282 BACTERIAL DISEASES OF PLANTS 



The disease is first visible on the leaves a few days after 

 infection (Fig. 213) is the form of minute translucent dots 

 which gradually enlarge (Fig. 214) and from being slightly protu- 

 berant become sunken and discolored, forming irregular reddish, 

 yellowish or brownish spots (Fig. 215). Frequently on the yellow- 

 ing leaves the green of the leaf persists around the spots (Fig. 216) . 

 As in cotton leaves attacked by Bacterium malvacearum, there 

 may be distortions of the leaves due to disease of the veins (Fig. 

 217). This occurs, so far as I have observed, only when infection 

 takes place very early, i.e., when the leaflets are quite small. 

 When older leaves are sprayed with a sus- 



J pension of this parasite they become badly 

 spotted but are not then distorted. 

 On the pods the spots appear the sixth 

 to eighth day as small (0.2 mm.) circular 

 ■^- areas centering in a single stoma and are 



deeper green than the surrounding tissue 

 (Figs. 218 and 219). These spots enlarge 

 slowly (Fig. 220) being level or slightly 

 protuberant at first, as on the foliage, then 

 sunken and discolored and showing some- 

 times a reddish border. As the center of 

 p ., , . y the spot shrinks (over the internal cavity), 



Fig. 2 15. — Bean ^ ' . '' ^ ^ 



leaflet attacked by Bac- from destruction of the subjacent tissues, 

 terium phaseoli. From bacteria from this cavity (as in the black 

 a garden in Washington, ^ ^^ ^f ^^le plum) are forced through the 



D. C, June, 1908. One- i i i • n i i 



half natural size. stomata abundantly, especially when the 



spots are on the pods (Figs. 221 and 222). 

 These extrusions appear in the form of yellowish cirri, if the 

 surface is dry, and of expanded masses or crusts, if the surface is 

 alternately wet and dry. 



The bacterial multiplication in the leaves being less abundant 

 than in the pods there is less surface ooze, but almost always 

 there is some. Even in early stages of the leaf-spot, the bac- 

 teria in the tissues are very abundant as shown in Figs. 223, 

 224. The leaf-spots which are circular at first frequently coalesce 

 as they enlarge, forming irregular areas which are often of con- 



