974 MICROBIAL DISEASES OF PLANTS 



METHOD or INFECTION. Experimental evidence goes to show that 

 infection takes place through stomata as well as through wounds 

 produced by insects, or by other mechanical injuries. 



PATHOGENESIS. According to Berger, the following citrus varieties 

 are subject to citrus canker: Pomelo, citrus trifoliata, wild lime/ Navel, 

 sweet seedlings, Satsuma, tangerine, King orange and lemon. 



CONTROL. Removal of the affected parts of the tree by pruning has 

 proven a complete failure as a control measure, and the only practical 

 means of handling the disease appears to be the prompt and complete 

 destruction, by burning, of all stock that shows the slightest trace of 

 infection. 



ANGULAR LEAF-SPOT OF CUCUMBERS 



Pseudomonas lachrymans Erw. Smith and Bryan 



HISTORY AND DISTRIBUTION.- -The angular leaf-spot of cucumbers is 

 a widespread disease occurring in many of the Eastern and Middle 

 Western States. It has been recognized in the field for more than 

 twenty years, but it was not until 1914 that the causal organism was 

 isolated. 



SYMPTOMS. The disease is characterized by the " numerous, often 

 confluent, angular, dry, brown spots which tear or drop out when dry, 

 giving to the leaves a ragged appearance. In the early stages a bac- 

 terial exudate collects in drops on the lower surface during the night 

 and dries whitish," * and because of these tear-like drops of exudate the 

 specific name lachrymans has been suggested for the causal organism. 

 The young stems and petioles may become soft-rotted and crack open, 

 but there is little evidence that the fruit itself suffers from the disease, 

 other than indirectly from lack of nourishment resulting from the 

 destruction of the active leaf surface. 



CAUSAL ORGANISM. Pseudomonas lachrymans is a short rod with rounded 

 ends, motile by means of 1-5 polar flagella. No spores have been observed; capsules 

 are formed on agar and in milk. It is Gram-negative and is not acid fast. 



On agar, the growth is smooth, shining, transparent, white; agar colonies, 

 two to four days old, exhibit opaque white centers which spread in radiating lines 

 into the thin margin. Gelatin is liquefied slowly, and as the liquefaction progresses 

 the upper part becomes stratiform, the lower part bluntly funnel-shaped. In 



* Smith, Erw. F. and Bryan, Mary Katherine; "Angular Leaf-spot of Cucumbers," Jour. 

 Agr. Res., Vol. V, No. n, pp. 465. 475, 1915. 



