22 BULLETIN 934, U. S. DEPARTMENT OF AGRICULTURE. 



the conditions which might increase resistance of one host might 

 very easily decrease its resistance for a host with different environ- 

 mental requirements. To illustrate by an extreme example, the 

 piiion (Pinus edulis) of the arid or semiarid region might remain 

 resistant in soils in which Picea engelmanni of the high mountain- 

 stream bottoms or Picea mariana of the northern swamps might be 

 low in vigor and easily attacked. In the second place, it is to be 

 expected that species with a certain order of relative susceptibility to 

 the parasites which predominate at one nursery may exhibit a very 

 different order of susceptibility to the different combination of para- 

 sites which might be prevalent in another locality. 



The only individual species on which there are a sufficient number 

 of reports and a sufficient agreement between the reports are the two 

 common western spruces, Picea pungens and P. engelmanni, which 

 (at least as compared with Picea excelsa} seem rather susceptible, 

 and Pinus ponderosa, which (as compared with most of the other- 

 species of the Abietoideae) is to be regarded as generally more re- 

 sistant than the average. Within each of the larger genera of this 

 group it seems evident that susceptibility is extremely varied and 

 that no statement as to the relative susceptibility of the genera them- 

 selves can therefore be made. The only group generalization that 

 is perhaps permissible is derived from the consideration of the 

 Cupressoideaa. In this group, out of 23 reports, 16 are in the " not 

 susceptible" or "least susceptible" columns and only one indicates 

 more than intermediate susceptibility. Of 163 reports pertaining 

 to the Abietoidese, only 26 place them in the " not susceptible " or 

 " least susceptible " columns and 63 in the classes of more than inter- 

 mediate susceptibility. The general feeling among nurserymen 

 seems to be that serious damping-off need not be feared among the 

 cedars and their relatives. The data at hand tend to justify this 

 confidence. 



CONTROL OF DAMPING-OFF. 



Early efforts to prevent damping-off were chiefly directed to the 

 avoidance of excessive moisture in either the air or soil. A means 

 to this end, which has been observed more or less by nurserymen for 

 many years, both in the United States and elsewhere, is the applica- 

 tion of small quantities of dry sand to the seed beds after the disease 

 becomes noticeable (18, p. 166; 83). This is sometimes applied hot 

 (101, p. 43-44; 145), though even this procedure does not result in 

 very great advantage. Surfacing with hot sand can not always be 

 counted on to give any measurable advantage over untreated beds (67, 

 p. 3). The use of sand (25) or sterile subsoil (101) instead of ordi- 

 nary soil in covering seed at the time of sowing has been advised. 

 Johnson (82) did not secure satisfactory results with sand in tobacco 



