. Bulletin of the Department of Agriculture. Agriculture; Agriculture. 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) N
. Bulletin of the Department of Agriculture. Agriculture; Agriculture. 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 by 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 developme
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