INDIVIDUAL AND VARIETAL RESISTANCE— WHAT CONSTITUTES IMMUNITY? IMMUNE 

 VARIETIES— INTRA- VARIETAL SELECTION-CROSS-BREEDING FOR RESISTANCE-IS 

 IT POSSIBLE BY SPECIAL FOODS TO OBTAIN RESISTANT PLANTS? 



There is a good deal of scattered evidence going to show that resistance to fungous 

 and bacterial diseases differs greatly within the species, i. e., different varieties react vari- 

 ously. The writer has also seen some things which lead him to believe that there is also 

 variation in resistance inside of the variety, particular individuals being more resistant than 

 their fellows. 



In pear-blight it is a well-known fact that certain varieties are very sensitive to 'the 

 disease, for example, Clapp's Favorite and Bartlett blight badly; other varieties are more 

 or less resistant, e. g., Duchess, Ejefer, and Seckel. The same is true of apples: Craig has 

 made Hsts. It was formerly thought that Le Conte was a blight-proof variety of pear, but 

 this was due to incomplete observation. The Le Conte was grown principally at that time 

 in the southern States where the blight organsim was not very widely distributed. Since 

 then it has become distributed in those locahties and the Le Conte has suffered severely, 

 whole orchards being destroyed. The Idaho pear also was advertised at one time as bhght 

 proof, but many trees of this variety have since been destroyed by blight including the 

 original tree from which the variety was multiplied. The tree was never resistant but, hke 

 the Le Conte, in the early years of its cultivation it was not much exposed to the disease. 

 Formerly all pears in California were exempt for the same reason. Now, however, the 

 disease is widespread and destructive. The only evidence of this sort worth very much is 

 that to be obtained in mixed orchards in regions where the disease prevails annually. 



Dr. Halsted, in New Jersey, observed that bean-varieties were susceptible in very 

 different degrees to Bad. phaseoli. Deane B. Swingle, then of my laboratory, observed the 

 same thing on Arlington Farm, in Virginia. Delacroix has reported the same thing for the 

 bean disease prevalent near Paris. In the matter of potato-rots it has been observed both 

 in Germany and in this country that some varieties are much more liable to attack than 

 others, and when attacked rot worse (see plate 1 1). In Holland some varieties of hyacinths 

 are known to be much more subject to the yellow disease than others: Consult lists of 

 susceptible and resistant hyacinths under Yellow Disease of Hyacinths. For studies of 

 resistance in varieties of sweet corn, and for a statement respecting variation in suscepti- 

 bility of sugar-cane to Cobb's disease, see Vol. III. 



It has been observed in California in seedling orchards of Persian walnut (Juglans 

 regia) that some individuals resist the bacterial walnut blight better than others. The 

 same thing has been observed in the eastern United States in patches of tobacco attacked 

 by the bacterial wilt. The writer has also observed indications of intra- varietal resistance 

 in potato tubers inoculated with Bacillus phytophthorus. 



We may as well admit that we do not know what constitutes immunity. It is a good 

 subject for study. The factors underlying it are probably partly physical, partly chemical. 

 When they have been brought into clear relief, and this can be done only by the laboratory 

 method, we shall be in a much better position to cope "with these destructive diseases, in 

 the presence of many of which we are now so helpless. 



Variation inside of the variety suggests that careful selection may in the end lead to 

 the production of many resistant sorts. This has been accomplished already for certain 

 fungous diseases, e. g., cotton- wilt.* 



There are perhaps no absolutely immune races or varieties, but in case of many bacte- 

 rial diseases there are some which approximate this desirable condition. Generally speaking 



*Orton: U. S. Dept. of Agriculture, Farmers' Bulletin 302 (1907), and Farmers' Bulletin 333 (1908). 



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