A Bacterial Disease of Stone Fruits 419 



twig experiments moisture did not influence the period of incubation to 

 so marked an extent as in the experiments with leaves and fruit. 



Infection. — Infection occurs soon after the bacteria pass through the 

 stomata into the substomatal chambers, where they multiply and secrete 

 a cytolytic enzyme which soon breaks down the cell walls of the neighboring 

 cells, giving the organism access to an abundant food supply. 



Serial sections made from infected plum fruit show that the organism 

 at first occupies merely the substomatal chamber, but the neighboring 

 cells are soon broken down, and as the bacteria multiply, the deeper tis- 

 sues become involved. In about six or seven days a small cavity filled 

 with bacteria is formed, and the first evidence of disease appears in the 

 form of a water-soaked spot just beneath the stomata. A cross section 

 through one of these spots shows the cavity filled with a compact mass 

 of bacteria (Fig. GO, page 415). In about ten days or more, depending 

 much on the weather conditions, the bacteria escape to the surface through 

 the stomata or a slight central rupture. In some cases the bacteria bur- 

 row deeper and spread laterally, forming a somewhat circular, sunken, 

 black area. The bacteria reach the surface through the various stoma and 

 central rifts. 



The tissues of the leaf are invaded in much the same way as are those 

 of the fruit. At first the parenchyma tissues are those principally in- 

 volved, but finally the vascular bundles also may be attacked (Fig. 68, 

 page 414). The external signs of injury sometimes appear a day or two 

 earlier on the leaves than on the fruit, and the ooze may also be evident 

 a little earlier. The area in which the cells have been broken down as- 

 sumes a more or less circular form and soon becomes separated by a line 

 of cleavage from the healthy tissue. The injured tissue within this area 

 soon contracts, dries out, and falls away, leaving a fairly clean-cut hole in 

 the green tissue. 



The tender tissues of the young shoots are also injured in much the same 

 way, but the incubation period in the older tissues of the twigs is apparently 

 longer and the development of the water-soaked spots is often slower. 

 A cross section through one of these spots also shows a cavity filled with 

 a compact mass of bacteria. The black-spot condition on the twig appears 

 in from twenty to thirty-five days. Many of the phloem cells of the 

 sunken black area arc broken down and the entire tissue is more or less 



