1920.\ FBOGHOPPEB BLIGHT OF SUGAB-CANE.] la 



restricted to a temporary arrest of development due to the blighting of 

 the leaves, together with the death of the young leafy shoots at the base 

 of the stools. 



10. It will be seen that the actual nature of froghopper blight is the 

 same in all these cases. With a field in a given condition the effects of 

 a heavier or lighter infestation result in the scaling upwards or down- 

 wards, among the shoots of differing ages, of the degrees of damage 

 described above, while from an infestation of given severity the amount 

 of damage is governed by the condition of the stool attacked in respect 

 of the degree of development of its canes. 



11. The associated condition of the underground stems and roots 

 does not, as I have seen it, suggest that it has any important causative 

 relation to the symptoms above ground. A large proportion of dead 

 roots may be found in the affected stools, but this appears to be equally 

 the case with many uninfested ratoon fields in poor or compacted soils, 

 and several badly bhghted stools examined had quite as much or more 

 development of fibrous roots than is seen to suffice for the maintenance 

 of stools which are, and remain, healthy and vigorous. Moreover the- 

 younger canes, in the stools dissected, have had the better roots, while 

 showing the worst effects of the trouble. 



12. The foregoing observations are supported by the behaviour of the 

 affected stools. When favourable conditions have prevailed after the 

 passing of the infesting brood an exceedingly vigorous new growth 

 has been seen to appear over areas where no green leaves were left and 

 where most of the short bunches were killed to the ground. Even- 

 during the course of an infestation, where soil conditions are good, the 

 retained vigour of the underground system finds expression in the 

 forcing of giowth from several or many of the eyes of the 

 develo,)ed cane-. 



]:-;. The exp^imental production of froghopper blight has been 

 limited to the leaf effects observed on plant canes caged with adult 

 froghoppers. Pending the results of further experiments, conclusions as 

 to the manner of production of the condition under discussion have to be 

 drawn from observation of tli.; course of attacks and from consideration 

 of the symptoms of affected sto )ls. By these respective lines of investi- 

 gation Mr. Williams and I independently reached the conclusion that 

 the condition could be largely and perhaps completely attributed to the 

 effects of the injuries caused by the feeding of the adult froghoppers on 

 the leaves. How far the feeding of the nymphs on the roots may 

 function as a contributory cause there is at present no evidence to show. 



14 The experimental evidence illustrates the possibility of the 

 causation of failure in the leaves by adult froghoppers ; it has been 

 fur/ther shown that the condition of the individual stalks is closely 

 associated with leaf failure. The production of the unhardened spindly 

 canes can be directly attributed to the failure of the food supply 

 from the leaves, whife inabilij;y of the soft and ill -developed younger 

 canes to maintain themselves during a period of arrested food supply 

 may be readily concsived to be responsible for the degeneration of their 

 upper nodes and eventual dying-back. 



