GENERAL CONSIDERATIONS 83 



pen. In a few cases such material should be promptly burnt, 

 but as the loss of organic matter is to be avoided in arable agricul- 

 ture under tropical conditions burning should only be resorted to 

 when specifically advised. 



Means of avoiding damage. 



As the incidence of many diseases is unpreventable by the 

 means so far discussed, and in many more is usually unpre- 

 vented, consideration passes to the means which may be adopted 

 for control. 



Resistance. 



Of these the most effective and the least troublesome, because 

 it is automatic, is that which lies in the resistance of the plant 

 to infection or to serious infestation. This, it has been pointed 

 out in a previous chapter, may be due to some hereditary quality 

 or character, or it may depend on the condition of the plant as 

 determined by its environment. Some degree of this acquired 

 resistance can be had in all crops and under all conditions by 

 giving the best care possible under the circumstances, but its 

 extent is limited by the possibilities of soil, situation, and climate, 

 which may not be favourable to the best development of the 

 plant. This kind of resistance is effective against most types of 

 disease caused by saprophytes with weak powers of parasitism, 

 such as root disease of sugar-cane and the Diplodia diebacks, 

 and also against the numerous class of special parasites which 

 principally develop on senescent leaves but advance the time of 

 their attack on weakened plants. The West Indian leaf-mildew 

 of cotton is a good example of this, as are several of the true 

 rusts and many leaf -spot parasites. A vigorous plant moreover, 

 even when not resistant, may keep pace with the development 

 of the disease and produce a crop in spite of it. 



It happens in some cases, though they are naturally difficult 

 to recognise, that a dangerous degree of susceptibility in otherwise 

 weU-cultivated plants may arise from the shortage of some par- 

 ticular substance in the soil. There are several well-marked 

 instances on record where this condition has arisen through 

 a shortage of lime. In others it has been shown that resistance 

 could be increased by the application in the particular case of 

 potash or of phosphates. 



It must be fully recognised that there are many diseases 

 against which the type of resistance just discussed is not effective, 

 in which in fact the parasite may flourish all the better for the 

 abundance or succulence of its food supply, or in which, as in the 

 Rosellinia diseases, the conditions which favour the most perfect 

 development of the host are those which also best suit the 

 parasite. 



Inherent (hereditary) resistance, on the other hand, may be 

 effective against infection or infestation with the most virulent 



