326 DISEASES OF CROP-PLANTS 



stantly develop reddish dots thickly scattered, but as a rule 

 the variation is only from dark to pale yellowish or whitish 

 green. Most of the doubtful cases can be determined by ex- 

 amining the young unfolding leaves, which always show symp- 

 toms if mosaic disease has infected that particular shoot. 



In many varieties of cane the shoots when thoroughly infested 

 show distinctive markings on the surface of the cane itself. On 

 the younger joints (intern odes) these are usually in the form of 

 streaks or stripes, which may be pale yellow on green, or red on 

 yellow. On older joints the surface may be marked with irregular 

 whitish stripes, the development of which is followed by cracking 

 and often by local shrinkage and drying out. The joints of 

 infested canes tend to be short, and where the symptoms just 

 described are prevalent there is great loss of weight. 



In plants from infested cuttings a further symptom appears 

 in the form of white opaque spots and streaks in the light-coloured 

 areas on the leaves. These have no pigment whatever and range 

 from pin-point spots to streaks several inches long. This white 

 tissue remains firm and does not turn red or brown as do the pale 

 spots caused by the feeding of froghoppers. 



Fields planted with infected cuttings soon show pronounced 

 dwarfing of growth, and a similar result is seen in ratoons of the 

 more susceptible varieties which have become infected after 

 planting. 



Influence of External Conditions. 



The disease occurs under all conditions of soil and climate. 

 Cultivation, manuring, and the use of lime do not notably 

 decrease its prevalence, but rather tend to bring the leaf markings 

 into prominence. 



Losses due to the Disease. 



Regarding the losses which have occurred in Porto Rico, 

 F. S. Earle stated in 1919 : " We are safe in concluding in a 

 general way that when infection reaches an average of 60 to 80 

 per cent., losses of sugar will be from 30 to 40 per cent." A 

 table published in the Hawaiian Islands in 1912 showed losses 

 due to this disease varying from 0.5 to 34 per cent., according to 

 variety. In Jamaica S. F. Ashby records tonnage reductions, 

 by comparison with healthy canes, of 33 per cent, in D 116 and 

 27 per cent, in Purple Ribbon. 



The losses in advanced cases of the infestation of highly 

 susceptible varieties extend to the complete failure of the crop ; 

 in others to reduced yields in all degrees according to the pro- 

 portion of stunted stools and rotted stalks. The affected canes 

 have a tendency to be pithy and poor in juice, and on account 

 of changes which follow the splitting of the internodes the 

 behaviour of the juice is liable to cause trouble in the factory. 



