522 Journal of Agricultural Research voi. xv.No. lo 



may attack seedlings in this manner, but that they also very frequently 

 attack the seedlings in other ways, while the cases which most closely 

 agree with the older descriptions of damping-off are often not caused by 

 parasites, and may occur under hot, dry conditions. The term "damp- 

 ing-off" as here used will include all cases of the death and early decay 

 of seedlings less than 2 months old, resulting primarily from fungus 

 invasion. Consideration of the disease as thus limited will therefore 

 involve the discussion both of the work of several different parasites and 

 of several symptomatic types. Many of these types intergrade to such 

 an extent that there is no possibility of giving them separate treatment 

 as distinct diseases. The different types recognized are described in the 

 following paragraphs. 



NORMAIv DAMPING-OFF 



Normal damping-off is caused by Pyihium debaryanum, Fusarium 

 ftioniliforme, F. ventricosum, Corticium vagum (the common American 

 Rhizoctonia) , and several other fungi. The still succulent seedlings are 

 invaded by the parasites at any point on the root or lower part of the 

 stem, ordinarily a short distance below the ground surface. The para- 

 sites spread rapidly, especially through the root tissues, and the seedlings 

 fall over. The fall of the seedlings is not usually due to stoppage of water 

 supply and consequent wilting; it most commonly occurs when the 

 tissues of the hypocotyl just above the soil surface become involved in 

 the decay, and while the rest of the stem is still green and turgid. This 

 type of disease is shown in Plate B, figure i. It may be fairly well con- 

 trolled by soil disinfection. 



GERMINATION tOSS 



This type of damping-off seems in most cases to be caused by species 

 of Pythium and Corticium rather than by Fusarium. The radicles are 

 killed soon after emerging from the seed coats and before the seedlings 

 appear above the ground. Two-thirds of the seedlings are sometimes 

 destroyed in this way in nursery beds. There is no essential difference 

 between this type and the normal type, except that this type occurs 

 earlier in the life of the seedling and is ordinarily not detected. A poor 

 initial stand of seedlings due to this type of damping-off is commonly 

 attributed by the nurseryman not to damping-off but to poor germina- 

 tive capacity of the seed. This early type of damping-off, like the 

 normal type, is invariably fatal. It can be much more completely pre- 

 vented by soil disinfection than can the more familiar normal type. 



It is often stated by practical men that seedlings from seed with low 

 germination percentage are especially liable to attack by damping-off 

 fungi. This is very likely true within certain limits. However, in a 

 great many cases in which apparently poor germination has been fol- 

 lowed by especially heavy damping-off losses, the sequence is to be 



