294 DISEASES OF CROP-PLANTS 



contour of the ground or the nature of the soil. Where its 

 origin is due to drought the windward edges of the field are often 

 worst affected. The predisposing causes of its prevalence may be 

 summed up under the headings : dry weather, poverty of soil, 

 defective aeration, and insect injuries. 



Soil poverty and drought are much the same in their results, 

 and root disease as the effect of the former most frequently 

 becomes evident as rain fails. On the other hand, it is possible 

 by thorough cultivation, by heavy manuring, by reducing ratoons 

 and by rotation of crops, to so increase the resistance of the cane 

 and reduce the quantity of fungus that the effects of a low rain- 

 fall are largely offset. It is from this point of view that the 

 intensive agricultural methods characteristic of Barbados must 

 be regarded. 



Defective aeration, which confines the roots to the upper 

 layers of the soil, reducing their feeding range and leaving them 

 susceptible to the first influence of dry weather, may be due either 

 to want of tilth in the soil or to the saturation of the soil with 

 water. Both causes operate to induce root disease. The retention 

 of tilth depends, (a) on the physical consistency of the soil, so 

 that some soils are for this reason less suited to ratooning than 

 others, (6) on its content of organic matter, (r) on the frequency 

 and thoroughness of cultivation, and ((/) on restriction of the 

 operations of implemental tillage to suitable weather. The watP'^ 

 logging of the soil is usually due to imperfect provision 1 

 drainage, but in some cases is attributable to the highly retenti; 

 nature of certain soils, which in wet weather refuse to drain at l 

 adequate rate even on the margin of the trenches. At the same 

 time that the drying out of water-logged soil causes an improve- 

 ment in canes suffering for that reason, dry weather takes effect 

 on those in light or compacted soils and makes their condition 

 worse. 



Insect injuries and root disease may be intimately associated, 

 as in the froghopper bhght of Trinidad. The fields attacked by 

 froghopper are usually those which by their age or situation are 

 most liable to root disease, and after they are damaged by the 

 insect the fungi get a hold on them which prevents recovery when 

 the insect infestation has passed. In this way an end result of 

 whole fields or tracts appearing completely crippled by root 

 disease is produced in the situations where the froghopper has 

 been prevalent. The infestation of stools with root grubs also 

 produces or increases susceptibility to root disease. 



So far the discussion has emphasized the harmfulness of root 

 fungi, but an account of root disease would give a wrong im- 

 pression which did not indicate the ability of plants growing under 

 good conditions to make excellent growth in spite of heavy 

 infestation of the stools. This is best seen when canes in good 

 soil, infested as a result of drought, resume growth with the return 

 of rains. Stools which appeared dead will send out vigorous 



