220 DISEASES OF CROP-PLANTS 



spots, and, although this condition does not directly affect the 

 edible portion, the fruit is not only rendered so unsightly as to 

 be unsaleable, but is very liable to splitting and to the inroads 

 of secondary fungi, which cause rotting. 



The very heavy monetary cost of the disease in Florida has 

 been due to the wholesale destruction of infested groves in the 

 attempt at complete eradication. 



Causative Organism. 



The disease is due to the action of a specific bacterium, 

 which has been described under the name of Pseudomonas Citri, 

 Hasse. The organism is yellow, one-flagellate, is variable in 

 shape and size, but as obtained from young cankered tissues is 

 usually a short rod with rounded ends, which measures 

 1.5-2.5 X 0.5-0.75 microns. No host-plants are known outside 

 of the order Rutacese. 



Mode of Infection. 



Infection occurs through natural openings and through 

 wounds. Drops of dew or rain on affected organs contain the 

 bacteria in large numbers, so that the disease is very readily 

 transferred by physical or animal agency from plant to plant. 

 The disease is, in fact, extremely infectious and has been seen 

 to spread with incredible rapidity. 



Fallen leaves are believed to carry over the infection, and 

 inconspicuous lesions containing the parasite can occur on 

 mature wood. H. A. Lee has determined in an extensive series 

 of experiments that the organism quickly disappears from 

 unsterilized soil. Dried on cloth, according to Stevens, it was 

 still alive after five weeks. 



Influence of External Conditions. 



The disease is most prevalent and destructive in warm humid 

 weather, and is checked by cool dry periods. It is much more 

 active on thrifty growing trees than on those in poor condition, 

 and on the latter it may exist in a suppressed or dormant con- 

 dition for long periods. The frequent flushes of growth on 

 limes in Dominica and St. Lucia would, so far as can be foreseen, 

 afford very favourable conditions for the development of the 

 disease should it be introduced. 



Control. 



Reports as to the effects of spraying are contradictory. In 

 Florida cutting grapefruit trees down to stumps and coating 

 them with Bordeaux mixture did not prevent the new growth 

 from being infected. In South Africa an outbreak was success- 

 fully controlled by pruning and three sprayings with Bordeaux. 



