■122 hROoiiopr'ER T,Li<;iiT of sugar-caxk. 



sECTioN^ ix:. 



THEORIES OF DAMAGE AND GENERAL PROBLEMS. 



In previous Sections we have given an account of the froghopper 

 and its habits ; its natural enemies ; the root fungus ; the effect of frog- 

 hopper and root fungus injury on the cane ; and the way in which this 

 is influenced by the environment of the cane field. In the present 

 section an atteuipt will be made to discuss some of the general problems 

 arising out of these facts. 



THE CAUSE OF THE BLIGHT. 



The most important of these problems is the determination of the 

 relative importance of the different factors concerned in the production 

 of blight and the way in which they are related to each other. 



For many years there has been discussion on the parts played by 

 root disease and froghopper, and this question has been examined and 

 summarised in several recent reports by Mr. Nowell and myself {see 

 WilHams 1919, Nowell 1919 and 1920, and Nowell and Williams 1920). 



We are concerned with the adult and nymph froghoppers, root fungi, 

 some other insects and diseases, and a variety of external conditions. 



These factors may be inter-related in many ways to produce the 

 final visible effect. 



The damage may arise : — 



(1) Through the sucking of the froghopper, adult or nymph. 



(2) Through the destruction of the roots by root fungi. 



(3) The previous attacks of froghopper may weaken the canes and 



produce outbreaks of root disease. 



(4) An outbreak of root disease may render the canes more 



attractive to froghoppers. 



(5) The environmental conditions may act directly on the frog- 



hopper or indirectly on it through its natural enemies. 



(6) They may act directly on root fungus. 



(7) They may act directly on the cane rendering it less able to 



resist injury. 



The Part Played by the Froghopper. 



There is now no doubt that the sucking of the froghopper is by far 

 the most important immediate cause of blight. The chief evidence for 

 this is as follows : — 



(a) The constant presence of the froghopper in blighted fields in 

 numbers which, in the early stages of damage at least, bear 

 an approximate relation to the amount of damage. 



(6) The constant presence in blighted canes of discoloured streaks 

 on the leaves, which have been shown experimentally to be 

 produced by the sucking of the adult froghopper. 



(c) The occasional occurrence of severe blight in fields, usually of 



plant canes, in which root fungi and other pests and diseases 

 are almost absent and the abundant froghopper is the only 

 apparent cause. 



(d) The first occurrence of blight in the wet season is about the 

 time of the first brood of fi'oghoppers and is early in those 

 years in which the froghopper broods are early and late when 

 they are late. 



