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DISEASES OF CROP-PLANTS 



If the affection on the leaves can be held to be the main source 

 of material for infection producing black-arm and bacterial boll 

 disease, which is a natural conclusion, its presence becomes a 

 matter of serious concern. Frequently, however, there is no 

 obvious evidence of correlation in the prevalence of these 

 affections. 



Influence of External Conditions. 



The affection is so common in the cotton fields of the islands 

 that with little qualification it could be described as universal, 

 but there are wide variations in the degree of its prevalence and in 

 the period at which it becomes abundant. Isolated plots some- 

 times remain practically free of it throughout the season. In 

 agreement with the theory of its spread noted above, its occur- 

 rence is closely associated with rainy weather. When frequent 

 showers fall during the growing period it is usually well distributed 

 through the fields by the time the bolls begin to mature. It has 

 been seen to be abundant on or absent from the leaves of the 

 secondary growth according to the weather in which they are 

 developed. 



Inherent Resistance. 



There are undoubtedly degrees of susceptibility and resistance 

 to this affection in various types of the cotton plant, and even 

 what appears to be complete immunity. Attempts carried on 

 over several years to develop resistant strains from the Rivers 

 type, which is the basis of most of the Sea Island strains in 

 cultivation in the West Indies, have not been successful, but in 

 St. Vincent strains evolved from the Superfine type, which is in 

 general less susceptible than the Rivers, are reported to be re- 

 sistant to a considerable degree. 



In hybrids the character of resistance is irregularly distributed 

 and its method of segregation has not yet been worked out. 



Control by Spraying. 



F. M. Rolfs has reported that in South Carolina angular spot 

 has been found to be controllable by six sprayings with Bordeaux 

 mixture. Experiments with Burgundy mixture have been made 

 in St. Vincent and reported on by S. C. Harland. Spray fluid 

 containing 2 per cent, of copper sulphate was used in 1917-18, 

 but caused very obvious injury to the plants. In the following 

 year a one per cent, mixture was used in the first two applications, 

 and a further reduction was then made to 0.8 per cent. The 

 plants were sprayed 13 times in 5 months. The control of angular 

 spot was almost complete, but there was no significant increase 

 in yield. 



The injury caused by spraying chiefly affects the small buds, 

 many of which shrivel and turn black. The amount of this was 



