66 Diseases of Greenhouse Crops 



These experiments seem to indicate that light is of 

 itself able to modify plants. 



Effect of Light on Disease Resistance 



It has already been seen that light is an important 

 factor in plant culture. It alone seems able to 

 change the form of a plant as well as its color. 

 Moreover, the resistance of a plant to a disease may 

 be modiiied by light. Damping off, for instance, 

 a prevalent disease in greenhouses, is more virulent 

 on dark cloudy days, when the light in the hothouse 

 is abnormal and weak. A temporar}* change in the 

 normal functions of the metabolism of the plant 

 occurs and is followed by a sudden lowering of 

 vitalit}% The greenhouse man cannot overlook the 

 importance of the light requirement of plants under 

 glass. Experience has shown that white glass is 

 the only one capable of furnishing the necessary 

 light rays to the plant. Needless to say, that only 

 the best quality should be secured, for economies in 

 the qualit}^ of glass may not always be the wisest nor 

 the cheapest in the end. 



Electro-Culture 



The field of greenhouse culture is a plastic one. 

 Electricity is undoubtedly capable of influencing 

 plant growth. The greenhouse man who is conscious 

 of the possibilities of control in his plastic domain 

 will not neglect this phase of plant culture. 



