NON-PARASITIC, OR PHYSIOLOGIC PLANT DISEASES 579 



in some cases becoming long and sinuous. As the plants approach 

 maturity and become infected they develop into the characteristic 

 "gray-top" or "mottled-top." The incubation period of lo or 15 days 

 is followed especially in the hot sun by a very noticeable wilting of the 

 upper leaves which become finely mottled. The mottling is due to the 

 distribution of the dark-green shades along the fine anastomosing veins, 

 while the lighter shades occupy the small inclosed areas. The roots of 

 mosaic plants appear superficially quite normal but it is probable they 

 are impaired, in form and function. It is however in the leaves that the 

 disease is most manifest, which become blotched and mottled accom- 

 panied by distortions which produce at times fantastic leaf forms. The 

 lamina is suppressed at times so that the leaf is reduced to a twisted 

 midrib. Sometimes long sinuous ribbon-like leaf blades are found. 



The flowers of diseased plants are characterized by the presence of 

 the normal pink color in lines, specks, or conspicuous blotches, usually 

 of very irregular distribution. A rather striking and symmetric color 

 character is the occurrence of the pink color as a fine line in the sinus of 

 each corolla lobe. Some blossoms are entirely devoid of color and have 

 a blanched appearance. 



Various solanaceous plants are susceptible to the mosaic. Such are 



many species of tobacco, tomato varieties. Petunia, two distinct garden 



varieties of Physalis, Datura, Hyoscyamus, Solarium (2 species), and in 



. several varieties of Capsicum. It is probably distinct from the mosaic 



of pokeweed. 



The incubation period of the mosaic disease is variable, depending 

 upon conditions favorable or unfavorable to the growth of the plants. 

 Eight days is the shortest period recorded. The mosaic virus permeates 

 all parts of the plant, including the roots and corollas as well as the 

 foliage, but it does not infect the embryos of seed produced by mosaic 

 mother plants, and, therefore, such seeds produce healthy plants. The 

 sap of mosaic plants after passing through a filter still retains its infec- 

 tious properties and mosaic material ground and dried retained its 

 virulence one and a half years. The virus preserved by ether, toluene 

 and glycerin was virulent four months later, as was also the original 

 juice, which had been allowed to undergo natural fermentation during 

 that time. Certain species of aphides are active dissemmators of the 

 mosaic disease. 



"Various theories have been advanced to explain the primary origin 



