286 



THE AGRICULTURAL NEWS 



September 2, 191L 



FUNGUS NOTES. 



THE DIE-BAOK FUNGUS OF PARA RUBBER 

 AND OF CACAO. 



In an article in the A(jricultural News, Vol. X, p. 7S, 

 attention was called to a few of the fungi which are parasitic 

 both on cacao and on rubber plants. Among the.se was includ- 

 ed Lasiodiplodia t heohromae — the die-back fungus well 

 known as a cacao parasite in the West Indies. Recentlj-, 

 important investigations on the life-history of this fungus and 

 on its parasitism have been conducted by Mr. C. K. Bancroft, 

 Mycologist on the Staff of the JJepartment of Agriculture of 

 the Federated Malay States. The results of these investiga- 

 tions have been published in Bulletin No. 9 of that Depart- 

 ment, entitled The Die-back Fungus of Para Rubber and of 

 Cacao. These are of considerable importance, as they not 

 only do much to clear up the confusion in the nomenclature 

 of this fungus, but throw considerable light on its method of 

 attacking its host and on the factors governing the extent of the 

 damage cau.sed. A short summary of this work is given below. 



The die-back fungus is distriliuted throughout practically 

 the whole of the tropical zone. It occurs in the West Indies, 

 tropical America, San Thoniu, tropical Africa, Ceylon, the 

 Malay Peninsula, the East Indies, the Philippines and Samoa. 

 It is found on a very large number of host plants, though it 

 is of greatest importance as a parasite on cacao and Para 

 rubber, since on many of the others it would appear to be 

 mostly only saprophytic. Bancroft records it on dead shoots 

 of cofi'ee, cocoa-nut, camphor, Castilloa elaslica, and on dead 

 .stems and roots of tapioca, or cassava. 



The symptoms of the disease on Para rubber are those 

 usually associated with die-back, though it is worthy of note 

 that in early stages it is rarely found that more than one or 

 two shoots are affected simultaneously. Infection usually 

 takes place first on a branch at a point varying in distance 

 from its apex. The terminal portion dies first, owing to the 

 stoppage of its food supplies, and the disease then spreads 

 downwards towards the base of the branch. When the trunk 

 is reached, the fungus may continue to extend to the roots. 

 Frequently, however, if the tree is healthj', the spread of the 

 fungus in the branch is checked before it reaches the trunk, 

 and growth is continued by a healthy shoot arising below the 

 dead portion of the branch. The disease may also be arrest- 

 ed in its course, upon reaching the trunk of the tree. When 

 it spreads down the trunk, all the branches above the point 

 of entry die from want of food, and eventually the tree is killed. 



The wood is the part in which the spread of the fungus 

 principally takes place. This is turneil to a uniform grey 

 colour, owing to its presence, while in the parts le.s.s 

 affected the change of colour takes place in the medullary 

 rays, which are turned lirown. After the wood is dead, 

 the outer tissues still yield .some latex, but these die 

 eventually; the bark becomes greyish in colour and peels off, 

 while the cambium layer is reduced to a black mass. In 

 cases where the growth of the fungus is slow, and where 

 infection has occurred on an older part, a cankered appear- 

 ance may be produced. It is worthy of note that the line 

 of demarcation between healthy and dead tissue in the wood 

 is often fairly definite, but that the hyphae of the fungus 

 actually extend for a distance of 4 or 5 inches below the 

 point where the wood can be seen by the naked eye to be 

 infected. 



The ordinary fructifications of Lasiodiplodia theohramae 

 are well known and need not be described here ; they are 



illustrated in the Ayricidtural News, Vol. IX, p. 174. 

 Bancroft has, however, found two other forms of spores that 

 are produced by this fungus. One is the ascospore, of which 

 eight are formed in a definite sac or ascus, the asci being 

 contained in black perithecia: the other is a small colourless 

 spore, produced like the T^asiodiplodia spores, in a black 

 pycnidium ; this form lielongs to the genus Cytospora. The 

 discovery of the a.5cospore form was recorded in the number 

 of the Agricultural News to which reference is made at the 

 beginning of this article, and it was mentioned there that 

 the name IViyridaria tarda, given to it by Bancroft, is that 

 by which the fungus in all its forms must now be known. 

 This is so because the ascomyretous stage is regarded as the 

 most highly developed form of the fungus. 



The perithecia containing the asci were produced on 

 specimens of diseased cacao branches, sent to Kew from West 

 Africa, They were borne on the same stromata as the 

 Lasiodiplodia form, and contained asci in which eight, 

 3-septate dark-coloured spores were produced. 



On the other hand, the pycnidia belonging to the Cyto- 

 spora stage were produced on dead l>ranches of Hevea, in the 

 Federated Malay States. They also arose on stromata which 

 had previously carried the Lasiodiplodia form. As has been 

 stated already, they produced numerous elliptical-oblong, 

 small, hyaline spores l)orne on the ends of sterigmata. 



Inoculation experiments with the Lasiodiplodia spores 

 on Hevea plants gave rise to interesting results. In 

 the first place, it was found that no infection could be 

 produced at an uninjured surface, either of the leaf or the 

 stem; and that even very young leaves were not attacked, 

 although Ridley has claimed to have infected them directly. 

 (See Agricultural News, Vol. IX, p. 382.) Further, it was 

 observed that infection did not occur at small wounds or on 

 well tapped surfaces. This was the case, as long as the wound 

 was not deep enough to expose the wood and did not pene- 

 trate to the inner bark, or to the cambium. The bark tissues 

 possess the power of rapidly forming a corky layer, which 

 excludes the fungus; while the wood, when exposed, presents 

 a dead surface, on which the organism can grow vigorously. 

 Infection resulted in every case from inoculating wounds that 

 were deep enough to expose the wood, though the death of 

 the plant when it was in a healthy state did not always follow. 



Inoculations on Hevea plants, with the ascospores formed 

 on cacao, produced the usual symptoms of die-back on the 

 Hevea, in each case, and eventually pycnidia of Lasiodiplodia 

 were formed on the dead portions of the rubber plant; these 

 were identical in appearance with those on cacao. This places 

 the identity of the fungus on cacao and rubber beyond dis- 

 pute. No inoculation experiments with the Cytospora spores 

 have yet been recorded, though further work on this form is 

 being undertaken by Bancroft. Finally, it may be stated 

 that the pre.sence of Gloeosporiuin alborubruui., said by 

 Fetch to be frequently responsible for the initial stiiges of 

 this disease in Ceylon, has never been recorded in the Malay 

 States. 



Jn regard to remedial measures, Bancroft emphasizes the 

 need for thorough sanitation, good cultivation and careful 

 pruning on rubber estates, in order to check the spread of the 

 disease —measures long advocated by this Department for the 

 same purpose in regard to cacao. He also stales that the 

 attacks of other fungi, as well as the presence of wounds and 

 ill-health due to physical causes, may predispose a tree to 

 disease; con.sequently, it is extremely likely that the die-back 

 fungus is not always the primary cause of the trouble, and 

 that by no means all the effects that have been attributed to 

 it are really due to this fungus. This is an opinion which 

 receives the support of this Department. 



