EIGHTEENTH ANNUAL MEETING. 49 



cry. Sinitli, and later, Woods, of the U. S. Bureau of Plant 

 Industry, seem to have been the first to suggest a definite 

 physiological cause for the trouble. Personally I thoroughly 

 believe in the physiological nature of the disease. Briefly 

 expressed, my views at present are as follows : Winter injury 

 is primarily at the bottom of the trouble as a first cause or 

 starting point. Certain winter-injured trees, especially under 

 subsequent unfavorable seasonal conditions, such as drought, 

 may have set up in their tissues unusual chemical activities 

 that result in the formation of deleterious enzymes or toxins. 

 That these enzymes or toxins are spread through the tree by 

 its sap and are capable of producing the disease in healthy 

 trees through bud propagation and possibly through other at 

 present unknown means. Little peach I believe to have a 

 somewhat similar origin. In other words, the winter-injured 

 trees may develop yellows, little peach, some non-infectious 

 trouble, or may entirely recover, depending on the varying 

 conditions that exist with each individual tree. My reasons 

 for* these views are as follows: 1st, peach yellows is primar- 

 ily, if not entirely, a disease of our northern, semi-northern, 

 and southern mountainous regions, where the possibility of 

 winter injury exists. 2nd, the yellows comes in waves and 

 at irregular periods, just as do winter injuries, and there seems 

 to be some sort of connection between these, as illustrated by 

 the present wave, which seems to be following up the recent 

 winter and drought injuries. 3rd, there are certain features 

 in common with the yellow^s and the chlorosis of variegat- 

 ed cultivated plants, recently studied by Baur of Germany, 

 and with the calico of tobacco, studied by various experi- 

 menters, including myself. These troubles are now gen- 

 erally admitted to be jihysiological diseases produced by 

 enzymes or toxins rather than due to germs. For instance, 

 Baur has shown that certain variegated shrubs (those with 

 mixed green and white or yellowish marked foliage) are 

 infectious, and others non-infectious when budded on the 

 normally green varieties, or vice versa, when a bud from 

 the infectious variegated variety is grafted into the nor- 

 mally green variety the subsequent growth from the green 



