A Bacterial Disease of Stone Fruits 421 



New infections occur only on the current season's growth. The or- 

 ganism is most aggressive in the young, tender tissues of the twigs. As 

 the tissues harden, the influence of the organism in many cases becomes 

 less marked and the blackened areas gradually become lighter in color; 

 by the close of the season these areas have in many cases assumed much 

 the same color as the old bark. During the second year a large propor- 

 tion of the twig cankers thus disappear. A cross section of a twig through 

 such cankers shows that no new bacterial pockets have appeared near the 

 cambium zone and the old cavities have gradually been crowded into the 

 outer bark. The organism, however, in some cases at least, is still alive 

 and active, and no doubt often serves as a source of inoculum. 



effect of environmental factors 



The condition of the weather is the most important environmental 

 factor with respect to this disease. This is particularly true as regards 

 moisture, but temperature is also of marked consequence. The shot- 

 hole condition does not usually appear until about the middle of May or 

 I he first of June. In a very warm, damp spring, however, it may appear 

 :i- early as the first of April, while in other years severe outbreaks may not 

 appear until in August or even later. A temperature of from 20° to 28° 

 ( '. is the best for tin; growth of the bacterium. Cold weather not only 

 retards its growth, but also checks the activities of insects, which are 

 the important agents in disseminating the organism. In the writer's 

 spring and fall field work, it was frequently observed that when a cold 

 period followed immediately after a series of inoculations had been made, 

 the growth of the organism was retarded, and in some cases three, and even 

 four, weeks passed before signs of disease appeared. The warm, slow, 

 continued rains of the summer furnish the best conditions for the rapid 

 spread of the disease. Heavy, driving rains of short duration followed by 

 sunshine and winds are not favorable to its spread, since many of the bac- 

 teria are washed to the ground, and the leaves are quickly dried off and 

 the lew bacteria that may be spread will be quickly dried and killed. 



The foliage of eight three-years-old peach trees, which were growing 

 in boxes in the open,' was inoculated with pure cultures of the organism. 

 In three months the foliage on all the trees was dotted with brown spots. 

 Four of the ivcr^ were then moved into the greenhouse and water was 

 applied only to the roots. No new spots appeared on the old leaves 



