. 7 



38 PEACH YELLOWS. 



The experiments also throw considerable light upon the nature of 

 the disease but do not settle the question as to whether the disease is 

 latent in the whole tree when symptoms appear in any part of it. This 

 will require additional experiments of another kind. 



IV. OBSERVATIONS AND EXPERIMENTS BEARING UPON IMMU- 

 NITY. 



(1) The question of immunity is a very important one. If we could 

 somewhere find peach trees hardy enough to resist this disease, a great 

 practical problem would be solved. It has been said that seedlings are 

 much less subject to yellows than budded trees. In the localities where 

 I have studied this disease seedling orchards are infrequent, but from 

 what data I have been able to gather I should say that seedling trees 

 derived from budded fruit are equally subject. 



On the Delaware and Chesapeake peninsula the belief is also pretty 

 general that budded trees imported into diseased districts from healthy 

 localities are as much subject to yellows as those grown at home, or at 

 least do not escape the disease. On the other hand, I have been told 

 by growers that they have had little trouble from yellows since they 

 have formed the practice of selecting their own buds and stocks. The 

 well-known habit of certain nurserymen to buy trees wherever they 

 can, and sell them again as their own stock, obscures what would 

 otherwise be a somewhat simple problem. In many cases in studying 

 orchards it has been difficult to determine the origin of the stocks, 

 and impossible to learn anything about the location or character of 

 the trees from which the buds were taken. Because nursery stock 

 was grown in a locality free from yellows it by no means follows that 

 the buds came from the same locality. Buds are often procured directly 

 from a distance, or are taken from young trees recently introduced into 

 a neighborhood. In case of new varieties, I have several times known 

 trees to be propagated from young trees recently introduced from 

 localities where this disease is very prevalent. In buying trees much 

 depends upon the integrity of the nurseryman, which is often an un- 

 known factor and sometimes a negative quantity. 



The fact that growers in the diseased districts have bought trees 

 from a good many nurserymen with pretty much the same results is 

 tolerably strong evidence, although not entirely conclusive. 



A knowledge of these facts led me to plan an experiment in which 

 I should know in advance the character of the stocks and the buds, 

 and be able to watch the behavior of the parent trees as well as the 

 progeny so long as might be necessary. 



(2) EXPERIMENT 1. The trees for this series of inoculations were 

 1,000 Mariana plums rooted by J. W. Kerr, at Denton, Maryland, in the 

 spring of 1888 from cuttings made in his own orchards. They were 

 divided into five lots and inoculated in August, 1888, with buds taken 

 from five healthy peach trees also in Mr. Kerr's own orchards. These 



