568 



TEXTBOOK OF BOTANY 



fectious diseases in both plants and animals was established at about the 

 same time. 



Fig. 256. Fire blight of apple. Gelatinization of the cell walls results in a slime 

 which oozes to the surface of the host, and in the above pictures appears as dark 

 blotches on the leaf blades, petioles, and twig. At right a photomicrograph of 

 bacteria in ooze as seen when magnified about 2000 times. Photos from U. S. 

 Department of Agriculture. 



While the early contributions to our knowledge of plant diseases were 

 mostly by European workers, the study of plant diseases and their con- 

 trol is now progressing most rapidlv in America. Research in plant path- 

 ology is being carried forward not only by the universities and colleges 

 but also by the United States Department of Agriculture and by every 

 state agricultural experiment station, as well as by several privatelv en- 

 dowed institutions and industrial laboratories. 



Plant diseases are so diversified that no single statement satisfactorih 

 defines them as a distinct group of phenomena. When parasites partially 

 or wholly invade and infect a host, the resulting injuries or alterations 

 in development, behavior, and well-being may be referred to as "in- 

 fectious diseases." Agents, such as fungi, bacteria, viruses, nematodes, 

 and certain insects, which cause infectious diseases are called pathogens. 

 Examples of easily observable symptoms of plant diseases are galls, 



