CH. VII, 5] NATURE OF PLANT DISEASES 369 



Unknown causes, apparently not parasites or any known 

 kind of physiological disturbance, produce the important 

 diseases known as Peach Yellows, the Mosaic Disease of 

 Tobacco, the Curly Top of Sugar Beets, and others, these 

 names sufficiently describing the characteristic symptoms. 

 Perhaps a wholly new type of . disease-cause awaits dis- 

 covery. 



It is not usual to include insect ravages among plant 

 diseases, but rather to reckon them in a category of pests, 

 although their effects come often very close to those of the 

 true diseases, and they are combated by much the same 

 methods. Insects do damage in three principal ways : 

 first, they eat the leaves of plants of which the food-making 

 power is thus damaged. Second, they bore into and feed 

 upon fruits or other parts, which thereby are rendered re- 

 pulsive even if not extensively injured. Third, they lay their 

 eggs in plant tissues under conditions which promote the 

 formation of galls, already described (page 203). Insects are 

 chiefly combated by poisons, either in sprays, as against the 

 Elm Beetle, or else in gas, as in the fumigation of green- 

 houses, or of fruit trees under specially made tents. 



The great number, variety, and ubiquity of plant diseases, 

 together with the excessive loss they entail every year upon 

 agricultural and horticultural interests, have long stimulated 

 their practical study, with a resultant development of elab- 

 orate methods for combating them. In the past these 

 methods have mostly centered in the effort to find a sub- 

 stance fatal to the parasite and not to the host, and such is 

 the principle of spraying plants with solutions like the well- 

 known Bordeaux mixture. Another method, of which the 

 importance has more recently become manifest, consists in 

 determining and applying to plants the conditions requisite 

 for keeping them in fullest vigor, for the condition of bal- 

 ance between parasite and host is often such that a healthy 

 individual can resist while a weakened one cannot. This 

 principle becomes clear in connection with the ventilation 

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