146 Diseases of Greenhouse Crops 



Diseases of Lettuce 

 Top Burn 

 Cause, physiological. 



Symptoms. In plants suffering from top burn a 

 withering of the leaves is followed by the curling 

 back of the tips and margins of the outer leaves, the 

 affected areas becoming blackened to a distance of an 

 inch or more from the margin. The condition great- 

 ly disfigures the lettuce head and reduces its mar- 

 ket value. 



Causes. The trouble may originate on a bright 

 day following a cloudy spell. The greenhouse air 

 is then saturated with moisture and consequently 

 the lettuce leaves transpire very little. With the 

 sudden appearance of strong sunlight, there is a 

 rise in temperature, and a loss of moisture in the 

 greenhouse air. Under these conditions, the foliage 

 will transpire heavily. If the water given off by 

 the leaves is greater than the roots are able to re- 

 place, the leaves will wilt, and if this is continued 

 for any length of time, the tissue along the leaf 

 margin will wither and die. 



ControL A practical remedy for top bum is to 

 saturate the greenhouse air with moisture on bright 

 days which follow cloudy spells. This will prevent 

 the undue transpiration of the leaves and its subse- 

 quent bad effect. 



