DISEASES OF PLANTS. 393 



variety of Avays ; it may be through excessive humidity, as deter- 

 mined by the stagnation of water through imperfect drainage, or the 

 natural condition and position of the water-table, or it may be through 

 the mechanical condition. While these conditions may not actually 

 cause disease, they will certainly promote it when once developed, 

 and we therefore find a certain part of remedial measures to consist 

 in thorough drainage and cultivation. But more than this, we find 

 in sijecial or general exhaustion of the soil a fruitful source of disease. 

 Lands which have been cropped for a long period become at least 

 specially exhausted, and in such case usually in the direction of that 

 food-element most essential to the growth of the plant which has 

 brought about the exhaustion. There is thus developed a debilitated 

 condition of the entire system, by means of which the normal func- 

 tions are impaired, and this in itself constitutes a disease. But the 

 debilitated state permits the operation of other forms of plant-life 

 which would otherwise 'be unable to develop readily, and also allows 

 certain abnormal physiological and chemical changes to occur, all of 

 which promote secondary features and thus bring about complication. 

 This, it seems tolerably certain, is the case in peach-yellows, and 

 may also prove to be the case in other diseases such as pear-blight. 

 Diseases developed in this way, however, are most diflicult to treat, 

 because the entire system is involved. Remedial measures must 

 therefore be directed toward — 1. Removing the cause ; 2. Building 

 up the general system ; 3. Restoring to a normal condition the dis- 

 ordered organic function. In the case of peach-yellows, the results 

 of chemical analysis, as well as the changes produced by special treat- 

 ment, show that in all probability the specific is chlorine as con- 

 tained in muriate of potash, while a general toning of the system may 

 be accomplished by the judicious application of a complete food as 

 determined from the ash composition. 



Atmospheric conditions are largely, if not wholly, beyond the control 

 of man. They include, of course, the varying conditions of heat and 

 moisture, and are thus either highly stimulating and favor the excessive 

 growth of weak structure and parasites, on the one hand, or they are 

 depressing and cause a stagnation of vital activity, and thus injure the 

 plants, as through excessive drought ; while this, in turn, leads to the 

 development of parasites, which would not otherwise gain a firm hold. 

 We can not expect to modify the conditions which produce these re- 

 sults ; we can only hope to so prepare the plant, by judicious treat- 

 ment, that it will suffer no material injur}' from the peculiar meteoro- 

 logical conditions in which it is placed. With this in view, we would 

 doubtless find it 'svise to apply strong food, which will retard the vege- 

 tative process, and tend to the more solid maturity of the parts already 

 formed. Nor must we neglect the importance of a judicious course 

 of irrigation during drought. Doubtless the time will come Avhen 

 every man who depends upon the growth of plants for his living 



