Plant Diseases 



42.7 



ErwinF. Smith, U.S.D.A. 



Fig. 265. Tumors developed on leaf of tobacco by inoculating with Bacterium tumijaciens. 

 From the tumors, leafy shoots are growing. 



(i) Malformations of leaves, stems, roots, and fruits. Galls, 

 knots, " witches' brooms," and curled and wrinkled leaves are 

 common examples. 



(2) Cankers, or rough sunken spots on stems and on branches 

 of trees. They may be caused by frost injuries. Sometimes 

 they are produced by a fungus, as in the brown rot of the peach 

 tree and the black rot of apple . 



(3) Blight is the term applied to the sudden dying of leaves, 

 shoots, and blossoms. The fire blight of apple and pear are 

 known wherever these trees are grown in America. 



(4) Leaf and fruit spots are the result of local injuries to leaves 

 and fruit through the growth of a parasite in or under the surface. 



(5) Wilts include various diseases that are first noticeable by 

 a sudden wilting of the leaves or of the complete plant. Cucum- 

 bers, cotton, cowpeas, and watermelons are particularly subject 

 to diseases of this type. 



(6) Yellowing of foliage or development of yellow spots and 

 blotches characterizes a disease of peaches and the mosaic disease 

 of tobacco. 



