SECTION II.-- NATURE OF THE " BLIGHT." 27 



in each leaf from the centre roll outwards at various intervals after ;i 

 froghopper outbreak. The height of the vertical lines represents the value 

 to the plant of the successive leaves. A, shows a healthy plant in 

 which there were ten leaves all of which were completely green. 



B, shows a plant one month after being slightly tlamaged by froghopper-^. 

 The five most recent leaves are quite healthy while the next seven all 

 have their surface more or less reduced by the brown discoloration of 

 the streaks, the greatest loss, however, not being beyond 20 per cent. 



C, shows a plant at the same period which had been originally 

 much more seriously damaged and has only four young green leaves 

 undamaged and the next six damaged up to as much as 80 per cent, of 

 their surface. D, shows a plant three months after severe damage in 

 which all but the outermost leaf have been replaced and are quite free 

 from injury. 



The production of new leaves has been estimated by several other 

 methods with similar results. 



A.S the time taken to produce a complete new set of leaves by a 

 plant is greater than the average time between successive outbreaks 

 (about two inonths) it follows that a plant damaged in one brood has 

 not regained its normal vigour on the outbreak of the next attack, which 

 will account for the greater severity of the damage in second and third 

 attacks, even when the number of froghoppers present appears to be 

 more or less similar. 



VARIATION IN INTENSITY OF BLIGHT. 



It has been pointed out that the blight occurs first about two months 

 after the advent of the wet season, and then recurs periodically at 

 intervals of about two months. These times correspond with the 

 successive flights of the adult froghoppers, the life cycle of this insect 

 occupying about two imnths. The greatest visible blight however, is 

 not at the time of greatest abundance of adult froghoppers, but con- 

 siderably after this. In the broods of 1919, when careful observations 

 were made on this point, the interval was about two or three weeks. 

 This is due, as has been mentioned above, to the time taken for the 

 spread of injury from the original punctures on the leaves. 



About the time of the second or third brood the short dry period 

 known as the Indian summer occurs. If this is severe and long it may 

 stop recovery and the canes graluallv go from bad to worse. Farther 

 discussion on this point will be found on p. 96. 



The distribution of damage in various years varies greatly. Some- 

 times the blight is not severe bub widely distributed ; at other times it is 

 very severe in small well defined areas immediately surrounded by quite 

 healthy canes. These patches are usually related to the contours of the 

 ground, outcrops of different soil, different ages of canes, or different 

 cultivation. In some cases the attack spreads into an otherwise healthy 

 field from a diseased area alongside. A fuller discussion of these 

 problems will be found in sections VIII and IX. 



SIMILAR INJURIES FROM OTHER CAUSES. 



Root Disease and froghopper injury are frequently intimately 

 associated and the condition called " blight" is usually due to the com- 

 bined effects of the two. The relative part played by each is discussed 

 later. 



At the same time there are quite distinct and recognizable differences 

 between the uncomplicated forms of each. The most obvious of these, 

 apart from the presence or absence of the insect, is the absence of any 



