130 Diseases of Truck Crops 



plants in the field may be detected by their stunted 

 growth, smallness and paleness of foliage. 



Cause and Control. The cause of this trouble is 

 not a bacterium or a fungus but a minute micro- 

 scopical worm, Heterodera radicicola. For further 

 description of the parasite and for methods of control 

 see p. 49-51. 



DISEASES OF SPINACH (Spinacia okracea) 



Spinach is an important truck crop in the United 

 States but one subject to numerous diseases. 



MALNUTRITION 



The result of an excess of acidity or of a lack of 

 soil humus. 



This condition causes great losses in those districts 

 where commercial fertilizers are used exclusively, at 

 the expense of organic manures. 



The margins of the veins of the leaves become yellow 

 and the central part takes on a mottled appearance. 

 The outer leaves are usually the first to suffer; soon, 

 however, the entire plant exhibits similar symptoms 

 and ceases to grow. 



Control. Spinach, like lettuce and other crops 

 which are eaten for their foliage, cannot be sprayed 

 with poisonous fungicides. The trucker must look 

 to other sources for relief. The methods for control- 

 ling malnutrition have already been considered on 

 pages 81-82. As regards the other diseases men- 



