CONCLUSION FROM EXCISIONS. 37 



disease. The symptoms appeared again the following season in all but 

 two of the trees, and in these two they came back the second season. 

 Even when the entire top was removed, as by the tornado in G 17, the 

 disease quickly reappeared. 



The inference that the disease was really cut out and that all these 

 cases are reinfections is inadmissible, because nothing of a similar 

 nature occurred elsewhere in the orchards. To establish this we have 

 only to trace the history of these trees for 1 year from the date of the 

 excisions, and to compare that with the history for a similar period of 

 other trees in each orchard, i. e., those which were healthy when the 

 excisions were made. 



In Mr. Greene's orchard 100 per cent of the excised trees showed fresh 

 symptoms of the disease in 1888, but only about 12 per cent of the 

 others became affected ; L e., the new cases in 1888 amounted to only 

 12 per cent of the whole number of healthy trees. In Mr. Harper's 

 orchard 95 per cent of the excised trees showed fresh symptoms of the 

 disease in 1889, but only about 2 per cent of the others became affected. 

 Most of the excisions were made in these two orchards, and no record of 

 cases in the other orchards was kept for comparison. 



In a few instances I have known 50 per cent of the trees of an orchard 

 to become affected with yellows in 1 year, but never 100 per cent, and 

 very rarely more than 15 to 25 per cent, even in the worst affected dis- 

 tricts. It would seem, therefore, that the disease persisted in these 

 trees, i. e., that the symptoms subsequent to the excisions were not the 

 result of reinfection. 



It remains to ask whether the excisions exerted any retarding in- 

 fluence. One year from the date of the excisions the trees in Mr. Har- 

 per's orchard were freer from symptoms of yellows than had been those 

 in Mr. Greene's orchard (Experiment B), and the same relative differ- 

 ence was apparent at the end of the second year. It might not be 

 proper, however, to draw the inference that the development of the 

 disease was slower on account of the excisions. There is some doubt as 

 to whether the excisions retarded the progress of the disease, for the 

 following reasons : (1) The trees manifested only very slight symptoms 

 to begin with, i. e., much slighter than the trees in Mr. Greene's orchard j 

 (2) They bore no fruit in 1889 or 1890, whereas the trees in Mr. 

 Greene's orchard experienced the strain of a large crop in 1888, and of 

 a partial one in 1889 ; (3) The natural progress of this disease is some- 

 times very slow, requiring a third season for the development of the 

 symptoms in all parts of the tree. It is therefore not impossible that 

 the disease might have progressed in these particular trees with the 

 same slowness, had no limbs been removed. The most that can be 

 said is that the severest excisions, e. #., those in Mr. Harper's orchard, 

 appeared to exert a retarding influence on the progress of the disease. 



Of course, the results obtained do not preclude the possibility of cut- 

 ting out the disease in some cases. The evidence to the contrary is, 

 however, reasonably conclusive, and sufficient for all practical purposes. 



