74 



BULLETIN 9&i, 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 banksiana.. 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 very great increase in the ease of spread 

 ^___^___ i ______ i ________________ 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 COO 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 XT. 



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. 



6oo j,2oo /,eoo z/t-oo 3,000 



NUMBER OFSESOS dO/YA/ FS/7 SQU#R£ FOOT OF 0£D 



Fig. 19. — Diagram showing the extent of damping-off in 

 •drill-sown Pinus banksiana in. plats with different seed 

 densities. The regular' seed density at this nursery was 

 600 seeds per square foot. 



