New York Agricultural Experiment Station. 399 



the best of care, protection against insects, diseases, adverse 

 soil or adverse climate are the means of preventing individual de- 

 generacy which so many fruit-growers confound with degenera- 

 tion of the variety. 



NATURAL RESISTANCE TO DISEASE IN APPLES. 



Notes on resistance to the various troubles of apples have been 

 taken in the Station orchard for a number of years and while 

 these, when compiled, make no great showing, yet they do have 

 some value to apple-growers. It means much in selecting varieties 

 to know which are immune or susceptible to an uncontrollable dis- 

 ease, as fire blight ; or in the planting of home orchards, where it 

 may not be feasible to spray, a man may well select the sorts that 

 are least susceptible to scab, whereas this disease counts for almost 

 nothing to those who spray. The subject, as one can see after a 

 moment's thought, is a most important one to plant-breeders. 



Immunity to contagious disease, or the fact that some animals 

 and plants are more or less secure against infectious germs to 

 which their near of kin are subject, is elementary knowledge alike 

 to those who have charge of the health of humans or of lower 

 forms of life. In spite of a wealth of recent discoveries the causes 

 and conditions of immunity are not well known. With plants, 

 especially, knowledge of causality and condition is a thing of 

 shreds and patches. It is known, however, that there are two 

 kinds of immunity; that which is acquired and that which is in- 

 herited. 



Immunity in animals is acquired in several ways ; as, by having 

 the disease, of which smallpox and measles are examples ; by being 

 inoculated with attenuated virus or with some toxic product of the 

 bacteria; and by injections of the serum of some other immune 

 animal. Immunity in plants takes a different turn and it is not 

 known that it can be acquired. Man has smallpox but once, but 

 there is no known parallel in the plant kingdom ; though there are 

 cases, as that of pear blight, in which a disease seldom attacks old 

 plants which must have had the disease in their youth. Neither 



