Bacteria and Fungi 



255 



U. S. Dept. of Agriculture 



FIG. 150. The same field shown in Figure 140 planted with seeds from plants that 

 survived the attack of the disease. 



plant diseases, however, are produced by fungi. Of the 

 bacterial diseases of plants, the twig blight of pear and apple, 

 the cucumber wilt, and the crown gall of various plants are 

 perhaps best known. 



In crop plants subject to disease, it has been found possible 

 in some instances to breed varieties that are immune to a 

 particular disease. By selecting seed from plants that were 

 least affected, or not at all injured, in a diseased field, the 

 characteristic which gave that plant immunity may be pre- 

 served. In this way wilt-resistant cotton was bred in the 

 South. In fields where the wilt disease had killed nearly all 

 the cotton, it was noticed that an occasional plant suffered 

 scarcely at all. Seedlings grown from these plants were 

 found to be highly resistant. 



By hybridizing with related species or varieties that are 

 immune, and then selecting the desired offspring for producing 

 seed, it is possible to transfer the immunity to the crop plant 



