DAMPING OFF. 271 



Humphrey 30 advocates the entire removal of the soil, whitewash- 

 ing the beds, and the introduction of fresh soil. 



In houses heated by steam if it were possible to have, without 

 too great expense, a steam chest where the pots and seed pans 

 which are used could be placed and the soil thoroughly steamed 

 for several hours it could be sterilized, and the finer and more 

 delicate seedlings be grown then with little danger if subsequent 

 care was used to not introduce soil from the beds. In testing the 

 virulence of the Artotrogus debaryanus (Hesse), and of the sterile 

 fungus, several experiments have been made by steaming pots of 

 earth, growing seedlings in them and then inoculating some of the 

 seedlings with the fungus while other pots were kept as checks, 

 and all were under like conditions with respect to moisture, tem- 

 perature, etc. The seedlings which were not supplied with the 

 fungus remained healthy while those supplied with the fungus 

 were diseased and many killed outright (see frontispiece). 



CONCLUSIONS. 



Damping off is caused by the growth in the seedlings or cut- 

 tings of fungus parasites which themselves are plants but micro- 

 scopic in size. The plants when affected frequently present a 

 paler green color. The tissues become soft at the surface of the 

 ground, the plant falls over and dies. No one fungus is concern- 

 ed even in the soft rot of seedlings. In related cases the plant 

 may show a brownish ulcer at the surface of the ground which 

 frequently increases in size until the plant is severed at this point 

 and then dies. 



Too great a moisture content of the soil, air, high temperatures, 

 close apartments, and insufficient light not only favor the rapid 

 growth of the parasites but they also induce a weakly growth on 

 the part of the seedling so that it cannot so readily resist the 

 disease. 



The parasites can grow and multiply on decaying vegetable 

 matter which is in the soil. 



When once in the soil they can remain alive for months even 

 though the soil become dry or frozen. 



Soil used in seed beds or cutting beds should be free from de- 



3 Mass. State Agr. Exp. Sta. Bull. 402, 1891. 



