270 



THE AGRICULTURAL NEWS. 



August 20, 1910. 



FUNGUS NOTES. 



DIE-BACK DISEASES OF HEVEA, CACAO 

 AND MANGO. 



The die-back disease of cacao is one that has long been 

 known in tlic West Indies, and has been the subject of 

 several publications, not only from this Department, but 

 from many parts of the tropical world. Recently, a disease 

 of Hevea, characterized by symptoms somewhat similar to 

 those exhibited by cacao, has been described by Fetch from 

 Ceylon, in Vol. IV, Xo. 23, of the Circulars and Agricultural 

 Jimriml of the Royal Botanic Gardens; a similar disease 

 attacking mangos has been observed in Barbados, and could 

 probably be found in most of the West Indian islands, especi- 

 ally on young grafted trees of the more highly developed 

 Tarieties. Although the disease upon this last host plant 

 has not been fully worked out, enough has been seen to make 

 it worthy of mention here, more especially in the light of 

 what has recently been foand to be the case in Hevea 

 disease in Ceylon. In order to explain the situation clearly, 

 each disease will be treated separately, commencing with 

 that on Hevea. 



HEVEA BRASiLiENSis. The disease was first investigated 

 at Peradeniya in 1905, at which time it was essentially 

 a disease of young trees from one to two years old. During 

 1909, however, it was found on trees from nine to fourteen 

 years old, and in some instances these were killed very 

 rapidly. The symptoms of the disease on young trees are 

 as follows : a brown patch, which is frequently rather soft, is 

 formed on the leading green shoot about the middle of its 

 length. This patch extends over the whole shoot, and the 

 leaves fall off as the disease reaches them. As the causative 

 fungus spreads in the tissues, the parts first attacked 

 dry up and become grey in colour. If the diseased 

 parts are removed at this stage, very little harm results, as 

 growth is continued from the up[iermost remaining bud; but 

 if the disease is neglected, the secondary stage sets in, and 

 the tree dies. The fungus which causes the first stage of 

 the disease is known as Gloeosporium alborubrum. Its fruc- 

 tifications appear as minute swellings, produced immediately 

 beneath the epidermis of the dead shoot. These burst at the 

 top, and thin tendrils of pink or white spores are extruded. 

 The second stage of the disease is due to Lasiodiplodia theo- 

 bromae, which is better known in Ceylon as Botri/odiplodia 

 elasficae, as this is the name under which it was originally 

 described, in that island. On older trees, the diagnosis of 

 this disease is a matter of more difficulty than in the 

 case of the young plants, as the shoot may die off, from 

 numerous causes. It may, however, be recognized by 

 the fact that once the second stage has commenced, the 

 whorls of branches are killed off' in succession. In the case 

 of both young and old trees, the second fungus, Lasiodi- 

 plodia theobromae, can only obtain a hold on the tree 

 after the leading shoot has been killed by the Gloeosporium. 

 Having once obtained a hold as a saprophyte, it can spread 

 rapidly, and can attack the hard, woody parts which are able 

 to resist the Gloeosporium. In this way, it brings about the 

 death of the host, often in a fairly short time. 



MANGO. In the Cuba Review, Vol. VIII, No. T), a short 

 account is given of a bloom blight, the symptoms of which are 

 as follows: as the blo.ssoms open they rot, becoming black 

 in colour, and finally dry up and fall ott", leaving only 

 the central stalk. This also dries up and turns brown. 



The di.sease, in Cuba, is due to a fungus known as 

 Gloeosporium manyiferae, and is related to that found on 

 Hevea. Recenjjy, portions of diseased mango, including 

 the inftorescence, were examined at the Head Office of 

 the Department, and it was found that the external symp- 

 toms were very similar to those just described. When some of 

 these twigs were kept in a damp chamber, they developed 

 numerous fructifications of Lasiodiplodia theobromae, and it 

 seems probable, though this has not been definitely estab- 

 lished, that the latter fungus was of secondary origin, and 

 prevented the true cause of the disease from appearing. 

 Young mango trees are subject to a disease which attacks the 

 green shoots, causing them to become brown and hard, as in 

 the case of Hevea; though up to the i)resent, the disease has 

 not been sufficiently carefully investigated to make it possible 

 CO state definitely at what point the attack commences. One 

 or two instances of this disease have been noted in Barbados. 

 In view of the facts mentioned, it seems possible that there 

 is here a case parallel to that described by Fetch, namely that 

 a Gloeosporium starts the disease, and that if no remedial 

 measures are undertaken, this is followed by Ladodiplodia 

 theobromae which can then complete the destruction of the tree. 

 CAf'AO. 'l"he die-back disease on this host plant is too 

 well known to require much detailed description. As far as 

 is known at present, only one fungus, Lasiodiplodia theob) oniae, 

 is concerned with the spread of this disease, and there is no 

 previous direct attack by any other. In consequence, it is found 

 here, that the disease is practically confined to trees, the young 

 branches of which have been damaged by wind, bad drainage, 

 unsuitable soil conditions, excessive sunlight and similar causes. 

 The same was'^also found to be the case in Surinam, by 

 Mrs. van Hall and Mr A. W. Drost. (See Agricultural A'eivs, 

 Vol. IX, p. 46). Once established on dead and dying twigs, the 

 fungus can spread downwards into the branches and main 

 stem, and will eventually kill the tree. 



The remedial measures in the case of the rubber and 

 cacao diseases, and probably also in that of young mangos, 

 are exactly similar. They consist in the careful removal of 

 diseased twigs and branche.s, followed by the careful tarring of 

 the wounds made. In removing large branches, Fetch suggests 

 that three cuts should be made with a saw. The first is made 

 1 foot from the stem on the under surface of the branch, and 

 about half through it. The second is made about 3 inches 

 further from the trunk on the upper surface, and is continued 

 until the branch breaks oft'. Finally, the stub must be sawn 

 off quite close to the stem, by a cut running parallel to the 

 stem. Before the tar is applied, the cut surface should be 

 allowed to become as dry as possible, but it should not be 

 exposed for more than twenty-four hours. All the parts 

 removed should be burnt. The bloom blight of mangos can be 

 controlled by two thorough sprayings with Bordeaux mixture 

 at a fortnight's interval. The first .should probably be done 

 just as the flowers are commencing to open. 



In conclusion, it may be stated that dying back of twigs 

 both in Hevea and in cacao may be due to many other 

 causes besides those mentioned, notably to root disease. In 

 this ca.se, an examination of the roots and collar will 

 frequently reveal the true cause of the symptoms observed. 

 In the case of cacao, die-back due to physical or physiologi- 

 cal causes can usually be distinguished from that due to 

 Lasioliplodia, by the fact that the line of demarcation 

 between healthy and unhealthy tissue in the first case is quite 

 sharp, while 'in the second it is indefinite. It is often 

 difficult to distinguish between the diseases originally fungoid 

 in origin and those primarily of a physiological nature; but in 

 either case, removal of the dead parts, especially if they 

 become numerous, is the wisest course to adopt. 



