274 DISEASES OF CROP-PLANTS 



selection of resistant varieties or by spraying, have been made 

 indirectly by adopting these measures for the control of angular 

 leaf-spot, in the St. Vincent spraying experiments a 3 per cent, 

 increase in the percentage of bolls to flowers was obtained in 

 1917-18, which is too small to be regarded as significant. The 

 subject is one which needs further study. 



Soft Rot of Bolls. 



History and Distribution. 



The soft rot of bolls, which is caused by one or more species 

 of Phytophthora, does not appear to have been met with outside 

 the West Indies. It was reported first from Montserrat by W. 

 Robson, in 1910. From specimens then forwarded the nature of 

 the disease was tentatively determined by F. W. South. In 

 1916 the writer was present in St. Vincent towards the end of a 

 period of extremely wet weather, and found the disease to be 

 very destructive so long as this prevailed. In 1917 the course of a 

 small outbreak was followed in the same island and cultures of 

 the fungus were obtained, but its specific identity has not yet 

 been determined. The presence of the disease in the other 

 islands is suspected. 



Recent observations indicate that boll rot is not always 

 caused by the same species of Phytophthora,. A form isolated 

 from bolls in Montserrat was apparently Ph. parasitica Dast, 

 while a species isolated from bolls in St. Vincent on which 

 it had developed sporangia luxuriantly proved to be closely 

 related to the Phytophthoras of cacao pod-rot and coconut 

 bud-rot. 



Symptoms. 



The first appearance of the disease is shown by a darkening of 

 the colour of the boll, which proceeds from a green darker than 

 the normal to blue-black or black. By the time this is completed 

 the boll and its contents are more or less soft-rotted and com- 

 pletely spoiled. The fungus appears externally on the boll as a 

 white cottony envelope of mycelium, or as a white mealy covering 

 of conidia. No other part of the plant has been seen to be at- 

 tacked. 



Influence of External Conditions. 



The development of this disease is closely dependent on a high 

 degree of humidity. It makes its first appearance on bolls near 

 the ground, especially amongst crowded bushy plants after 

 heavy rain. Only in very wet, cloudy periods does it attack bolls 

 on the higher branches. At such times it is responsible for a very 

 large part of the heavy losses which occur. Severe outbreaks 

 pass off very quickly with the succession of dry sunny weather. 



