2 BULLETIN OF THE BUSSEY INSTITUTION. 
(a) Researches of cryptogamic botanists, particularly those of 
Dr. E. F. Smith, show that all cases of the disease cannot be 
caused by microbes, although the microbes are often present and 
may assist in weakening the host. 
(b) (c) Investigations of Dr. Smith and others lead to the con- 
clusion that, although mechanical injuries and improper conditions 
of the soil may aggravate the symptoms of the disease, neither of 
these accidents can be its primary cause.* 
(d) Since diseases of a similar nature have been shown to be 
caused by animal parasites, such as aphides, mites and thrips on 
carnations,t and by mites on palms,{ it has been suggested by 
Mr. Cook that possibly ‘‘ yellows”? may be caused by mites or 
other parasites. The probability of this view is strengthened by 
the fact that mites of the genus Phytopetus have frequently been 
found on the trees affected with the ‘‘ yellows.” It may well be 
that occasionally the injurious effects of this disease have been 
increased by the parasites; but as mites or other parasites are 
not found in all cases of the disease, this hypothesis leaves many 
cases unexplained, including those met with in the experience of 
Mr. Hersey, as will be stated further on. 
(e) The idea formerly prevalent that the disease is due to 
climate has gradually been given up by most observers who haye 
attempted to solve the problem, though, as will be shown directly, 
it is evident that certain phases of the subject have been over- 
looked, and that there are many facts on record which must be 
explained rationally before we can reach the conclusion that 
climate is not the predisposing cause of the disease. Among 
these facts are the following : — 
(1) So far as I have been able to ascertain, peaches grown 
under glass are not affected. 
(2) The disease has not appeared in California, in Europe, or 
in any country besides the United States. § 
(3) Comparatively little harm has been caused by the *‘ yellows” 
in the Southern States, or in States west of the Mississippi River. 
* Bull. No. 9, Bot. Div. U.S. Dept. Agr. (1888), and Bull. No. 4 (1893), 
Div. Veg. Path. 
+ Wood, Bull. 19, Div. Veg. Path. U. S. Dept. Agr. 1900. 
{ O. F. Cook, ‘‘ Science,” 1900, p. 875. 
§ Bull. No.1, Div. Veg. Path. U. S. Dept. Agr., 1891, p. lig 
