22 BULLETIN 934, IT. 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 
pihon {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 Abietoidea?) 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 
Cupressoideas. 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 
