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 
