CROP-PLANTS 357 



has been made with rotation of crops, but this is mainly of local 

 application and the best rotations have to be worked out for each 

 area. The object of rotation is to enable crops to draw uniformly 

 on soil constituents, but in some cases it has proved of value in 

 controlling disease. 



This introduces another botanical aspect of cotton research, 

 concerning the alternative host plants of pests. Thus during 1934 

 it was shown at the Mazabuka Station in Northern Rhodesia that, 

 though climate and soil may be quite suitable for cotton cultiva- 

 tion, this will be successful only in districts where the food sequence 

 of the cotton stainer is absent. A complete sequence exists in most 

 of Northern Rhodesia owing to the prevalence of a perennial 

 host plant, Thespesia rogersii. Again in parts of South Africa the 

 American boll-worm is so much attracted by maize that a suitable 

 rotation of cotton with maize leads to a reduction of the worm's 

 attacks on cotton; and the beetle pest, Sygarus, can be controlled by 

 a one-yearly rotation. In South Africa, Nyasaland and the 

 Rhodesias jassid resistant strains have been produced and brought 

 into general cultivation. Valuable work has been done by the 

 Empire Cotton Growing Corporation in the study of crops suitable 

 for rotation with cotton. 



The times of planting, uprooting and burning of cotton plants 

 are of great importance. By such means, not only can the best use 

 be made of a small rainfall, but contact of the plants with the 

 major wave of a pest may be avoided. A Belgian worker, Paul E.A. 

 Jansenns (1932), has gathered a mass of information on cotton in 

 tropical Africa into a most useful reference work. 



Cotton has been a special subject of study in the French Sudan 

 in connection with the Niger irrigation schemes. Forbes (1928) has 

 described the studies carried out in early years, and further informa- 

 tion is to be found in the unpublished reports of the Office du 

 Niger and in a paper by Roberty. 



In the Belgian Congo the two main areas of cotton are along 

 the northern frontier, mainly in Uele district, and along the rivers 

 Sankuru and Kasai on the western border of the Lusambe Pro- 

 vince. Native production is actively stimulated, marketing being 

 organized through the Compagnie Cotonniere Congolaise. By 1931, 

 cotton cultivation occupied over a million hectares, and 900,000 



