are always 
kept in dark, cool, well-ventilated cellars or specially constructed potato 
houses, usually the latter. In these houses considerable attention is 
paid to ventilation. If possible, no excessive moisture is allowed to 
develop, and the temperatures are held as low throughout the winter as is 
compatible with safety to the potatoes. In laboratory experiments the 
organisms in beef-broth cultures were found alive after a period of io 
months, or until nearly all of the moisture had evaporated from the cul¬ 
ture tubes. Temperatures a few degrees above freezing, or in the ice 
box, have been found to be most favorable for keeping this and similar 
organisms alive for long periods of time in cultures. Under these condi¬ 
tions the organisms retain their vitality, but multiply at a compara¬ 
tively slow rate. Therefore it seems probable, if the bacteria are able to 
enter the interior tissues of the tubers either by natural infection .in the 
field shortly before harvesting, or through wounds or cracks made in har¬ 
vesting, or even through lesions produced by other parasitic organisms, 
that, so long as they are supplied with a small amount of moisture, they 
will remain alive. The low temperatures of storage prevent their rapid 
multiplication and the resultant decay of the tubers. It is undoubtedly 
those only slightly affected potatoes which are responsible for the propa¬ 
gation of the disease. 
Unfortunately, where blackleg is observed for the first time, it is not 
always possible to trace the seed tubers to determine whether the disease 
occurred the season before on the field where they were produced. In 
all cases where this was possible the answer has been found to be in the 
affirmative. The only criterion by which to judge matters of this kind 
is whether or not the characteristic blackening and death of the growing 
stems occurred. As has already been pointed out, the fact that a large 
