SECTION IX. — THEORIES OF DAMAGE AND GENERAL PllOIiLEMS. 125 



saliva is pumped into the wound. He sa3's that it is full of enzymes, 

 which aid digestion, and also prevent the' coagulation of the sap. (More 

 probably the original effect is to facilitate the extraction of the sap by 

 action on the protoplasm which lines the cell walls). He also mentions 

 (1914 p. 70) that the glands differ somewhat in the nymph and adult. 

 If such a structural difference were reflected in the composition of the 

 saliva, it might account for the greater destructiveness of the adult. 



There is also to bfi considered tlie possibility that some infection is 

 introduced into the wound. This is known to be the case with some 

 insects (e.g. the leaf-hopper which transmits "curly leaf" of sugar- 

 beet, and various insects suspected of transmitting " mosaic disease " of 

 sugar cane), and in the present case local infection may be set up in the 

 neighbourhood of the puncture by a similar process. Further investi- 

 gation along these lines is needed though there is at present no evidence 

 supporting such a view. 



The Part Played by the Root Disease. 



The evidence that root-fungi play a material part in the production 

 of blight is as follows ; — 



(a) The almost invariable presence of root-fungi in blighted fields, 

 usually in greater quantities in severely blighted fields. 



(6) The close dependence of the blight on weather conditions, 

 particularly drought, which is known in many other countries 

 to be correlated with severe root disease attacks. 



(c) Although the blight is periodic up to September or October, 

 after this month in bad years the fields frequently continue to 

 get worse, or at least fail to improve even after the froghoppers 

 have disappeared. There must be some other cause holding 

 them back, and of these the most apparent is the diseased 

 condition of the roots and the visible presence of root-fungi in 

 large quantities. 



{A) In some fields the damage appears to be greater than the 

 number of froghoppers present would be likely to cause, but 

 great care has to be used in estiniating this, as the planter 

 does not usually draw attention to a field until the damage is 

 severe and this may be when the number of froghoppers has 

 already greatly diminished. 



(e) When there is noroot disease present, recoTery is very much 

 more rapid than when it is widespread. 



Relation between Root Disease and Froghopper. 



From the evidence given above it will be seen that typical blight can 

 exist without root disease, but not without froghopper, and further that 

 the chief evidence of the destruction by root-fungi is in connectibn with 

 the later broods. 



From these and many other observations in the field one is led to 

 the conclusion that the attacks of the froghopper are the prime cause of 

 the blight, bvt that they produce in the cane a weakened condition that 

 results in the rapid, spread of root-fungi, which in turn become des- 

 tructive, and in some years and localities may finally cause greater 

 injury than that originally caused by the froghopper. 



