DAMPING-OFF TN FORKST NURSEBIES. 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 recognised the frequence of 



FIG. 3. Normal type of damping-off of Pinus ponderosa. At the left ig 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 Tip 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 aiul ^insusceptibility 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 sylrestris, P. bankxiana, /'. nigra autttriaca, and P. 

 niyra poirctiaua to survive such infections, even when more than half of the 

 ro;t system has been destroyed, by transplanting such root-sick seedlings and 



