DISEASES OF COTTON 271 



not large with the 0.8 per cent, mixture, and it is compensated 

 for by the lengthening of the flowering period which follows 

 shedding. This point is of importance since spraying may still be 

 found desirable on account of soft rot. 



Black Arm. 



Infections with Bacterium malvacearum occur on stems, twigs 

 and petioles of the cotton plant, giving rise to the affection known 

 as black arm. It is characterised by the formation of dead patches 

 in the cortex, which may be small or may extend for several 

 inches, girdling the organ on which they occur. On vigorous 

 plants the effect is small, but on plants which have made poor 

 growth owing to drought or unfertile soil it can be severe, leading 

 to the drying up of the fruiting branches. The v/eak spots pro- 

 duced by this disease in stems and branches often lead to break- 

 ages when the plants are struck by a gust of wind. 



Bacterial Boll Disease. 



The disease of cotton bolls arising from infection with 

 Bacterium malvacearum is frequently the cause of very heavy 

 losses in these islands. In the experience of the writer, at least 

 a large part of the damage generally attributed to anthracnose 

 has had its origin in such infections, the spots produced having 

 afforded a foothold to one or other of several fungi, of which 

 that of anthracnose is under some conditions the most frequent. 



History and Distribution. 



There is very little literature dealing with this disease. It 

 appears to have been first noticed by E. F. Smith in the United 

 States in 1901, it was referred to as a disease of Sea Island cotton 

 by W. A. Orton in 1907, and is discussed at some length, with 

 records of inoculations, by C. W. Edgerton in two papers pub- 

 lished in 1912. As described by that author the position in 

 Louisiana with regard to the disease and its relation to other boll 

 rots closely resembles the local one. 



Regarding the disease in the West Indies there is a doubtful 

 reference by L. Lewton Brain in 1904 to a bacterial boll rot 

 attacking the inner tissues and contents of the boll, which seems 

 rather to refer to what was afterwards described by the same 

 writer as black boll and is now called internal boll disease. In 

 1909-10, F. W. South recognized the disease and published an 

 adequate description of its typical appearance. He records it as 

 apparently prevalent throughout the cotton-growing islands, 

 causing losses estimated to vary from 2 to 20 per cent. This is 

 about its present status, the variation occurring under the 

 influence of external conditions as discussed below. 



The disease has been recorded (1910) as destructive to Egyp- 

 tian cotton grown in low-lying wet districts in Nyasaland. 



