60 TBINIDAD AND TOBAGO BULLETIN. [X/X. «». 



according to the length of time the attack persists. Damage is usually 

 most conspicuous in dry weather. If conditions become normal recovery 

 takes place rapidly and diseased canes may throw out quite healthy 

 shoots. Root disease is almost alwaj^s more serious in ratoon canes- 

 than in those recently planted. 



FACTORS INFLUENCING THE PREVALENCE OF BLIGHT. 



Rainfall. It has been shown in a previous report that the distribu- 

 tion of the rainfall during the 3'ear has an important effect on bliglit. 



(1) Alternation of wet and dry periods during the growth of the cane 

 is an important factor in determining the prevalence of blight. 



(•2) A long dry season followed by unusually heavy rains in June and 

 July is frequently connected with wide spread blight. 



(3) The most important climatic condition however appears to be 

 the severity of the "Indian Summer", a dry period in September or 

 October. A severe Indian Summer is a general condition botli of 

 localities damaged by blight in one year and of j'ears of widespread 

 blight in one locality. 



This effect is brought about in several different ways : 



(1) The effect on the froghopper itself. 



(2) The effect on its enemies. 



(3) The effect on the cane. 



(4) The effect on root disease of the cane, 



(1) Except in damp localities, the conditions during the dry season^ 

 are below the minimum moisture required for the froghopper to breed. 

 With the first rains of the wet season all tlie eggs hatch and breeding 

 starts. The dates of tlie broods ai-e determined by the date of the first 

 rains. The first brood is about 57 daj's after the beginning of the 

 rains, and the second and third at successive intervals of about 58 days.. 

 The continuation of the rainfall late in the year does not cause the 

 production of a fourth brood so frequently' as might be expected, even 

 when, as indicated by the large first brood of the following year, many 

 eggs are present. 



(2) The vermilion egg-parasite can carry on at a lower degree of 

 moisture than that requii'ed b}' the froghopper, and has been captured 

 during the dry season in places where the froghopper has ceased to 

 breed. The Syrphid fly disappears in the dry season, and appears to 

 require a higher minimum of moisture than the froghopper. The severe 

 fires that occur in prolonged dry seasons destroy large numbers of the 

 lizards, frogs, toads, spiders, etc. that feed on the froghoppers. 



(3) The cane is checked by a severe drought during its growing 

 period. 



(4) The root fungus is always more severe in canes suffering from the 

 effects of dry weather. 



Contour. Low-lying districts are in general more liable to attack 

 than hilly ones. In some cases the blight is worse at the top of a slope 

 than at the bottom, in other cases the reverse is found. This is nearly 

 always a question either of the distribution of different soils at different 

 levels or of drainage and surface washing. 



