27 



their contents soluble, thus causing the actual destruction of the liviao- 

 tissue. Many of the species of fuagi that are normally strictly sapro* 

 phytic at times develop this power of killing and disintegrating livino" 

 tissues. Most of the timber rots so destructive to forest trees and to struc° 

 tural timbers, belong among these facultative parasites. 



It is only after the cause of a disease is thoroughly understood that w e 

 can begin iatelligeatly to seek a remedy. The anaaal losses from plant 

 diseases are so great as to be beyond computation, but it is safe to say that 

 ■they reach many millions of dollars for the State of New York alone. 

 Unfortunately, too, all of these losses come from what should be the 

 farmer's profits, for it costs the same to prepare the land, plant and harvest 

 the grain crop when the yield is half smut as it does when it is all clean 

 sound grain. The question of the prevention of plant diseases is thus one 

 of very great practical importaace. Vegetable pathology is one of tha 

 newest of the biological scienc .s. What we know of it has practically all 

 been learned during the past thirty years. I remember that when in 

 College during the seventies the only known remedies for plant diseases 

 were that sulphur sprinkled on rose bushes and grape vines would 

 to some extent, prevent mildew, and that soaking seed oats in a weak solu- 

 tion of copper sulphate would prevent smut in the following crop. At 

 least there was a popular impression that these were facts, but°no conolu- 

 sive experiments in regard to them had been recorded. Now the list of 

 preventable or partially preventable diseases is a very long one. The 

 number of remedial measures used is also considerable. 



With the environmental diseases the obvious remedy is to correct the 

 unfavourable conditions. If the ground is too wet, drairi it. If too dry, 

 irrigate it, or cultivate so as to conserve moisture. If poor in plant f >od' 

 fertilize it. Or if a certain crop is not suited to the prevailing conditions 

 grow some other crop that will find them congenial. These I say are ob- 

 vious methods for preventing troubles of this kind and yet the problem is 

 by no means a simple one. In only too many cases we are unable to pre- 

 diet without actual trial whether or not a given crop will thrive under new 

 and untried surroundings. 



Our knowledge of the functional diseases is not yet sufficient to permit 

 the suggestion of renedies. They must still, for the mjst part, be classed 

 as incurable. We may know, as in the case of the aster "yellow disease," 

 that an insufficient s-^cretion of diastase prevents the assimilation of the 

 f.tarch grains but what cause prevents this normal secretion is as yet ua- 

 guessed and consequently is unpreventable. No group of diseases is more 

 urgently in need of further investigation than these. 



It is in the controlling and preventing of parasitic diseases that modern 

 progress has been most marked. Remedial measures that may be em- 

 ployed against them can best be considered under the headings, hygiene, 

 topical applications, and heredity. 



Under hygiene are included cultural methods that aid the plant in re- 

 sisting disease; the establishment of crop rotations so that plants 

 liable to the same diseases shall not follow each otner in the same field ; 

 the prevention of contagion by the destruction of diseased plants or parts 

 of plants, and methods of pruning and training whether for removing 

 diseased portions, as is often practised with pear blight and plum black ' 

 knot, or for regulating exposure to sun and rain as in some methods of 

 training grape vines. A good example ot the effect of cultural methods 

 in controlling a disejse is furnished by the so-called "black rust" of 

 ,cotton, which often causes serioiis losses on light sandy lands. Espari- 



