Vol. VII. Xo. 162. 



THE AGRICULTURAL NEWS 



2-.'l 



THE PRINCIPAL FUNGUS DISEASES OF CACAO, 



Un<li'r the above title a leaflet gfving in simple 

 latii^uage partiri'lars of the occurrence, ."sytiiptoiiis, and 

 methods of tveaftrifnt ofthe chief fungus diseases affect- 

 ing cacao in the West Indies, has just been issued by 

 the Imperial J)epartment of Agriculture. More detail- 

 ed information may be found in the paper read by th(- 

 Mycologist bt'fore the last West Indiab Agricultural 

 Conference, and shortlv to be published in the Wf-sf 

 Indian Bullrlin Vol/lX, No. 2. 



The first portion of the leaflet is here reprinted, 

 and the concluding portion will be given in the next 

 issue of the Ayrk'ultiiral Ncw>i: — 



Oci-iirycm'f. — .-Vll cacao-growing districts. 



Si/mjtfoiiix. — The best time tn look t'i»i- caiikei' would 

 appear to be in the dry season immediately after a shower of 

 rain. The browiiisli-coloured bark of the cankered ai'eas 

 does not dry as quit-kly as the unaffected portions of the bark, 

 and the limits i)f affected portions may generally easily be 

 ascertained. If several of these spots are carefully watched 

 until ijuite dry, a keen observer may notice that the liark of 

 cankered areas presents a peculiar dry brownish a[j[)earance. 

 Subsequently these spots .split or crack, a'n'cl'allow a lii-(.iwnish- 

 red gununj- fluid to ooze out. When this gum dries, it gives 

 a dark rusty appearance to the bark. This is known as the 

 ' bleeding ■ stage and the' cbsease is now well estaldished. 

 Every effort should therefore be made to detect the disease, 

 if possible, before the bleeding stage makete its ap[>eai-ance. 



In Dominica, cacao plants arc often noticed that produce 

 on them a very large number of flowers whifeh never set fruit, 

 and what is known as the ' flowering ' dfeease is observed. 

 This in IJominica may be taken to be one of the first symptoms 

 of canker, for it has been sho>-n that Such trees always 

 eventually develop the disease; 



If a cut is made into a 'canker ' spot with a kinfe, it is 

 found that the bark is dark claret in colour, and is moist and 

 soft to the touch. The canker, as indicated by this discolora- 

 tion, may spread round the stem or branch, or may extend in 

 all directions. The discoloration extends to the wood of the 

 tree, and it has frecjuently been noticed that the darkening 

 of the wood may extend for some distance under what appears 

 to be healthy bark. 



When the rains .set in, small pustules make their appear- 

 ance and white, yellow, or red spores may be found. Tliey can 

 readily be distributed by wind or rain, ami when they gain 

 entrance to a wound they may set up disQ,ase. 



Canker may atfoct a branch or a stem, and fre(|uently 

 several sjiots may be noticed on the same jtree. When these 

 diseased areas become n-umerou.s, the leaves be<.'ome small and 

 assume a yellowish hue. Branches freipK^ntly die off through 

 having become 'ringed,' and not uncomn)only whole trees, 

 particularly v.'heji the diseased' area' is near, the surface of the 

 ground, are killeil. , 



Canker isgent-ndly to be noticed mt)re frequently among 

 old trees, for woujids on such trees are^generallv the more 

 iiumer' ais. ' '• " 



Sjjrend. — Canker is catised by seveiul fungi {Ncctria, 

 Caloner.tr id, etc,) that have been .shown to be wouikI 

 parasites. The dis.ea.-;e .makes its entrance into a liraiicfi oi' 

 stem through a wound. ' 



Remedies. — The best time for canker work is dniing the 

 dry season, but the disease should be coiistantly looked for 

 and when the canker spots are detected, the diseased bark 



and wood shoulil lie cut out with a sharp pruning knifi'. 'f'lie 

 wr)und should lie thoroughly ti-eateS with far, or with a nu.x- 

 tnre of resin oil and manjak. After 'a free has been treated, 

 it should l)e marked l\y a ring of TOite-wa.sh or whiti' paint 

 around one of the branches in luller that it may lie kei)t 

 under careful supervision. The ilileased portions that have 

 beei< cut out should be collected ai^ff- burned. They should 

 not be allowed to remain on the-ground at the foot of the 

 tiee. All dead trees or branches shtjuld be cut out, removed 

 and burned. All treated trees should be inspected periodically, 

 and further careful search for canker spots made. It is a<lvis- 

 able that the knives u.sed for cu.tting out canker should not he 

 used for pruning, and they should be disinfected b(>fore they are 

 put aside. As the fungi that are associated with canker are 

 wound pai-asitcs, care must be taken with all wounds. ( 'nts 

 made in pruiung or while picking the (a'op should be failed 

 over. A joint of bamboo may be a convenient form to carry tar 

 when pruning operations are being carried on. This maj- be 

 carried in the left, hand, and after a branch or snclcer has 

 been removed, tar can innuediately be applied. 



Experience has .shown that the above remedial measures 

 liave proved successful, for estates, on which carefully con- 

 ducted measures against canker have been carried on, have 

 given increased crops and the number of cases of i';ii l,er lias 

 been very largely reduced. 



DIE-BACK. 



Oci-in-rnnct. — This di.sease is common 

 occurs in St. Lucia, Dominica, Grenada, 

 St/niptoinx. — The disease coibmences 



111 some Islands; 

 and St. Vincent. 



the younger 

 twigs and spreads from .these , to the larger branches. The 

 trees on which the younger twig.s'have been killed, present 

 what is known as a ' stagheaded ' appearance. If one of 

 these dying twigs is. cut lengthways by a knife, it will lie 

 seen that the end of the twig is black and is quite dead, and 

 tbere occurs a brownish-coloured dying zone before the living- 

 tissues of the branch are reached. There is no sharp line of 

 demarcation between dead and living tissues, as .is noticed 

 when twigs are killed bj' exposure to wind, etc. This inter- 

 mediate zone of dying tissue makes it possible to i-eadily 

 detect the die-back disca.se. - 



In the rainy season,. or even after a good shower, if these 

 <liseased twigs are closely examined, there will be noticed 

 small black pustules breaking through the bark, from which 

 a black dust — the .spores of the fungus — may be obtained. 



Spread. — The fungus (Dip/odia cacaoicolo) that cause.s 

 this disease is a facultative parasite and can live on dead cacao 

 wood The spores are spread by means of wind and rain. 

 There is also some connexion between attacks of this disease 

 and cacao ' thrips.' ^ ' . 



/tpz/ffrfc.-;. --This disea.se does? not readily attack trees ia 

 a vigorous conditi'm of irrowth. Every effort should, there- 

 fore, be given to tli'irough cultivation, all diseased branches 

 and twigs should be cut out and burned, and all wounds thus 

 made .should be t'cllow.'d by an apiMication of coal tar or some 

 sinular substance. It has been Hemonstrated in St. IjUcia 

 that this disease can be entirely eradicated froni an estate by 

 high cultivation, manuring, and attention to careful pi-uning. 

 Pen mamires and mulchings should be applied, and all weeds 

 should be carefully buried. 



The husks or shells of all cacao pods .should be buried 

 with lime, for it has been .shown .-piat heaps of old pods lying 

 unburicd abottt a' plantation serve'as centres of infection for 

 the spread of the fungus that caiises this disease. 



