128 VEGETABLE FORCING 



so as to immediately stop the further progress of any 

 disease that may be present. 



Soil selection. Unless the most thorough steam sterili- 

 zation is practiced, too much care cannot be exercised in 

 the selection of soil which is not infected with disease 

 germs of the crops to be grown. For example, it will be 

 folly to select a garden soil in which lettuce, tomatoes 

 and cucumbers have been grown for many years when 

 these same crops are to be grown under glass. When 

 new ranges are constructed, it may be possible to select 

 soils so free from infection that radical measures of con- 

 trol, such as steam sterilization, may not be necessary 

 for several years. 



Manure selection. Infection of greenhouse vegetable 

 crops may easily occur through stable manures. For 

 this reason, soil sterilization, whether steam or formalin, 

 is used, should be practiced after the manure is applied. 



Infected plants. Diseases are often introduced when 

 infected plants are purchased or transferred from other 

 houses. In new establishments, where there may be no 

 evidence of fungous or bacterial troubles of any kind, it 

 is highly undesirable to take chances in buying plants 

 from any district, even under the assurance that infection 

 does not exist on the premises of the grower who offers 

 the plants for sale. 



Influence of light. Practically no experiments have 

 been made upon the influence of light in relation to the 

 development of parasitic fungi. It has been observed 

 that shading or the reduction of light hinders the progress 

 of certain diseases of plants grown in the open. For ex- 

 ample, Duggar calls attention to the fact that ginseng 

 growers have found that lath screens are valuable in 

 preventing sun scald on the margin of the leaves, and, 

 inasmuch as a serious blight is supposed to gain entrance 

 through the tissues thus affected, shading actually di- 

 minishes the infection from this disease. It is possible 

 that shading is sometimes beneficial in the control of 



