DAMPING-OFF IN FOREST NURSERIES. 9 
ing-off and of these nonparasitic troubles. The detailed descriptions 
will not be repeated here. A brief summary of the different types 
of disease recognized as included in damping-off follows : 
(1) Germination loss: The radicles are killed very soon after the seeds 
sprout and before the seedlings can appear above ground. This is an important 
type, which can be caused probably by any of the organisms commonly capable 
of causing the better known types of trouble (61, 63, 68, 137). 
(2) Normal damping-off (figs. 3, 4, and 5) : The seedlings are killed by fungi 
invading either the root or hypocotyl after the seedling has appeared above the 
soil and while the stem is still dependent largely on the turgor of its cortical tis- 
sues for support. In sandy soils root infection is more common than hypocotyl 
infection, though the latter is the type most emphasized in the early horticul- 
tural descriptions. Biittner (26) some time ago recognized the frequence of 
Fig. 3. — Normal type of damping-off of Pinus ponderosa. At the left is a damped-off 
seedling or root sprout of the southwestern ragweed (Ambrosia psilostachya) . (Photo- 
graphed hy S. C. Bruner.) 
root infections. Damping-off in beds out of doors is primarily in most cases a 
root rot, either of this type or of the types preceding and following. 
(3) Late damping-off includes cases of the root-rot type occurring only after 
the seedling stems have started to become woody and the cortex has begun to 
shrivel. The damping-off parasites, or at least part of them, continue to kill 
seedlings by rotting their roots for some time after the stems become too woody 
to be decayed. The seedlings affected do not fall over till a considerable time 
after death. For convenience, all cases of this sort up to the purely arbitrary 
age of two months are classed as damping-off. However, in weather permitting 
of average speed of development the seedlings are usually able to resist attack 
before they reach this age. Seedlings at the marginal age between suscepti- 
bility and nonsusceptibility to killing infections are found often with the 
younger parts of their roots killed, but with the older portions apparently able 
to resist invasion by the fungus, recovery taking place by laterals. Dr. 
R. D. Rands and the writer in 1911 established the ability of seedlings from 
43-day-old beds of Pinus sylvestris, P. banksiana, P. nigra austriaca, and P. 
nigra poiretiana to survive such infections, even when more than half of the 
root system has been destroyed, by transplanting such root-sick seedlings and 
