STEM AND LEAF DISEASES 153 



dense pile of fine hairs bearing tiny globular heads, the sporangia 

 of the alga. The latter when ripe discharge upon being wetted 

 numerous biciliate zoospores, which after a short active period 

 come to rest and are capable of germination. The ripe sporangia 

 are capable of being broken off and transported by wind before 

 this discharge occurs. 



A second type of sporangium, essentially similar, is formed 

 in the body of the thallus as it occurs on the surface of leaves. 



The alga is itself liable to be invaded by fungus hyphae, the 

 combination producing for a time a type of structure resembling 

 that of a lichen. 



Nature of the Attack. 



On various leaves the alga exists in all conditions between 

 the common case of mere attachment to the cuticle and complete 

 penetration leading to the development of thalli (cellular ex- 

 pansions) on both sides of the leaf. The epidermal cells beneath 

 an algal patch are usually but not invariably discoloured and 

 dead. The only importance attributed to the occurrence of the 

 organism on leaves lies in the provision of material which can in- 

 fect the stems. 



Infection of the twigs appears to take place from germination 

 of the zoospores in the surface cracks which develop in connection 

 with the formation of the first layers of bark. The alga pushes 

 in among the cortical tissues, successive layers of which are 

 sloughed away. The irritation caused by the presence of the 

 parasite sometimes results in hypertrophy (abnormal swelling) 

 of the twigs. In severe cases the separate patches first occurring 

 on the twig may coalesce and completely envelop it for some 

 distance. 



If growth is not sufficiently vigorous to throw off the infection 

 by new and deeper bark formation the result is the failure of the 

 leaves and the death of the twig, and even if this does not occur 

 the lesions in the bark are liable on weakly trees to afford access 

 to the dieback fungus, Diplodia. 



Influence of External Conditions. 



The accounts of the diseases caused by this organism agree 

 as to the dependence of serious injury on lack of vigour in the 

 host. On tea the disease occurs suddenly and constantly when- 

 ever bushes become unhealthy from any cause whatever. On 

 cacao most harm has been observed where the trees are in poor 

 condition as a result of exposure or insufficient drainage, or in 

 young plantations where favourable conditions have not yet 

 become established. 



The reasons for this are believed to lie in the inability of 

 weakly twigs, as indicated above, to get rid of the parasite, and 

 in the greater susceptibility to infection of twigs in which bark 



