Applied Biology 789 



V. DISEASES PRODLICED BY PLANTS 



No attempt can be made to discuss fully the diseases which are pro- 

 duced by plants. Approximately 200 diseases of animals and man are 

 produced by bacteria, while many others are produced by pathogenic 

 yeasts and molds. Approximately 200 diseases of plants may be caused 

 by pathogenic bacteria. In general, the majority of infectious diseases 

 of man in this country are caused by bacteria rather than by protozoa. 

 This is true because climatic conditions prevent many of the protozoa 

 from existing in this area. The bacterial diseases of animals are con- 

 sidered elsewhere, but if further information is desired, the reader is 

 referred to the many textbooks in bacteriology. Besides the many bac- 

 terial diseases of living organisms, there are several produced by yeasts. 

 Included in this group are thrush (parasitic stomatitis), a disease of the 

 mouth, and blastomycetic dermatitis, which is an infection of the skin. 

 Certain types of fungi produce such typical disorders as dermatomycoses 

 of the skin; ringworm, a skin disease produced by at least two varieties of 

 fungi; sporotrichosis, a disease of the skin characterized by multiple ab- 

 scesses; "lumpy jaw" or "wooden tongue" of cattle; actinomycotic in- 

 fections of man. 



The problems of toxins, antibodies, split proteins, allergies, and 

 hypersensitiveness are considered in more detail in other chapters. In 

 addition to the large number of diseases of plants and animals produced 

 by bacteria, yeasts, and fungi, there are certain higher plants, such as 

 poison ivy, poison sumac, nightshade, and similar forms, which are harm- 

 ful to man. Some of these must be taken internally to produce harm, 

 while mere contact with others will produce characteristic disorders. 



VI. DISEASES CAUSED BY VIRUSES 



Viruses are also known as "inframicrobes," "ultramicroscopic or filtra- 

 ble viruses," "microplasms," etc. A brief summary of some of the char- 

 acteristics of viruses are as follows: (1) they are assumed to be protein 

 in nature^ because all other living things are, and because they can serve 

 as antigens, and, according to present data, only proteins, or things com- 

 bined with proteins, can stimulate the production of antibodies; (2) some, 

 but by no means all, have been crystallized which differentiates some of 

 them from other infective agents; (3) they are regarded by some investi- 

 gators as a form of life in which one molecule, or aggregation of mole- 

 cules, of the living protoplasm composes a unit just as a cell forms a unit 

 of higher life, thus giving these chemical molecules the ability to repro- 



