SECRETARY'S PORTFOLIO. 383 



THE PEACH TREE YELLOWS. 



Prof. T. J. Burrill writes as follows to Science ; 



A peculiar disease of the peach, known as the yellows, has long been the 

 scourge of the principal peach growing districts of our country. Its appear- 

 ance somewhat recently in Michigan caused much alarm, and since its occur- 

 rence throughout great orchards in some of the best fruit districts of the State, 

 special attention has been called to it. 



In Science for September 25, 1880, page 162, there appeared an abstract of 

 a paper read by me before the American Society of Microscopists at Detroit, 

 upon the blight of pear and apple trees. In this paper I expressed the opinion 

 that the " yellows" of the peach tree would be found due to an organism sim- 

 ilar to that found to be the cause of the pear tree blight. This opinion was 

 based upon my knowledge of the latter disease, upon the thoroughly confirmed 

 contagious character of the " yellows," and upon the failure of competent in- 

 vestigators to find, after extended research, anything like the ordinary para- 

 sitic fungi. It was long ago conceded by entomologists that the disease did 

 not arise from the depredations of insects. 



I am now able confidently to assert that this devastating disease of the peach 

 is caused by Bacteria ! 



These minute, moving, living things are found in great numbers within the 

 cells of the diseased tree. They are apparently specifically different from those 

 of the pear tree, being comparatively more slender. What I take to be the 

 typical form — all very considerably — is very nearly 1 u by 3.5 u (.0000343 in. 

 by .0001202 in.), made up of several not very evident articulations. They rest 

 in some stages nearly or quite motionless, and in this condition show a curious 

 peculiarity of lying in ranks, side by side. In other periods of development 

 they move in an unsteady, undulating manner, with considerable rapidity; they 

 turn, twist and tumble on their sides, on end, now drifting with the current, 

 now swarming in an inextricable maze in the field of a first-class one-tenth 

 objective. 



As the Bacteria increase, the starch grains, stored by the tree for its own 

 nourishment disappear, and I doubt not further investigation will prove that, 

 as in the blight of the pear and apple, butyric fermentation takes place. The 

 diseased tree probably suffers in other ways from the presence of these minute 

 parasites, but we may say. with truth that it really starves to death. Its food, 

 gathered from the earth and air, assimilated by the leaves and stored for 

 immediate or future use, is ruthlessly seized upon and destroyed. No doubt 

 this takes place at all times of the year, when the temperature of the surround- 

 ing air is considerably above the freezing point; but the Bacteria are probably 

 most active in the summer time. 



Judging from my experiments upon the pear tree, the destroyers only gain 

 entrance to the tissues of the tree through wounds in the epidermis or bark] 

 but it is possible that at the time of flowering they penetrate by way of the 

 stigma, which is not protected by an impervious coating. 



The cellulose tissue of the tree is not destroyed, and it is still a puzzle how 

 the Bacteria, minute as they are, pass from cell to cell. As in the pear, it is 

 probably a very slow process, and is not connected with the circulation of fluids 

 in the tissues. 



The discovery of Bacteria as the cause of disease in plants may prove a nota- 

 ble contribution to the "germ theory" of disease in animals. 



