268 DISEASES OF CROP-PLANTS 



migrate in swarms from one district to another these measures 

 would need to be general over an island. 



Natural control of the green bug may be assisted by the 

 transfer of parasitized egg clusters to affected localities. 



Angular Spot. 

 History and Distribution. 



The affection produced by Bacterium (Pseudomonas) mal- 

 vacearum E.F.Sm. on the leaves and bracts of cotton was first 

 described, under the name of angular spot, by G. F. Atkinson in 

 Louisiana in 1891. Some ten years later E. F. Smith isolated 

 and described the causative organism and made successful 

 infection experiments. 



The disease is common in the United States on both Upland 

 and Sea Island cotton. In the West Indies it is general on Sea 

 Island in most districts. Some of the perennial types are still 

 more susceptible, others are highly resistant. Its distribution in 

 other parts of the world is imperfectly recorded. 



In the active stage of the disease the spots are not conspicuous. 

 They appear as small water-soaked areas thickly scattered on the 

 undersides of the leaves and on the bracts, irregular in shape, 

 generally angular through being bounded by the smaller veinlets, 

 and not usually more than 3-5 mm. in diameter. A succession 

 of them, more or less in contact, often runs along by the main 

 veins. They quickly penetrate to the upper surface of the leaf, 

 and then undergo no further development. At this stage a slightly 

 yellowish varnish or thin crust of bacteria can often be found on 

 their surface. The tissue involved soon dies and turns brown, 

 and then first becomes very noticeable. As this condition of the 

 spots naturally persists for the remainder of the life of the leaf, 

 a good deal of misapprehension is caused as to the actual time of 

 occurrence and duration of attacks. 



Recent observations in St. Vincent in the month of November 

 showed that the disease occurred in fresh condition in periodic 

 bursts which appeared to be connected, at an interval of about a 

 week, with a previous showery day. Studies by R. C. Faulwetter 

 point to the conclusion that the predominant factor in the 

 distribution of the disease is rain splashed from the leaves and 

 blown as spray by the wind. This accords with the known ability 

 of the bacterium to produce infection on healthy leaves provided 

 a iilm of water is present. The organism is demonstrably present 

 in the film consequent upon heavy dew. Infection appears to 

 take place as a riile by way of the stomata. 



Effects. 



The disease in itself is not usually considered to be of much 

 importance, though severe attacks, which involve the loss of a 

 good deal of effective leaf-area, cannot be regarded as neghgible. 



