SUMMARY, 151) 



Presence of Pests and Diseases. As a general rule 20 froghoppei-^ 

 per stool will produce visible signs of damage, and 50 to 100 per stool 

 will produce severe damage. Fields in poor condition suffer more froui 

 a small number of insects than better fields do from a larger infestation 

 (p. 120). 



Root fungi are constantly found in blighted fields as well as various 

 insect pests of cane which weaken the cane and make it less resistant 

 to froghopper attacks (p. 120). 



A-Uowing for changes in season and age of plants a field attacked one 

 year is more liable to be attacked in the following year than one 

 previously free (p. 121). 



GENERAL PROBLEMS. 



The question of the relative importance of the adult and nympji 

 froghopper and of the root disease is discussed. 



The froghopper is considered as the most important cause of tlie 

 blight because of (1) its constant presence in blighted areas, (2) the 

 constant presence of streaks on the leaves in severe blight, (H) the 

 occurrence of blight in plant canes when no other complicating disease 

 is present, (4) the variation in time of the first appearance of blight 

 according to the date of the first brood, (5) the periodicity of tlie 

 blight (p. 122). 



The adult is considered as more important than the nymph because 

 the blight is never severe until the streaking of the leaves has developed 

 ;ind is at its worst shortly after the greatest flight of adults. Symptoms 

 of blight have been produced in canes, which have never had nymphs, 

 by caging on them a number of aJults (p. 12o). 



In addition several of the symptoms of blight, including in particular 

 the narrowing of the cane, have been produced in normal canes l)y 

 imitating the leaf destruction of adult froghoppers by removing the 

 greater number of leaves from the cane (p. 124). 



The spread of injury in the leaf from the original point of infection 

 indicates the possibility of some toxin or enzj'me being introduced into 

 tiie plant (p. 124). 



Root disease is considered as playing its most important part after 

 the froghopper has damaged the cane and particularly after the second 

 brood, when the damage may cease to bj pGrloJle aiiJ coutiauo tj 

 increase entirely owing to the effect of root fungi (p. 125). 



The attacks of root disease follow on the froghopper attacks which 

 are the prime cause of injury (p. 125j. 



There is no [definite evidence as yet that canes damaged by root 

 fungi are more attractive to froghoppers. On the other hand the 

 conditions which lead to root fungus attacks are nearlj' always those 

 which allow the froghopper to breed more freely (p. 126). 



Influence of Environment. A. study of Section IX indicates that 

 the most important factor in determining the prevalence of blight is the 

 humidity just above and just below the surface of the ground. 



(1.) Extreme drj'ness in dry season tends to increase blight. 



(2.) Moderate dx-yness in dry season tends to reduce blight. 



(3.) Dry weather in wet season tends to increase blight. 



(4.) Normal weather in wet season tends to reduce blight. 



(5.) Very wet weather in wet season tends to increase blight. 

 In Table VII. an attempt is made to explain these facts by the 

 known effect of moisture on the cane, the root fungi, the froghopper and 

 its enemies (p. 126). 



