382 



THE AGRICULTURAL XEWS. 



November 26, 1910. 





FUNGUS NOTES. 



FURTHER OBSERVATIONS ON 

 DIE-BACK DISEASES 



Diplodin rapax. Massee, causes die-back of young 

 Hevea trees in the Straits and Federated Malay States, as 

 has been stated in the A;/n'cultural Seiv.t, Vol. IX, p. 318, 

 where a description of the disease, both on young and older 

 trees, is given. It may, however, prove of interest if a few 

 facts in connexion with this fungus are added here, especially 

 as, for reasons which will be given below, it .seems possible 

 that tlie information may have some application to cicao. 

 The additional point.s of interest are the outcome of observa- 

 tions conducted in the Straits l)y Ptidley. The account 

 of them "Was published in the Agricultural Bulletin of tlw 

 Straits and Federated J/alai/ State-', Vol. IX, pp. 290, etc. 

 In the first place, it was found that no spores of the fungus 

 were produced unless the atmosphere was moist, and that the 

 parts on v.'hirh the jncnidia occurred were always quite dead 

 and, frequently, commencing to dry. Infection experiments, 

 which could only be conducted on a small .scale, led to the 

 following conclusions. The fungus is unable to attack the 

 terminal bud directlv, but can do so if it is injured in any 

 way. It can al.so produce direct infection on young leaves 

 which are not more than half-grown. After infecting the 

 damaged terminal bud, the fungus spreads down the stem, in 

 the cambium, though, if the seedling is healthy, its progress 

 is eventually checked, apparentl}' by the formation of an 

 impervious corky layer. The same thing occurs in tlie case 

 of the 3'oung leaves. These are killed by the fungus, and 

 fall, being cut off from the plant by means of the usual 

 absciss layer, which prevents the entry of the fungus into 

 the tissues of the stem. If the .seedling is unhealthy, the 

 fungus may spread from the terminal bud right down 

 the cambium, and kill the plant completely in a few 

 days. When this happens, the most recently affectevl 

 parts of the plant do not show any very definite external 

 signs of disease, but the presence of the parasite in the 

 cambium is indicated, on examination by a red discolora- 

 tion of this tissue. Another result of the inoculation 

 experiments was to show that slight wounds on woody stems 

 do not serve as an easy means of entry for the fungus, as its 

 spread from these is very slow, even when it occurs at all. It 

 must however be borne in mind, that large wounds such as 

 cuts made in pruning, and small wounds on green twigs 

 permit the ready entrance of the fungus; and once it has 

 obtained a strong hold on the cambium of a woody stem or 

 branch, it spreads rapidly and the plant cannot stay its progress. 



The life-hi.story of the fungus occupies about seven days. 

 That is, infected portions of the attacked plants die 

 about six days after infection, and under favourable 

 conditions fructifications make their appearance one or 

 two days later. The spores from these can germinate 

 again, on a favourable surface, in twelve hours. The 

 ultimate fate of the plant on which they germinate depends 

 on its vigour, and on the part attacked, as has already 

 been indicated. The tree may lie killed out in a few days, 

 or may be able to throw off the disease. 



The fact that the Hevea tree sheds its leaves periodically 

 is naturally of great assistance to it in preventing any serious 



damage from fungus attacks on the leaves; so that this means 

 of infection is probal>ly not a very serious one, unless the 

 whole twig on which the leaves are borne is in an unhealthy 

 state from some other cause. 



It was stated al)Ove that these observations may possibly 

 have som.e bearing on die-back of cacao. The reason for this is 

 that very possibly Biplodia rapa.r may prove to bo identical 

 with Botryodiplodia elasticae, Petch, which occurs very fre- 

 quently on Hevea and other liosts in Ceylon Further, 

 Botryodiplodia elasticae is almost certainly the same as 

 Lasiodiplodi'ithenltrovMe, Griffon and Maublanc, which is the 

 West Indian die-back fungus of cacao. Consequently, 

 Diplodia rajM.r and Lasiodiplodia iheobromae may possibly 

 prove to be one and the same. In support of the theory that 

 Dip/odia rap'i r is very possil>ly the same as the Ceylon fungus, 

 Botri/odiplodia elasticae, Eidlej' puts forward the following 

 evidence. A fungus attacking Hevea in West Afiica was 

 identified as Diplodia rapa.r by Massee. Now, no specimens 

 of this rubber plant have been sent from the Straits to West 

 Africa, but specimens have been received from Ceylon, and on 

 one such consignment a fungus, which must have been Botryo- 

 diplodia elasticae, was found to develop in transit. This fact, 

 taken in conjunction witii the description of the two species, 

 makes their identity very proliable. Now, in Ceylon, die-l>ack of 

 Hevea is originated by the attacks of (rloeosporium alboruhrum 

 on the young shoots. When the top of a shoot has been 

 killed, Botri/odipidia elasticae obtains a hold, and comoletes 

 the destruction of the tree (Arjricultural Neivs, Vol. IX, 

 p. 270). In the Straits, the attack is originated by Dipio lia 

 rapa.r itself, without the aid of any species of Gloeosporium, 

 and the same thing appears Co be true with regard to die- 

 back of cacao here, ('onseqnently, Ridley's results may well 

 represent the state of things that obtains in the case of the 

 origin of the cac-io disease. Local experience tends to show 

 that this is actually so. On cacao, too, the disease certainly 

 starts on injured young green shoots or terminal buds, as 

 well as on big wounds. If the trees are in a healthy state, it 

 is improbable that the fungus will do much harm; while if 

 they are unhealthy from any cause whatsoevei', the disease is 

 likely to spread rapidly and prove serious, and if it is not 

 attended to, the tree will die. The necessary remedial meas- 

 ures are well known, and need not be detailed in this article. 

 It is generally found that die-back in the West Indies is 

 particularly prevalent in exposed situations, where the young 

 shoots are damaged by wind. It may, moreover, be added 

 that moist condition.s and excessive shade favour the spread 

 of the diseases lioth on rubber and cacao; while poverty of 

 the soil or insufficient drainage weakens the trees, and thus 

 encourages the growth of the f nn^i in the tissues of their hosts. 



ADDITIONAL NOTE ON PINK DISEASES. 



Ill the Agricultural yeics, Vol. IX, p. 2S6, mention was 

 made of a form of fungus closely resembling pink disea.se of 

 cacao. This occurred on a stem of guava which was used to 

 support a branch of cacao, and some of the cacao trees in the 

 neighbourhood were attacked by the true pink disease. Speci- 

 mens of the branches of the plants so attacked were recently 

 forwarded to the Head Office, and this enabled a comparison 

 to be made lietween the fungus on the guava and that on the 

 cacao. The conclusion arrived at was that the two species 

 were probably not identical; that on the guava, Corticium 

 laeiv, being probably sajirophytic only; the other on cacao 

 being Corticium lilarino-t'usruin, which is a partial parasite. 



