954 MICROBIAL DISEASES OF PLANTS 



typical lesions developing. Small, reddish spots appear which in- 

 crease rapidly in size and develop into watery, amber-colored blisters, 

 surrounded by a pink or reddish border. These blisters are filled with 

 myriads of bacteria, and in time, they dry down, forming a pale yellow 

 or amber-colored crust over the affected areas. Ultimately the dis- 

 eased leaves become brittle, ragged, and are worthless, while the pods 

 curl, shrivel, and rot. 



METHOD or INFECTION. It is believed that the disease is intro- 

 duced with the seed, and when once established, is spread from plant 

 to plant by rain, dew, and leaf-eating insects. 



CAUSAL ORGANISM. Ps. phaseoli Smith,* is a short, motile rod with rounded 

 ends, which produces a characteristic yellow growth on the different culture media. 

 Gelatin slowly liquefied. Milk becomes slowly alkaline, casein is precipitated by 

 lab ferment and partially redissplved. Very marked diastatic action on potato 

 starch. No gas from glucose, saccharose, etc. Aerobic. Uschinsky's solution, 

 growth feeble and retarded. Thermal death-point 49.5. 



CONTROL. Care should be taken to select seed from healthy fields 

 where the disease has never occurred. The disease has been partially 

 controlled by spraying with Bordeaux mixture when the plants were 

 2 to 3 inches high, again ten days later, and after blossoming. 



BLIGHT OF LETTUCE 



Ps. mridilimdum Brown 



The disease has been reported recently from the lettuce-growing 

 sections of Louisiana, and is described as producing a shriveled, dried, 

 burned aspect of the outer leaves, some of which may be in a soft, 

 rotted condition. The deeper leaves exhibit numerous separate or 

 fused spots with a water-soaked appearance; the center of the head is 

 not necessarily involved. 



CAUSAL ORGANISM. Miss Nellie A. Brownfhas described the causal organism, 

 Ps. viridilividum, as a short rod with rounded ends, motile by 1-3 polar flagella; 

 stains readily with the ordinary stains; is Gram-negative. No spores have been 

 observed. In young agar cultures, the growth is cream-white mottled with yellow, 

 the mottling disappearing with age. Gelatin is liquefied slowly. Nutrient broth 



* Smith, Erw., Proc. Am. Asso. Adv. Sci., 46, 228-290, 1897. 



f Brown, Nellie A., "A Bacterial Disease of Lettuce," Jour. Agr. Res., Vol. IV., No. 5. 

 P-t47S, 1915. 



