234 DISEASES OF CROP-PLANTS 



fructifications at first predominate, succeeded later by the peri- 

 thecia. 



Contrary to experience with most diseases, infections are 

 most abundant in dry clear weather, and least in rainy weather. 

 This is attributed by Stahel to the more favourable conditions 

 for penetration provided by an all-night coating of dew than by 

 intermittent wetting from rain, owing to the fact that the process 

 requires some ten hours for its completion. 



Effects of the Disease. 



Severe infestation of the young leaves causes defoliation of 

 the trees, and infestation less severe hinders the growth of the 

 leaves and reduces their efficiency. The production of rubber 

 is in consequence reduced, and should the defoliation, as fre- 

 quently happens, be repeated, a severe dieback sets in, and. the 

 process may eventually result in the death of the top or of the 

 whole tree. 



Control. 



No practicable means of control has been found. For spray- 

 ing to be effective the young leaves would need to be coated at 

 least twice during the first four days, which is more difficult to 

 arrange for since the trees in a plantation come into leaf in a 

 scattered way, and even different parts of the same tree do not 

 come into leaf all at the same time. Moreover, the way the 

 young leaves hang makes them difficult to cover. 



Stahel has proposed keeping the trees artificially bare of 

 leaves for three to four weeks to prevent the formation and 

 dissemination of conidia, and has also suggested the use of 

 smoke clouds at night to prevent dew formation, a method 

 practised in vineyards against threatened frosts,' 



