82 DISEASES OF CROP-PLANTS 



2. The parasite may be introduced with the planting material. 

 In crops which are grown from seed a parasitic fungus or 



bacterium may be introduced with the seed, either adhering ex- 

 ternally or, if it is from an infested plant, as an internal infection. 

 The simplest means of avoiding this is to plant, when possible, 

 seed from a perfectly healthy source ; where this cannot be 

 guaranteed, some form of seed sterilization, the methods of which 

 are discussed on a later page, may be adopted. With some crops 

 the trouble this involves is exceedingly well repaid. The same 

 considerations apply with even greater force in the case of crops 

 planted from cuttings. It is often desirable to supply these 

 from nurseries, which may be established in favourable situations, 

 perhaps at a safe distance from the main crop, and can be given 

 special attention to keep them healthy. 



3. Infection may come from standing crops or allied plants. 

 It is commonly the case in the tropics that an older crop may 



be still standing when fields of the same plant are being estab- 

 lished. In this event the conditions are favourable to the 

 passage of spores from old to new, blown by wind or washed by 

 rain, carried by birds and insects, by labourers and by imple- 

 ments passing from one field to another. For this, if the co- 

 existence of crops of different ages is a necessity, as is the case 

 with sugar-cane, there is little remedy. Where the necessity 

 does not exist a voluntary or compulsory adjustment of the, 

 time of planting may be of the greatest value, as is demonstrated 

 in those islands where a close season for cotton has been adopted, 

 involving the destruction by burning or burying of all old plants 

 by a certain date some time before planting is allowed. There 

 is much that could be done of a related nature by the control of 

 plants, wild or escaped, growing by the sides of roads and traces 

 and on waste land. Frequently these plants serve to carry 

 over the pests and diseases of allied crop-plants. With some 

 initial trouble and a little care from time to time such situations 

 may be covered with a growth of some harmless and self-main- 

 taining plant, which may moreover serve a useful purpose as 

 fodder, mulch or firewood. 



An instance of the kind of action described is afforded by the 

 very effective control of the insect principally involved in the 

 transmission of internal boll disease of cotton, which has been 

 achieved in St. Vincent by eradicating two useless Malvaceous 

 trees on which the insect bred between cotton seasons. 



4. Infection may be introduced with manure. 



With most fungi, some of the smuts being exceptions, well 

 rotted pen manure may be considered safe. This cannot be said 

 for vegetable mulches when these are derived from plants which 

 are identical with or related to that composing the crop. Sugar- 

 cane trash, for example, may carry many of the diseases to 

 which that plant is liable, and the material from infested fields 

 should not be applied until it has been well rotted down in a 



