60 PROTECTION OF PLANTS, 1918-19 



3. The insect may be a simple mechanical carrier, that is, it may 

 carry the germs on its body from diseased to healthy plants and these germs may 

 be mechanically brushed on to the surface of the latter. The efficiency of this 

 method depends on the ability of the pathogen to enter the tissues of the host 

 through the epidermis of leaf, flower or twig, or on the accident of the germs being 

 deposited on adventitious wounds. 



4. The insect may by injuring the plant tissues provide points of 

 easy entry for disease germs which may be carried by the wind, insects or 

 other agency. 



The Beet Leaf-hopper and the Curly Top Disease 



For many years the beet crop of the South Western United States has 

 suffered from a blight known as "curly top," the characteristics of which are thus 

 briefly summed up by Ball^. "The first symptoms of 'curly leaf or 'blight' of 

 the beet is a thickening of the smaller veinlets of the leaf, giving it a roughened 

 appearance on the underside. This is followed by a curling of the edge and a 

 final rolling up of the leaf, the upper surface always being rolled in. As this 

 progresses the small veinlets grow still larger and more irregular, knotlike swellings 

 appear at frequent intervals, and in extreme cases little nipple-like swellings 

 appear, extending to a height of nearly one fourth of an inch . . . The beet 

 almost stops growing and a large number of fibrous roots are sent out. These 

 roots are not confined to two irregular lines as in a healthy beet ... In bad 

 cases it shrivels and dies . . . The sugar content remains very low." 



BalF and Shaw^ were the first to connect this disease with the presence of the 

 beet leaf-hopper Eutettix tenella Baker. By enclosing beets with this insect in 

 hopper-proof cages, they were able to reproduce the disease, but their evidence 

 was not absolutely conclusive and not universally accepted. Smith and Bonquet^ 

 repeated their experiment with similiar results. In addition, they planted sugar- 

 beet seed in four-inch pots; tw'O plants were allowed to grow in each pot, and the 

 whole kept "in a Eutettix-proof cage from the first. After each plant had about 

 eight or ten leaves, a single insect was confined in a vial upon one of the two plants 

 in each pot. In each case the plant thus infected showed the typical disease 

 after the usual incubation period (about two weeks in the greenhouse), while the 

 other in the same pot did not. 



"In applying the insect for different lengths of time it was found that as 

 short a period as five minutes is sufficient to produce the disease." 



^ Ball, E. D., The leaf hoppers of the sugar beet and their relation to the "curly-leaf" condition. 

 U. S. D. A., Bur. Ent. Bui. 66, pt. IV, 1909. 



3 Shaw, H. B. The curly top of beets. U. S. D. A., Bur. Pi. Ind. Bui. 181, 1910. 



^ Smith, R. E. and Bonquet, A. New light on curly top of the sugar beet. Phytopathology 

 V, pp. 103-107, 1905. 



