DAMPING-OFF IN FOREST NURSERIES. 77 



plats on which the seedling counts were conducted, each determina- 

 tion representing two, and in some cases four, points. The deter- 

 minations for the upper one-fourth inch of soil, made more frequently 

 than for lower levels, are connected in figure 20 by a dotted line, 

 which gives some idea of the amount of moisture in the surface soil 

 during the periods between determinations. The determinations were 

 too infrequent to permit anything more than an estimate of the 

 moisture conditions between determinations, but the writer, having 

 before him the records of the times and amounts of rainfall and 

 artificial watering as well as the evaporation and soil-moisture de- 

 terminations, is in a better position to make such an estimate than 

 the reader. The dotted line which gives this estimate should not be 

 depended on to show what the percentage of moisture was at any one 

 time, but is believed reasonably reliable as showing whether in gen- 

 eral the soil was wet or dry. In interpreting the soil-moisture rec- 

 ords, it should be kept in mind that the soil was very sandy, the 

 wilting coefficient of composite samples from various parts of the 

 nursery, as determined by the indirect method of Briggs and Shantz 

 in the Laboratory of Biophysical Investigations of the Bureau of 

 Plant Industry, being only 3.4 per cent. The hygroscopic moisture 

 in dry air for the soil of the plats actually under consideration was 

 indicated by repeated determinations for the surface soil on dry days 

 to be in the neighborhood of or slightly below 2 per cent. The 

 nursery is located in a region of large temperature fluctuations, where 

 the air during the day is generally dry, and consequently the dew 

 is heavy at night. 



The first result of interest is the difference between the damping- 

 off for the day and the night periods. In the records of every 

 day but two, more seedlings went down during the day period than 

 during the night, the differences in most cases being large. As the 

 evaporation and temperature showed similar day and night fluctua- 

 tions, it is difficult to say whether temperature or moisture condi- 

 tions were responsible. The other interesting result brought out by 

 the graphs is the sudden drop in the general level of the damping- 

 off graph following the rains of June 15, June 19-20, and July 3. 

 In each of these three cases the damping-off came up again only 

 after the soil moisture came down. 



The fact that in the daily fluctuations the damping-off varied 

 directly with the evaporation rather than inversely is an apparent 

 contradiction of the generally accepted doctrine that moisture favors 

 the disease. This contradiction is, however, only apparent. Dur- 

 ing the first part of the damping-off period, when the seedlings are 

 still soft, the recognition of damping-off depends on the decay of 

 that part of the stem just above the soil surface which allows the 

 seedling to fall over. This usually takes place at this nursery as 



