DISEASES CAUSED BY FUNGI ii 



is native on jungle plants in both the Eastern and the Western 

 tropics. 



Local Factors affecting Prevalence. 



An established disease may assume serious or epidemic pro- 

 portions under the influence of changes in local conditions. The 

 most general of these is the collection of plants previously- 

 scattered as individuals or small groups into large blocks of 

 cultivation, which happens when a new agricultural industry 

 arises, as in the case of rubber, or an old one is greatly developed, 

 as has been the case in recent times with cacao, limes, cotton 

 and coconuts in various parts of the West Indies. When a plant 

 is grown by the acre or the square mile in pure culture its parasites 

 are secure of their food in due season and the agencies of dispersal 

 have free play. The absence of serious diseases from scattered 

 plants is thus, as experience has regularly proved, no sort of 

 guarantee that wider cultivations will not suffer. A contributory 

 cause to the increase of diseases with the development of an 

 agricultural industry is the greater activity which arises in the 

 introduction of planting material from more or less distant 

 sources. 



■ i A well-marked class of root diseases, due to fungus parasites 

 similar in the nature of their attack though often widely different 

 in taxonomic position, accompanies the establishment of crops 

 on newly cleared forest land. The decaying stumps and logs, and 

 the accumulations of humus to which they give rise, support 

 large numbers of fungi, mostly pure saprophytes but usually 

 including one or more species which are able to extend to the 

 roots and collar of the crop plant introduced. In arable cultiva- 

 tion, with few exceptions (see arrowroot), the cause and the 

 effect quickly disappear. With tree crops the original cause is 

 more slowly removed and the effect may later be continued for 

 a time by its victims, while in cacao orchards where conditions 

 approaching those of forest are maintained by the use of shade 

 trees, one such disease has shown itself in the West Indies to be 

 capable of permanent existence in favourable situations. The 

 tendency of such diseases in general is to decrease with time ; 

 sooner if appropriate measures are applied, later and after heavy 

 losses if they are not. 



The reverse situation to this is seen in the increase of debility 

 diseases as soils decline in fertihty. A period in which they 

 are prevalent comes in the history of new industries and of the 

 exploitation of virgin soils before a tradition of manuring and 

 perhaps rotation is reluctantly acquired. The situation recurs 

 if agricultural practice declines for any reason. In the West 

 Indian sugar industry there has been evidence for many years 

 of a more or less general decline, slow but continuous, in fertility, 

 as shown by the shortening of ratooning periods and increasing 

 prevalence of root disease. This appears to be connected with 



