DISEASE 



1806 



DISEASE 



disease of fowls, and trichina causes trichinosis 

 of hogs. The latter, like many other diseases, 

 may attack man. Sheep are particularly sus- 

 ceptible to worms, and many flocks are de- 

 stroyed by the lung and stomach worms and 

 by the fluke-worm, which affects the liver. 

 Mange in all animals is caused by a tiny para- 

 site, the mite (see MITES). There are a few 



flies, notably the horse-fly, the bot-fly and the 

 screw-worm fly, which spend a part of their 

 short lives as parasites. Sometimes the activ- 

 ities of these fly-parasites have fatal results, 

 but usually they merely cause irritation and a 

 temporary depression due to loss of blood. 

 Ticks and fleas are also extremely irritating, 

 but not usually dangerous. 



Diseases of Plants 



It has been only in recent years that the 

 importance of plant diseases as related to 

 agriculture and to general economic conditions 

 has been recognized. Millions of dollars are 

 lost each year through rust and other diseases. 

 Wheat, potatoes and grapes are among the 

 crops which seem to suffer most. The easiest 

 way to learn the chief facts about plant dis- 

 eases is to divide them, according to their 

 causes, into four groups. Diseases are due to 

 (1) fungi; (2) bacteria; (3) insects; (4) func- 

 tional disturbances. 



1. Fungi. The diseases due to fungi (which 

 see) are the commonest and most destructive. 

 Practically every living thing in the vegetable 

 kingdom is susceptible to fungi, either through 

 the roots, leaves, stem or fruit, and sometimes 

 through all of them. The seeds or spores are 

 tiny, and are carried everywhere by the winds, 

 which thus spread the disease. The method 

 of attack varies. Some fungi, usually called 

 wound-fungi, can enter the plant only through 

 a bruise or cut in the skin. Some irritate the 

 plant's skin and cause fermentation, which 

 dissolves an opening, and still others worm 

 their way through the pores. Once the fungi 

 get a footing, the disease spreads rapidly, 

 either stunting the plant or causing knots, 

 warts or other peculiar swellings. Some of 

 the best-known fungi are potato rot, smut 

 or rust, grape rot, mildew and scab. 



2. Bacterial Diseases. These are usually 

 and quickly fatal. The only way to prevent 

 an epidemic is to destroy any infected plant 

 as soon as the disease becomes evident. The 

 bacteria enter the plants at any part, and 

 spread rapidly through the entire structure. 

 The blight of pears, cabbage rot, brown rot of 

 potatoes and tomatoes and various diseases of 

 corn, celery and apples are of bacterial origin. 

 Until recently the bacterial nature of these 

 diseases has been disputed, but is now generally 

 acknowledged. 



3. Diseases Caused by Insects. Plants in 

 greenhouses or under other artificial culture 



are more likely to suffer from insects than are 

 plants growing under natural conditions. Many 

 plants, however, are attacked by insect dis- 

 eases, even when growing under normal condi- 

 tions. Phylloxera, which attacks the roots of 

 grape vines, is the worst of these pests. Each 

 year it lays waste many rich acres in France, 

 Germany arid other European countries. 

 Aphides and eel worms are pests affecting 

 oranges, cucumbers, roses, carnations and other 

 plants. See PHYLLOXERA; APHIDES. 



4. Functional Disturbances. These are tech- 

 nically known as diseases due to physiological 

 causes. In other words, they are due to inter- 

 ference with the normal functions of the plant. 

 Too much or too little water, light or heat are 

 the commonest of these causes. They usually 

 affect the . whole plant and make it wither. 

 The remedies for the diseases are obvious; the 

 only problem is to find out in -time what is 

 wrong with the plant. J.H.K. 



Consult Newman's Bacteria and the Public 

 Health; Muir and Ritchie's Manual of Bacteriol- 

 ogy. For plant diseases, consult- Massee's Dis- 

 eases of Cultivated Plants and Trees; for animal 

 diseases, Mayo's Diseases of Animals. 



Related Subjects. The following articles in 

 these volumes deal either with specific diseases 

 or with some topic closely related to the general 

 one of disease, all relating to the human body : 



Abscess 



Adenoids 



Anaemia 



Angina Pectoris 



Aphasia 



Apoplexy 



Appendicitis 



Asphyxiation 



Asthma 



Astigmatism 



Atrophy 



Bacteria and 



Bacteriology 

 Baldness 

 Blindness 

 Boil 



Bright's Disease 

 Bronchitis 

 Bunion 

 Burns and Scalds 



Cancer 



Canker 



Catalepsy 



Cataract 



Chicken-Pox 



Chilblain 



Cholera, Asiatic 



Cirrhosis 



Colds 



Colic 



Corns 



Cough 



Croup 



Dandruff 



Delirium Tremens 



Diabetes 



Diphtheria 



Dropsy 



Dyspepsia 



Eczema 



