DISEASES OF PLANTS 629 



matter, and thereby discourage conditions which predispose plants 

 to disease. Thus the burial with lime of all diseased cacao pods, 

 as well as the husks of healthy pods, left after the beans have been 

 extracted, has been found to have an important effect in preventing 

 the spread of the pod-disease of Cacao. 



Conditions favourable to infection. In damp, warm weather 

 and in shady situations, spores of fungi stand the best chance of 

 germination. Conditions of the soil which are unfavourable to 

 plants, as defective drainage, etc., are sometimes conducive to the 

 rapid spread of a root disease. Crowding together plants of the 

 same kind also favours the spread of disease, or an epidemic or 

 extensive wave of disease can only occur where large numbers of 

 the same kind of plant are growing in close proximity, as in the 

 exclusive cultivation of single crops. 



Wounds a cause of disease. A large number of fungi can 

 bring about infection of their host-plants only through wounds or 

 breaks in the bark. Canker is generally formed by a wound 

 parasite, and in order to avoid conditions likely to bring about 

 infection by it, careful attention should be given to the operations 

 of pruning, fruit-picking, etc., as well as the tarring of all wounds. 



Isolation of affected areas. The spread of a root disease may 

 sometimes be effectually arrested by means of cutting a trench 

 round the affected area, the progress of the mycelium through the 

 soil being thus confined to certain limits. This prevents healthy 

 plants being attacked, and enables the affected areas to be cleared 

 and treated with large quantities of lime. 



Effects of rotation of crops. A fungus disease may often be 

 starved out by rotation of crops. This, however, is only possible 

 in the case of annual or temporary crops, like root products, etc. 

 In the case of permanent crops, as Cacao, Tea, Rubber, etc., when 

 a tree has died from a root disease, the diseased roots should be 

 carefully extracted from the ground and destroyed, the soil being 

 opened up and treated with lime, the cavity being allowed to 

 remain open for some time before the vacancy is supplied. 



Disease-resisting varieties. Some varieties or species of 

 plants are more or less immune from disease, while others, culti- 

 vated under the same conditions, are specially susceptible to it. 

 Therefore varieties which are the least susceptible, consistent with 

 other desirable qualities, should be selected for cultivation. The 

 raising or selection of disease-resisting varieties is now recognised 



