96 Diseases of Greenhouse Crops 



foliage, giving them a burned, contorted appearance. 

 Another effect of soot and smoke is to close up the 

 stomata or respiratory openings of the leaf, so that 

 asphyxiation results. The effect of smoke on plants 

 is a loss of leaflets in case of compound leaves, and 

 an abnormal curling and distortion. Lesions and 

 spots may be formed on the foliage as a result of the 

 sulphur dioxide which is present in smoke. The 

 spots are at first small, but soon enlarge and finally 

 involve the whole leaf, which dries and becomes 

 gray. Smoke injury, although of a mechanical na- 

 ture, may also be considered from a physiolo^cal 

 point of view. The after effect of smoke on plants 

 resolves itself into a question of insufficient food 

 supply and assimilation. This is indirectly brought 

 about by diminished illumination, interference with 

 the normal transpiration and the reduction of leaf 

 surface. 



Methods of Control. There is as yet no definite 

 method of control known, consequently all that can 

 be done is to avoid the smoke belts. The greatest 

 injury usually occurs in locations to the leaward of 

 smoky districts and when the soil is wet. As far as 

 possible, therefore, postpone irrigation during the 

 windy days. 



2. Physiological Diseases 



In this class are included disturbances which are 

 due to unfavorable conditions of nutrition. There 

 are numerous diseases of plants which are brought 



