56 Diseases of Truck Crops 



tration. The latter form has the advantage in 

 that it will not interfere with the box boards when 

 used on frames. The pipe, after entrance, should 

 be a T form, so that steam in being forced into 

 the pan when in place does not blow holes in the 

 soil." 



Surface Firing. This method of soil sterilization is 

 used only in the absence of steam facilities or where 

 formaldehyde cannot be obtained, which, however, 

 is seldom the case. It consists simply in producing 

 a hot fire for an hour or more over the bed to be ster- 

 ilized. A combustible material such as brush, straw, 

 or wood may be used for that purpose. The objec- 

 tion to it is that the fire may destroy the organic 

 matter in the soil. 



Roasting or Pan Firing. In this method the soil 

 to be sterilized is removed from the bed and placed 

 in a pan, underneath which fire is present. After 

 roasting the soil is returned to the bed and more 

 of it sterilized. This method is too slow and is 

 open to the same objection as the surface burning. 

 The advantage of steam sterilization and of the 

 "fire" methods consists in the destruction of all 

 weed seed, together with the fungi which cause 

 damping off. 



Other Methods of Control. Damping off may be 

 largely controlled by careful cultural conditions. 

 Unless the soil of the seed bed is to be sterilized, it 

 is never wise to sow the seeds in beds where damping 

 off was known to have occurred previously. Thick 

 sowing especially should not be permitted. In 



