March, 1909.] 



261 



Plant Sanitation* 



discussion of this phase of the subject. 

 Very little is really known about it. 



Some writers claim that in certain 

 cases anatomical characters have some 

 influence on the degree of attack by 

 fungus parasites. For example, a thin- 

 skinned potato may be more subject to 

 scab than a potato having a thick skin. 

 This point does not seem to be satis- 

 factorily proven, The researches of 

 Marshall Ward and others tend to show 

 that immunity of the host plant to some 

 true obligate parasites is not due to 

 any peculiarities of anatomy, as abund- 

 ance of plant hairs, thickness of cuticle, 

 or number of stomates per square inch, 

 but has to do with the attraction or 

 repulsion offered by the cells of the 

 plant to the feeding organs of the 

 attacking organism. A fungus may 

 enter the plant but not be able to 

 develop after entrance. This attraction 

 or repulsion offered to the feeding 

 organs of the fungus is believed to be 

 due to certain little understood chemical 

 substances present in the cells of the 

 plant. In selecting disease-resistant 

 varieties, we are probably selecting 

 those which contain chemical substances 

 which inhibit the growth of the organism 

 we are endeavouring to evade. 



There is need of much more investiga- 

 tion and study on this phase of the 

 subject. When we know more about 

 these chemical substances we can, 

 perhaps, proceed more intelligently. 

 The recent work in breeding increased 

 protein in corn, sugar in beets, etc., 

 shows us that there is variation in the 

 amount of chemical substance in plants, 

 and that this variation can be directed 

 along desired lines by intelligent plant- 

 breeders. 



Artificial Immunity. 



If natural immunity is due to chemical 

 substances present in the cells of the 

 plant which prohibit the growth of the 

 disease-producing organism, the question 

 arises as to the possibility of producing 

 an artificial immunity in an individual 

 by treatment with chemicals which will 

 combine or mix with the cell sap and 

 act in the same way as the obscure 

 chemical substances present in naturally 

 immune individuals. Can we produce 

 an artificial immunity in a plant by 

 inoculating such chemical substances 

 into the tissues or inducing the plant to 

 absorb them through its roots ? 



There have been many experiments 

 conducted to test the effect of intro- 

 ducing chemicals directly into the tis- 

 sues, but so far without any practical 

 results. Interesting effects from the 

 plant physiologist's standpoint have 



been recorded, but no one has ever satis- 

 factorily treated any specific diseases 

 by these methods. 



There has been some work published, 

 notably that of Massee working in 

 England, and Beauverie and Laurent in 

 France, indicating that immunity to 

 specific diseases may be produced by 

 watering the soil in which plauts are 

 growing with solutions of chemical sub- 

 stances having fungicidal properties. 

 Massee succeeded in rendering tomato 

 and cucumber plants immune to certain 

 greenhouse diseases by watering them 

 regularly with a solution containing one 

 part of copper sulphate to 7000 parts of 

 water, later he increased this to the 

 proportion of 1 : 6000, 



Whether the study of the develop- 

 ment by artificial means of disease-re- 

 sistant individuals, as indicated, will be 

 productive of any practical results has 

 yet to be demonstrated. This work is 

 yet in the experimental stage. The 

 work that has already been recorded 

 needs verification, and there is much 

 more to be done. There is excellent 

 opportunity for investigation in this 

 field by the ptent physiologist and plant 

 pathologist. 



I hope that I have succeeded in shosv- 

 ing my audience some of the possibilities 

 of this phase of plant-breeding work, and 

 that I have not given a false impression 

 as to the ultimate utility of disease- 

 resistant varieties. I wish to lay parti- 

 cular stress on the objections which I 

 have mentioned. At this stage of the 

 work we cannot say whether the difficul- 

 ties will be surmounted, or whether they 

 will in time make the continuation of 

 the study impracticable. 



In resistance we have not a cure-all. 

 It is, however, a prominent adjunct to 

 the increasing number of methods of 

 treating plant-diseases. 



Some phases of the work of developing 

 disease-resisting varieties are very sim- 

 ple, and I hope that I have shown that 

 any intelligent grower may add to the 

 sum of our knowledge of this subject by 

 careful observation of varieties or by 

 selecting seed from individuals which 

 have survived an epidemic. 



Discussion. 



William H. Spooner remarked that 

 the fire blight of pears in this section of 

 the country seems to affect some varie- 

 ties more seriously than others. In his 

 own experience he had noted that 

 Superfin and Urbaniste were affected 

 the most, while Bartlett, Anjou, and 

 Merriam had very generally escaped the 

 blight. 



