96 FROGHOrPER BLIGHT OF SUG\R-CANE. 



SECTXON^ Ylir. 



THE FACTORS INFLUENCING THE PREVALENCE 

 OP BLIGHT. 



From year to vear and from ^\'eek to week — from district to district 

 and from field to field, the blight varies in intensity. It is the piu-pose of 

 the pi-esent section to discuss the influence of the factors of the environ- 

 ment that vai-y with time and locality. and so produce these local 

 difi'erences in the extent and severity of the injury. 



These factors may be conveniently grouped under four sub-headings : — 



(1) The j)hysical cnvironme)it, or those factors such as rainfall, 

 temperature, soil and contours of the land over which man has little or 

 no control. 



(2) The cujri cultural jJt'ocesscs such as drainage, tillage, rotation, 

 manuring, etc., by utilising which the planter can make the best of his 

 given physical environment. 



(3) Tlie characteristics of the cave itself which vary in different 

 varieties and produce greater or lesser resistance to injury. 



(4) The direct effect of the niimhcrs of frocfhoppers present and the 

 effects of root fungus and other diseases on the cane in weakening 

 resistance to further attacks or preventing the recovery from past injury. 



The methods by which these various influences may be utilised to 

 prevent or control blight will be discussed in Section X. Here we are 

 concerned chiefly with the degree of relation between the environment 

 and the damage. 



RAINFALL. 



The quantity and distribution of the rainfall during the year is 

 probably the most im[»ortant single factor in the environment of the 

 froghopper. 



From the vei'y first the connection between rainfall and blight has 

 been noticed, and Cruger in I860 considered the blight as due to "a want 

 of rain at a cex'tain period." Other observers have at intervals recorded 

 their opinions, some believing that an excess of rain favoured blight, 

 others that it was due to a deficiency, and others again that rainfall had 

 no effect. 



In 1919 I made a special study of the rainfall in relation to blight by 

 comparing it in good and bad years and on good and bad estates. It 

 was shown first that there is no relation between the total annual 

 rainfall and blight (see Fig. 20), but that there is evidence that the 

 distribution of the rainfall has a distinct effect. 



