78 BULLETIN 934, U. S. DEPARTMENT OF AGRICULTURE. 
a result of the extension upward of lesions which have started on 
the parts a little below the soil surface. It is supposed that such 
decay takes place most rapidly at high temperatures and that it is 
the temperature rather than the evaporation graph which the damp- 
ing-off is following in these early day and night fluctuations. Dur- 
ing the latter part of the damping-off period a dying seedling shows 
its first signs of distress in the drying up of its leaves, the stem being 
too stiff to go down until after the infection has gone far enough 
in the roots to cut off most of the water supply. It is, of course, 
under dry conditions that such a sign of distress will be most in 
evidence. During the latter part of the damping-off period it is 
therefore altogether likely that the day and night fluctuations are 
caused, at least in part, by the higher evaporation rate which ob- 
tains during the day. This is a relation not to the rate of progress 
of the disease, but rather to the rate at which the symptoms of dis- 
ease appear in plants already seriously affected. 
The drop in damping-off following the increased soil moisture of 
June 15, 19-20, and July 3 also apparently contradicts established 
doctrine. While it is ordinarily true that a wet soil is a cold soil 
and that in the rainy weather which causes wet soil the evaporation 
is usually low, it does not seem possible on Inspection of the graphs 
for these items to attribute entirely the reduction of damping-off 
during these periods of wet soil either to low temperature or to low 
evaporating power of the air. Lowered soil temperature probably 
had something to do with the reduced loss following the rains. It 
is also suggested that a sudden change in moisture content ma} 7 tem- 
porarily hinder a soil fungus by decreasing its air supply. In this 
sandy soil the fungi can work at very considerable depths during 
dry periods. Initial lesions have been found as much as 12 inches 
below the surface. If this soil is as completely changed in its aera- 
tion qualities by wetting as the sandy soil with which Buckingham 
(19) worked, a rain might result in a rather sudden change in the level 
at which the fungus is able to operate. 
On the whole, the graphs tend to confirm the common statement 
that high temperature favors damping-off. It must, however, be 
borne in mind that in uncontrolled field plats several factors vary 
simultaneously, and it is impossible to definitely attribute any ob- 
served phenomenon to any one of them. Furthermore, it is not 
possible to say for the seedlings at different ages just how long it will 
take a factor to exert an effect on the damping-off curve. An addi- 
tional consideration is that a method of investigation which gives 
entirely reliable information on the speed with which the disease 
develops does not necessarity throw light on the conditions under 
which the greatest total amount of disease can be expected before 
the seedlings become old enough to resist attack. High temperatures, 
