74 
BULLETIN 934, U. S. DEPARTMENT OF AGRICULTURE. 
DENSITY OF SOWING. 
The relation between the disease and thick sowing was strikingly 
indicated for tobacco seedlings in a single experiment by Johnson 
(82). For pines the only available information is from four experi- 
ments on Pinus hanhsiana. The results of the first two appear in 
figure 19. In both experiments there is an indication of an increase 
in the percentage of diseased plants as the seed density is increased. 
There is, however, no such marked relation as in Johnson's work. 
As the pines were sown in drills, they were so close together even in 
the less dense plats that no yqvj great increase in the ease of spread 
of the disease was to 
be expected from in- 
creasing the density. 
Greater differences 
should be expected 
in broadcast beds. 
That heavier losses 
have been found in 
drill-sown beds than 
in those sown broad- 
cast (69, 139) is pre- 
sumably explained 
by the fact that with 
equal numbers of 
seed per square foot 
of seed bed the seed- 
lings are much closer 
together in drills 
than in broadcast beds, and thus the spread of the mycelium of para- 
sites from one seedling to another is facilitated. 
Two tests of different seed densities were also made in 3-inch pots 
of autoclaved soil in the greenhouse. Each regular pot was sown 
with 28 seeds (equivalent to 600 per square foot). The pots were 
inoculated by adding to each a single small fragment of an agar 
culture of Pythium debaryanum. Uninoculated pots showed an emer- 
gence of approximately 50 per cent of the seed and were entirely free 
from subsequent damping-off in both experiments. The results ap- 
pear in Table XL 
In this case not only the damping-off after emergence but the loss 
before the seedlings appeared bore an apparent relation to sowing 
density. In the field experiments there was no evidence that the loss 
before the seedlings appeared was affected by seed density. 
GOO J t 2O0 {GOO Z/hOO 3,000 
A/t/MB£/? OFSE£OS dOtYA/ F£ff SQU#R£ FOOT OF B£0 
Fig. 19. — Diagram showing the extent of damping-off in 
drill-sown Pinus banlcsiana in plats with different seed 
densities. The regular seed density at this nursery was 
600 seeds per square foot. 
