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 leaver, 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 may 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 necessarily 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, 



