84 DISEASES OF CROP-PLANTS 



of parasites, but as a rule, where it is not due simply to inherent 

 vigour, it is narrowly restricted in range. Sugar-cane resistant 

 to red rot, or bananas resistant to Panama disease, for example, 

 are not necessarily protected against any other sugar-cane or 

 banana disease. Nor is resistance of this type necessarily 

 constant under varying environmental conditions, so that varieties 

 perfectly resistant in one locality have sometimes proved sus- 

 ceptible in another, and resistance which operates for the ordinary 

 prevalence of a disease may fail against the mass attack of an 

 epidemic. The utilisation of resistant varieties is also subject 

 to the disability that their produce may be unsatisfactory in 

 some commercial quality, as in the bananas resistant to Panama 

 disease, or may cover only one section of the demand for a 

 graded article such as cotton. 



It will be seen that dependence on acquired resistance, or 

 resort to inherently resistant varieties, is subject to many limita- 

 tions. The possibilities of these methods should, however, 

 be always kept in mind, and growers may usefully call attention 

 to apparently resistant plants when they occur ; the critical 

 testing of this quality is a technical matter best handled by the 

 pathologist. 



Disease Evasion. 



There are various ways in which disease may, under appro- 

 priate circumstances, be evaded. Land may be used which is 

 virgin to the crop, or remote from the same or related crops. 

 Of more general application is the timing of a crop to avoid the 

 seasonal prevalence of diseases ; thus cotton, which is highly 

 susceptible to damage from boll diseases in wet weather, may 

 be planted so as to bring the fruiting period within the edge of 

 the dry season. With short-term crops such as Indian corn 

 and many garden vegetables, the first plantings often escape 

 pests and diseases to which later ones are liable. 



Elimination Methods. 



The simplest kind of treatment for the control of a disease 

 which has gained access to a cultivation consists in the removal 

 of diseased plants or their affected parts : whole plants may be 

 pulled up or cut down and burnt or buried, diseased leaves and 

 fruit may be collected from the plants or from the ground, 

 diseased twigs or branches pruned away, affected bark or cankers 

 cut from stems ; while root diseases may be restricted by the 

 isolation of infected trees. Measures such as these are frequently 

 the only ones it is possible or profitable to adopt, and they are 

 commonly desirable as supplementary measures where spraying 

 or other treatment is applied. The effectiveness of elimination 

 in the control of disease varies widely. In the case of coconut 

 bud-rot thorough campaigns against diseased trees have been 

 reported very successful, and results may be similarly good in 



