BOTANY 1 75 



ments for practical purposes must depend largely on methods of 

 trial and error. 



Diseases of plants are caused by insects, nematode worms known 

 as eel-worms, fungi, moulds, and viruses. In addition, certain 

 deficiency diseases are known to be caused by lack of nutrients in 

 the soil. The effects of insects, both as direct agents in causing 

 disease and as vectors of viruses, etc., are considered in Chapter X; 

 most of the other diseases are dealt with in Chapter XII, and cer- 

 tain diseases of trees are mentioned in Chapter VII. A few general 

 remarks on the botanical side of pathology are suitable here. 



Cotton, as one of Africa's most important cash-crops, has been 

 subject to more botanical work than any other plant, both with a 

 view to the improvement of strains, and the control of disease. 

 The bacterial disease known as black-arm, caused by B. malva- 

 cearum, has received particularly intensive study. Notable workers 

 on this subject have been Massey in the Sudan and Hansford in 

 Uganda. 



On virus diseases of plants striking researches have been carried 

 out by the plant pathologist at Amani, Dr. H. H. Storey, who is 

 recognized as a leader in this field. On fungus diseases a consider- 

 able amount of work has been carried out both in tropical and 

 South Africa, the diseases of the most important crops having, 

 naturally, received first attention. In South Africa most of the 

 published work has been by Dr. I. B. Pole Evans, Dr. E. M. Doidge, 

 and Professor van der Bijl. With regard to the tropics, Mr. R. H. 

 Bunting and Mr. H. A. Dade, who formerly worked as mycolo- 

 gists in the Gold Coast, have published much, particularly on the 

 moulds which affect cocoa and other stored products. Moulds are 

 not well understood, but in the Gold Coast a number of strains 

 have been isolated and their reaction to artificial conditions has 

 been investigated. In general, the limiting factor to their growth 

 seems to be humidity. W. Small and C. O. Farquharson did pio- 

 neer work in Uganda and Nigeria respectively. A preliminary list 

 of fungi and plant diseases in Sierra Leone has been produced by 

 Deighton (1936) and one for Tanganyika has been published by 

 G. B. Wallace (1932 and 1936); J. C. Hopkins has published a list 

 of plant diseases in Rhodesia, and other such lists are in prepara- 

 tion under the auspices of the Imperial Institute of Mycology. 



