DAMPING-OFF IN FOREST NURSERIES. 81 
of the Bureau of Plant Industry. The estimates of the relative se- 
riousness of damping-off are very approximate, based in part on 
observation only. The stations at which damping-off is rated as 1 
are places at which it has been reported by nurserymen or foresters 
as negligible or absent. The estimates for stations 10, 11, 14, and 15 
are based entirely on the reports of others, and for station 5 on the 
basis of counts of clamped-off seedlings made by Mr. R. G. Pierce 
and Mr. Glenn G. Hahn. The writer personally has made the esti- 
mates or checked the estimates of the nurserymen at the other sta- 
tions. A considerable degree of correlation between the hydrogen- 
ion exponent and the amount of damping-off appears on the face 
of the graphs, the coefficient being 0.75 ±0.07. If the correlation is 
calculated with the H + concentration itself instead of its negative 
exponent, the coefficient, in this case itself negative, is not so high 
(— 0.5S±0.11). All of the above data on acidity relation have been 
picked up incidentally in connection with other work and are merely 
suggestive. The suggestion, however, seems sufficiently strong to 
warrant further experimental work directed specifically at the rela- 
tion between soil acidity and the disease. 
The indication in the graph that damping-off is not serious in 
soils in which the hydrogen-ion exponent (P H ) ° is less than 6 is of 
particular interest, in view of the experience of Hawkins and Har- 
vey (71) with cultures of Pythium debaryanum on potato juice. 
They obtained good growth through a range of acidity from P H 3.4 
to 5.8, with no growth or practically none at 3.06 or 8.4. If this 
represents the acid tolerance of the fungus in the soil solution, it is 
evident that ordinarily acid soils can not be expected to remain free 
from damping-off because of inhibition of this particular fungus. 
This suggests that the apparently salutary influence of soil acidity 
in decreasing the damping-off of some of the conifers may be ex- 
erted in the direction of increasing the resistance of the host rather 
than of inhibiting the parasites. In any case, it must be kept in 
mind that as the numerous conifer hosts commonly grown in nurseries 
have many different habitat preferences and many very different 
parasites of potential importance, it is not to be expected that there 
will be found any such constant relation between any factor and the 
amount of disease as would be expected in a disease in which only a 
single parasite and a single host are- involved. 
a Ph 6 is equivalent to a hydrogen-ion concentration, expressed in .mols per liter, of 
1X10- 6 or 0.000001. The higher the exponent, the smaller the hydrogen-ion concentra- 
tion. An exponent of 7 means approximate neutrality. In dealing with this exponen- 
tial form of expression, it should be kept in mind that P H 6 means ten times and P H 5 
one hundred times the hydrogen-iron concentration indicated by P H 7. Conversely, the 
concentration of hydroxyl-ions at P H 7 is one hundred times as great as at Ph5- 
19651°— Bull. 934—21 6 
