W. MCRAE 229 



monsoon the trees in this valley were badly infected, and both fruit-rot and 

 second leaf- fall were found on trees throughout the estate of about 1,000 acres, 

 though the attack was irregularly distributed and, on the whole, Ught. 

 Towards the end of the monsoon bark-rot appeared for the first time. This 

 is fairly typical of what occurs in the younger estates, that are being more 

 carefully observed in the light of better present knowledge. Some estates 

 seem to be particularly subject to bark-rot especially in part of the district 

 where the rainfall is high, and during the monsoon precipitation is ahnojst 

 continuous for weeks at a time. Here blocks of one hundred acres could be 

 selected in which about 50 per cent, of the trees are or have been affected by 

 bark-rot. Taking the rubber-growing districts as a whole, however, bark-rot 

 cannot be said to be a serious disease, though it nmst be classed as a 

 dangerous one. 



The disease caused by this species of Phytophlhora extends from the 

 southern limit of the area in which Hevea is grown to some distance north of 

 Calicut. Within this area every district is affected to a greater or less extent. 

 Further north the estates are not extensive, the rainfall is much lower, and, 

 though the fungus has been found there, as far as 1 know the phenomena 

 described above occur only to a limited extent. 



Microscopic characters in the tissues of the plant. 



Sections of the dull greyish spots on the leaf show hyphae in the mesophyll 

 passing into and through the cells as well as lying close alongside them in 

 the intercellular spaces. They also occur in the epidermal cells of both 

 surfaces. They pass outwards through these cells, through the stomata, and 

 through ruptures in the epidermis, and form on the surface a thin felt of 

 mycelimn bearing sporangia. When the spot has become just visible on the 

 leaf-surface, the protoplasm of the penetrated cells is still colourless. The 

 tissue as a whole, however, contracts slightly in diameter as the section 

 through the edge of the spot shows. The chlorophyll-granules lose colour, 

 and ultimately the cell-walls become yellow, and the cell-contents also 

 become yellow or even brown, and collapse so that the cells come apart 

 easily. Minute lesions appear into which latex exudes, and it escapes to 

 the surface in minute drops. Hyphse do sometimes penetrate the latex 

 tubes, but this has been observed seldom. Whether the latex escapes 

 from the laticiferous tubes at the points where they are penetrated by the 

 hyphae, or whether the walls of the tubes are disintegrated by the agency of 

 the fungus or otherwise, has not yet been determined. Yomig leaves bursting 



