io8 



FUNGOUS DISEASES OF PLANTS 



the suffusion of the plant with water, and even the extrusion of 

 droplets from the pores. Cool weather, warm, dry nights, and a 

 dry soil offer a check to the disease. Smith's careful study of 

 water pore infections has contributed greatly to our knowledge of 

 the method of bacterial attack. 



Symptoms. The first symptoms in the leaves are manifested J 

 " at the margins, and consist of yellowing of all the affected parts ■ 

 except the veins, which become decidedly brown or black [see 

 Fig. 24]. The leaves appear to have ' burnt edges.' From the mar- 

 gin of the leaf the progress of the disease is inward and downward 

 through the stem. It usually enters the latter through the leaves. 



Fig. 23. Black Rot of Cabbage. (Photograph by F. C. Stewart) 

 A, inoculated and diseased plant ; B, control, healthy 



Subsequently the disease passes out again from the infected stem 

 into healthy leaves and up into the center of the head. If leaves 

 diseased at the edges are pulled off and examined where they join 

 the stem, the groups of fibrovascular bundles, or leaf traces, in the 

 petiole, are seen to be either free from the disease, in the early 

 stage, or decidedly brown or even deep black from its presence. 

 Leaves attacked in this manner fall off prematurely one after 

 another, leaving in bad cases a more or less elongated stem cov- 

 ered with leaf scars and crowned with a tuft of small leaves. If 

 the disease has entered the stem only on one side, that side is 

 dwarfed and the head becomes one-sided." When young plants 



1 Smith. The Black Rot of the Cabbage, /. c, p. 6. 



