976 MICROBIAL DISEASES OF PLANTS 



SYMPTOMS. On the plum, the leaves and green fruit exhibit numer- 

 ous small, water-soaked spots; later the diseased tissue of the leaves 

 falls out, giving a shot-hole appearance, and the plums show black, 

 sunken areas and deep cracks. The spots may reach a diameter of 

 one-fourth to one-half inch. 



On the peach leaves, angular, purplish-brown spots one-eighth to 

 one-fourth inch in diameter are formed, which drop out giving the shot- 

 hole effect. The organism also attacks the young twigs and fruit. It 

 destroys the bark of the former, producing black, sunken areas, while 

 on the latter it causes small purplish spots over which the skin cracks. 



In both the plum and the peach, infection is believed to take place 

 through the stomata. It is primarily a disease of the parenchyma, 

 but the vascular system is invaded ultimately. 



CAUSAL ORGANISM. Ps. pruni Smith, is a small rod, motile by one to several 

 polar flagella. It grows readily upon the ordinary culture media. On agar, it 

 resembles Ps. campestris, producing a distinctly yellow pigment, but is distinguished 

 by its feeble growth on potato and by its growth in Uschinsky's solution, which is 

 converted into a viscid material like egg albumin. Gelatin liquefied slowly. Casein 

 of milk precipitated slowly and redissolved; litmus reduced but color restored later. 

 No gas produced. Thermal death-point 51. 



DISEASE OF SUGAR-BEET AND NASTURTIUM LEAVES 

 Pseudomonas aptatum Brown and Jamieson 



HISTORY. The bacterial leaf spot of sugar-beet and nasturtium 

 leaves was first observed in the summer and spring of 1908 on nastur- 

 tium leaves growing near Richmond, Va., and on sugar-beet leaves 

 obtained from Garland, Utah; more recently the trouble has been noted 

 in California and Oregon on the sugar-beet. 



SYMPTOMS. Affected nasturtium leaves exhibit water-soaked and 

 brownish spots from 2 to 5 mm. in diameter. The sugar-beet leaves 

 disclose "dark-brown, often black, irregular spots and streaks from 

 3 mm. to 15 mm. in diameter. They occur on the petiole, midrib, and 

 larger veins." Occasionally the discoloration extends along the veins, 

 and the tissue on either side is brown and dry; sometimes cork-like 

 protuberances occur at the central point of the spots. In badly dis- 

 eased petioles the tissue softens as though affected with a soft rot, but 

 where the infection is mild there is no indication of this condition. 



