78 



THE AGRICULTURAL NEWS. 



March 4, 1911. 



FUNGUS NOTES. 



SOME DISEASES COMMON TO RUBBER 

 AND CAOAO TREES. 



It has been suspected for some time that several of the 

 diseases afiecting Hevea and cacao trees are due to fungi 

 capable of attacking both host plants, and producing similar 

 symptoms ob both, while some of these fungi have also been 

 shown to be capable of infecting Castilloa trees. This point 

 was indicated in the A<jricidtural News, Vol. IX, p. 302, and 

 was also mentioned in the cases of pink disease and die-back 

 on pp. 270 and 318, while the identity of the brown root 

 disease of cacao, Hevea and Uastilloa in Ceylon is mentioned 

 on p. 302 of the same volume. Recent work of very consider- 

 able importance has added to this list; the suspicion in 

 the case of die-back is confirmed as regards the Federated 

 Malay States, and two new diseases, namely, canker and fruit 

 disease, have been added from Ceylon. These will now he 

 considered at somewhat greater length. 



PHYTopHTHOR.\ FABERi, Maubl. It has recently been 

 shown by Fetch, in Ceylon, that this fungus is the true cause 

 of canker and fruit diseases of Hevea, and of canker and the 

 pod disease, known in the West Indies as black pod of cacao. 

 A short reproduction of Fetch's work appear.s in the India- 

 Ruhher Journnl, Vol. XLI, No 2. This discovery constitutes 

 an interesting confirmation of Rorer's work in Trinidad, and 

 also emphasizes the importance of taking all possible precau- 

 tions to keep the diseases of cacao as completely as prissible 

 under control in any district where Fara rubber has been 

 planted. 



It has been found in Cejlon that the disease is only 

 active during the vv'et sea.son, while in dry weather the spread 

 of the mycelium in the tissues of the host is entirely arre.sted, 

 and there is no additional infection. As has been indicated 

 previously (see Af/ricultural Seti's, Vol. IX, p. 318), the 

 symptoms of canker in Hevea are not ea.sily detected, the 

 surest indication being the cessation of the How of latex from 

 all infected bark. When cuts are made into spots where this 

 phenomenon has been observed, it is found that the bark is 

 reddish purple in colour, while the cankered area frequently 

 has a well-defined black border. 



When cacao pods are infected by Fhytophthora, the 

 fungus is able to sjjread up the stalk into the cushion, and 

 cau.se canker of the cushion and of the surrounding bark. In 

 the case of Hevea, infection of the wood)' branches cannot 

 occur in this way, since the fruits are borne on the young, 

 green twigs only. The parasite may, however, spread back- 

 wards along these from the fruits and cause die-back, though 

 the extent of the tissue destroyed in this way is limited to 

 the end of the twig. 



The diseased Hevea fruits, like diseased cacao pods, will 

 naturally serve as a source of spores which, when carried to 

 the trunk of the tree during damp weather, are capable of 

 germinating and forming new areas of canker. (Consequently, 

 such fruits should be collected and burned, just as diseased 

 cacao pods should be buried with lime. 



One or two points liable to be overlooked are worthy of 

 careful attention. In the first place, it was formerly believed 

 that canker could only arise where the bark had been wounded. 

 This was due to the fact that the disease was universally attri- 

 buted to various sa])rophytic or semi-saprophytic species of the 

 genus Nectria, which were known to be unable to produce direct 

 infection, though allied to certain well-known wound parasites 

 occurring in temperate countries. This belief must now be 

 entirely discarded. The fungus in reality responsible for the 

 disease is a direct parasite, and does not recjuire the existence 



of wounds for its entrance, either on the fruits or on the stem. 

 The only requirement is the presence on the trees of sufficient 

 moisture to ensure germination of the spores. In the second 

 place, it is possible that a partly diseased cacao cushion might 

 produce pods. These, as the attack on the cushion developed, 

 would become infected by the mycelium of the fungus, as it 

 grew down into them from the cushion. In this way, more 

 diseased pods and more fungus spores would be produced. 

 Consequently, when a pod diseased at the stalk end has been 

 removed from the tree, a cut should be made into the cushion, 

 also, in order to determine how far back the fungus has 

 spread. All discoloured tissue from such diseased cushions 

 should be removed as carefully as the tissue from the more 

 usual cankered patches. This is important, not only for 

 the reasons stated above, but also because such cushions 

 give rise to as large areas of diseased bark, as do diseased 

 portions of the ordinary stems. Another point is that 

 pods which are discoloured from the pointed end upwards 

 for about half their length are often seen hanging on the 

 trees. Such pods should be removed from the trees when- 

 ever they are noticed, since in this stage of the disease the 

 mycelium of the fungus is unlikely to have penetrated as far 

 as the stalk and, consequently, if the pods are taken away 

 the cushion may be saved from infection. Lastly, it may be 

 recorded that the Immortel trees, Erythrina spp., largely used 

 as shade for cacao, are also subject to canker, and should be 

 carefully examined for this disease when growing in the 

 neighbourhood of badly infected cacao. 



The only remedial measure so far known for this 

 disease is excision. Fetch suggests a new form of subse- 

 quent treatment for small wounds made in this operation. 

 This consists of covering them with a dressing of cow dung 

 and clay, which promotes the growth of the covering bark. 

 Where the wounds are large, they should lie tarred or painted, 

 with the exception of a strip about 1 inch wide all round 

 the edge. This should be dressed with cow dung and clay as 

 is mentioned above, for the bark cannot cover the whole 

 wound, but will grow over the strip around the edge. In the 

 case of Hevea, tarring or painting the major portion of large 

 wounds is especially important, in order to prevent the entry 

 of boring beetles, which destroy the trees. 



L.\sloi)iPL0DiA TilKoBUOM.^E. This is the fungus respons- 

 ible for die-back and brown pod disease of cacao all through 

 the tropics, and for die-back of Hevea in the Malay States, 

 West Africa ;,nd probalily Ceylon. Bancroft, in the Agricul- 

 tiiral BuUi'tin of tin- Straits and Federated Malay States, 

 Vol. IX, p. 475, has shown that the die-back fungus in that 

 country, originally described by Massee as Diplodia rapa.r, is 

 identical with Lasiodiplodia theobromae, and probably with 

 Botryodqilodia elasticae, of Ceylon. Furthermore, he found 

 that cultures of the cacao fungus kept at Kew, developed 

 asci in the [jerithecia and that these asci contained eight, 

 3-septate, dark, oblong spores belonging to the genus 

 Thyridaria, of the family Sphaeriaceae. He has calK\l this 

 fungus Thyridaria tarda, and this is the name by which it 

 will probably be known in future. The discovery of the 

 ascomycetous stage is Important, as it should help in jirevent- 

 ing further mistakes in the identity of the dittorent forms of 

 Lasiodiplodia found on various host [ijants 



Ivorer has shown that, like Fhytophthora, the mycelium 

 of the brown rot fungus is able to grow from the pod into the 

 wood of the cushion and thus cause stem disease, so that in 

 advanced stages of this disease on the pods, the cushion 

 should also be examined. Unlike Fhytophthora, however, 

 this fungus is only a wound parasite, though once it has 

 obtained a footing it can cause serious damage very rapidly, 

 particularly on Hevea. 



