184 REPORT OF THE BOTANICAL DEPARTMENT OF THE . 
has been the custom to explain such cases by assuming that the 
rot in question was not the late blight rot, but the dry rot 
(Fusarium oxysporum) or the bacterial wet rot (Bacillus solana- 
cearum), two diseases which work only below ground and hence 
are not preventable by spraying. 
At various times during the past four years this matter has been 
brought to our attention by farmers who complained that they had 
failed to prevent rot by spraying and some of them even claimed 
that spraying encourages rot. Accordingly, we took advantage of 
the excellent opportunity afforded by the rot epidemic of 1905 to 
make some observations along this line. The result is that our 
views on the subject have changed somewhat. Briefly stated our 
conclusion is as follows: The general tendency of spraying is to 
reduce the amount of rot. In most cases the reduction is very 
marked ; in some cases there is no difference and occasionally spray- 
ing increases the amount of rot. It depends on weather conditions 
and the thoroughness of spraying. But whatever the effect on rot, 
sprayed plants invariably give a larger y1eld of marketable tubers. 
It must be admitted that under certain conditions spraying in- 
creases rather than diminishes the amount of rot. For specific 
instances see the Cortland experiment, p. 141, the Cassville ex- 
periment, p. 143, and Volunteer Experiments Nos. 5 and 7, page 168. 
The explanation seems to be as follows: Rot is brought about 
chiefly by spores which fall from the blighted leaves to the ground 
and are carried by rain down through the soil to the tubers. These 
spores are delicate and readily killed by drying. Unsprayed plants 
may die prematurely from blight with scarcely any rot of the tubers 
provided the soil remains fairly dry until after the plants are dead 
and dry; because under such conditions the spores dry up and die 
before they can reach the tubers. No matter how much rain comes 
later there can be no rot because there are no live blight spores to 
cause rot. Now, suppose these plants had been sprayed in the man- 
ner in which spraying is usually done by farmers. Their life would 
have been prolonged two or three weeks. Nevertheless, they would 
have been somewhat affected by blight because farmers rarely spray 
thoroughly enough to prevent blight completely when the disease 
is epidemic; and so long as the plants remain green live blight 
spores continue to be formed on the leaves and fall upon the ground. 
