416 



PLANTS AND MAN 



times injured — especially during wet weather — by escaping 

 electrical charges which pass down the tree trunk to the ground, 

 killing the cambium layer in much the same way as a small 

 lightning charge. 



In the case of parasitic diseases, some causal organism may 

 always be located. This may be a plant, an animal, or a virus, 

 which is an infectious principle so small as to pass through filters 

 which will screen out bacteria, and which is invisible with an 

 ordinary microscope. Numerous virus diseases of plants are 

 known, some of the more important being peach yellows, tobacco 

 mosaic — ^which causes a loss of over one million dollars per year; 



A B C 



Fig, 254. — Root parasites include (A) beech drops, (B) squaw root and (C) 



broom rape. 



curly top of sugar beets — which causes heavy losses in the beet 

 growing regions of the far west; and the mosaics of potatoes, 

 beans, and cucumbers; all of which cause large crop losses 

 chiefly through a reduction in the yield. Protection against virus 

 diseases is accomplished mainly through the use of disease free 

 seed and planting stock, varieties of plants that are very resistant 

 to attack by viruses, rotation of crops, and the controlling of 

 insects which are known to transmit the diseases from one host 

 plant to another by their feeding habits. 



The seed plants, which themselves constitute the most highly 

 parasitized group of the plant kingdom, include a number of 

 parasitic forms. While these do not rank in importance with the 

 virus, bacterial, and fungus diseases, brief mention of a few is 



