282 PLANT DISEASES 



those described above and again a mycelium is built 

 up. It will thus be seen that this fungus is well adapted 

 to living in the soil on dead material and young seed- 

 lings. It can grow rapidly where a great deal of mois- 

 ture is provided and can spread rapidly by means of its 

 spores. It is also so equipped that it can resist dry 

 or other unfavorable conditions. 



From what has been said of the life history of this 

 fungus the means of fighting the disease can easily be 

 surmised. Drainage of the soil and ventilation in the 

 case of greenhouses so that water will not collect at the 

 surface of the soil can both be recommended. The 

 sanding of seed beds also helps. This is commonly 

 practiced because it keeps the surface of the soil dry, 

 for it is close to the surface that the disease strikes the 

 seedlings. Since the disease occurs mostly in seed 

 beds of one sort or another, too frequent watering should 

 be avoided. In some cases the soil becomes so badly 

 infected with the disease spores that it is impossible 

 to grow seedlings. In such cases it is sometimes nec- 

 essary to sterilize the soil for these seed beds. This 

 is done by heating it, which kills off all the spores in 

 the soil. Large seed beds, such as those grown for 

 evergreen trees, are sometimes treated with solutions of 

 sulphuric acid, by which the soil is partially sterilized. 

 This has been found to give good results in certain 

 cases. In general, therefore, the treatment of this 

 disease lies in the proper treatment and handling of 

 the soil. 



What has been said about damping-off and its rela- 

 tion to soil treatment applies, at least in part, to a 

 large number of plant diseases which inhabit the soil 

 and which may attack mature plants as well as young. 



