and Magazine of the Ceylon Agricultural Society.— October, 1912, 323 



usually rough. When the scale dries, it fre- 

 quently cracks, so that the result is a cracked 

 scaly patch, quite different from the canker in 

 its progressive stage. It is often difficult to say 

 whether a scaly patch has been caused by can- 

 ker or not, once the scales have become dry. If 

 a patch of dead dry bark is found embedded in 

 healthy cortex, that is almost certainly a self, 

 healed case of canker. At the present time, it 

 would be preferable to treat all scaly bark as 

 possible old cases of canker. But the disease 

 should be found at an earlier state than this. 

 Why the fungus does not continue to grow and 

 ultimately kill the tree in these cases can only 

 be surmised ; personally, I should consider it a 

 result due to the weather, but it is open to any- 

 one to argue that the fungus has become less 

 virulent or the tree more resistant. 1 may 

 here remark that if the mycologist is to identify 

 a case of canker from a sample of bark only, he 

 must receive it within 24 hours of the time of 

 taking it from the tree. If the sample is left on 

 the verandah for two or three days, or if the 

 V A motors it round the district and posts it at 

 the end of the week, the diagnosis is a matter 

 of chance. Of course, it would be possible to 

 make a correct determination even in that case, 

 given about a month's work on the specimen for 

 fixing, embedding, section cutting, staining, 

 etc. ; but we cannot afford thai, and it is quite 

 unnecessary if the sample is posted immedi- 

 ately in a tin. 



The Fungus. 

 ThefuDgus which causes canker is a Phytopht- 

 hora, a member of the same family as the fungus 

 which is the cause of the most serious Potato 

 disease. It cannot be detected without a micros- 

 cope : it does not produce any large fructifica- 

 tion or strands of mycelium as Fomes semitostus 

 does. Here are drawings of various stages of 

 the fungus. It permeates the diseased tissue, 

 and ultimately puts out a small tuft of threads 

 which produce lemon-shaped bodies known as 

 sporangia. These sporangia become detached, 

 and are washed away by the rain or blown away 

 to other trees. If the sporangium falls on a wot 

 surface, its contents divide up into a number of 

 small masses of protoplasm. Then the apex rup- 

 tures, and these masses are extruded. Each is 

 provided with two threads which are in constant 

 movement, and with the aid of these it swims 

 about in the film of water. These bodies are 

 called zoospores. Ultimately they come to rest 

 round off and enclose themselves in a cell wall, 

 and finally germinate like ordinary spores. The 

 germ tube, or primary thread, bores into the 

 CCttex of the Hevea, and so reproduces the dis- 



ease. That is the typical life cycle, but there 

 may be variations. The sporangium in some 

 instances does not produce zoospores, but be- 

 haves as an ordinary spore. And under certain 

 conditions, the fungus produces " resting 

 spores," — thick-walled spores which preserve it 

 through the dry weather. Such resting spores 

 are formed within the diseased tissue. They 

 carry the fungus over from one wet season to the 

 next, but the chief means of spread during the 

 wet weather is the sporangium and the zoos- 

 pores. In addition to causing Hevea canker, the 

 same fungus 



ATTACKS HEVEA FKUITS, 



and produces the well-known pod disease. It is 

 to be expected therefore that an extensive attack 

 of pod disease will be followed by an increase of 

 "canker." Arguing from analogy with the cacao 

 disease, some part of the dying bark of green 

 branches which occurs in connection with the 

 pod disease is due to the same fungus, but that 

 has not been definitely established. The fungus 

 of Hevea canker is the same as that which 

 causes cacao canker, and the symptoms of the 

 two diseases are practically identical. In cacao, 

 however, where the fruits are borne on the old 

 wood, the disease travels from the fruit into the 

 stem through the Stem of the pod. That does 

 not occur in Hevea, or at least, the fungus does 

 not travel from the fruit, down the branches, 

 into the main stem. I am often asked why 

 "canker" should occur in districts where there 

 is no cacao? Well, why did it ever occur on 

 cacao ? There is no doubt that it is a native 

 fungus, and 1 believe that it is identical with the 

 species which attacks Breadfruit, but that 

 point has not been proved by infection experi- 

 ments. Another similar fungus attacks are.ca- 

 nuts in South India, but Coleman has proved 

 that that is not the same as the one on cacao. 

 The 



METHOD OF TBEATMENT 



advisedjic 1904 remains the only possible one. The 

 diseased cortex must be cut out and burnt. Where 

 the disease is discovered in au early stage, 

 before it has penetrated right through the cor- 

 tex to the wood, it is sufficient to scrape or cut 

 ofl' the cortex until healthy laticiferous tissue 

 is reached. Such scraping can be done with a 

 piece of hoop iron, If all the disease could be 

 discovered in that stage, it would do very little 

 damage. Where the wood beneath the patch is 

 discoloured a thin shaving might be removed, 

 but I do not think it is worth while to cut away 

 much of the wood : cutting away the wood, if 

 done smoothly, will not do any harm, but in all 

 probability it is not worth the time. In cases, 



