59 



heart wood, which has ceased to function, and to the fact that it 

 very seldom "rings" a tree but goes up to the collar on one side 

 only. 



In one field of rubber, 30 per cent, of the trees were affected by 

 the disease before its presence was even suspected. The disease is 

 usually discovered earlier on wet lands as owing to the poor develop- 

 ment of the root system, attacked trees blow over sooner. 



So far only one estate has had its young rubber badly attacked 

 by Poria ; this is not because young trees are immune to attack, but 

 because the laterals take time to establish contact. Isolated cases 

 can often be found on young rubber, and without doubt, many deaths 

 formerly attributed to Fomes or brown root disease, were due to the 

 then undescribed Poria. 



Treatment. 

 With few exceptions treatment of individual trees is im- 

 practicable, as the diseases are never discovered till too late. 



Attention must therefore be turned to prevention. 



Prevention. — Is of two kinds : (a) Total prevention of attacks ; 

 (fc) Prevention of spread from attacked areas. 



It has been shown that all the root parasites of Hevea also live 

 on dead wood, i.e., can be saprophytes ; and that it is from dead 

 wood that infections arise. The obvious remedy is to remove all 

 dead timber and stumps both of jungle and Hevea trees, i.e., to 

 adopt clean clearing : if this be done, it is not too much to say that 

 immunity will be ensured from Brown root disease and Poria, while 

 infections of Fomes, Sphxrostilbe, and Ustulina will be reduced 

 to those from spores blown or brought from neighbouring jungle 

 or plantations. 



It is usual to find that saprophytic fungi, which become 

 parasites, can only do so by gaining an entry through a " wound," 

 and the results of inoculation experiments support this as applied 

 to Hevea root diseases. It might be argued that if wounds could be 

 avoided, i.e., by covering all laterals, the roots would be immune 

 from attacks. Unfortunately a " Avound " in the technical sense, 

 includes any break in the protective covering of the plant. Such 

 breaks may arise in many ways, of which the deaths of very small 

 rootlets is one of the commonest. It follows that although exposed 

 wounds of animal origin should be avoided as far as possible, because 

 of the danger of spore infection, the root fungi cannot altogether be 

 prevented from entering by such precautions. 



Before work was carried out on Poria and Ustulina, it was 

 thought that clean clearing was not essential, the idea being, that 

 trees attacked by Fomes died in a relatively short time, so the 

 presence of the disease would be discovered, and the source removed 

 before neighbouring trees had time to establish contact. Also in the 



