3GG 



THE AGRICULTURAL NEWS. 



November 12, 1910. 



FUNGUS NOTES. 



ROOT DISEASE OF CACAO, LIMES, 

 AND OTHER PLANTS. 



The attention of the Mycologist on the staff of the 

 Department has been called, during the past year, to the 

 occurrence of root disease of limes in Sc. Lucia, Dominica, 

 Montserrat and Antigua, and of cacao more particularly in 

 8t. Lucia. Examination of the varidus specimens of lime 

 trees sent to the Head Office from the islands mentioned 

 Seems to indicate that the disease is different in symptoms 

 and origin in different localities. One form, however, which 

 occurs in Dominica, and possibly also in St. Lucia, appears 

 to be due to the same fungus as that causing the disease of 

 cacao. This similarity is manifested, not only in the nature 

 of the mycelia of the two fungi, but also in their method of 

 attack. In consequence of this, it is intended to deal with 

 the disease of these two host plants together, in this article, 

 and to leave the discussion of the other forms of lime root 

 disease for some future occasion. 



DESCRIPTION OF THE DISEASE. It would Seem that the 

 disease under consideration is the same as one which has long 

 been known in the West Indies; though certain characters of 

 the causative fungus, as recently examined, are not very clear- 

 ly indicated in the former publications on the subject, and 

 there is consequently a possibility that the tw'o fungi are not 

 entirely identical. (See West Indian Bulletin, \o\. IX, pp. 

 167-70, and Pamphlet No. 54 of the Department Series.) 

 The symptoms of the disease as recently met with are 

 briefly as follows : Badly attacked specimens, on examin- 

 ation, are seen to be entirely deprived of their small 

 lateral roots, and even the principal roots are dead. The 

 bark of the latter is blackened and shrunken, while the 

 wood is frequently grey in colour. On the surface of the 

 diseased roots is a mass of dark-grey or brown fungus myce- 

 lium, usually covered to some extent by earth. The external 

 symptoms often extend to the collar, which in some instances 

 is completely ringed. When this is the case, the tree 

 is dead. On removing the bark the position once occupied 

 by the cambium is seen to be filled with white, or yellowish, 

 fan-shaped strands of mycelium closely adpressed to the bark 

 on one side, and to the wood on the other. Inside this, the 

 wood is grey and often exhibits very narrow, longitudinal, 

 black streaks with a white centre, due to the presence of 

 narrow cavities lined by the black hyphae of the fungus, 

 the hyphae inside this lining being white. On the 

 broken ends of ro'jts, a web of grey or greenish-grey hyphae 

 develops, which certainly belongs to the sann fungus as the 

 other forms so far mentioned. Inside the dead bark, a black 

 stroma of the same fungus is often formed, but no fructifi- 

 cations definitely associated with this disease have as yet 

 been found. In addition to the mycelium described already, 

 loosely woven strands of hyphae of a yellow colour may also 

 be observed on the surface of some of the specimens. This 

 form appears to agree more accurately than the former with 

 the descriptions of root disease of cacao published previously in 

 the West Indi'iii Bullefiii. Lastlj', bright, pink, Stilbum-like 

 fructifications, surmounted by a spherical head of white spores 

 have been found. In one instance, these appeared to be 

 associated with a flat brown stroma about 3 mm. thick, 

 but varying ciisiderably in width and length, and occurring 

 on parts of the collar of the specimen, beneath the bark. It 

 could not be definitely determined if all these forms were inde- 

 pendent, or if they were all manifestations of the same fungus. 



GENEKAL coNSHiERATioNs. Tlus fungus has also been 

 found attacking'the immortel shade tree {Ervthvina relutina) 

 in addition to limes and cacao, and the pigeon pea (Cajanus 

 indicus), in St. Lucia. If, as is almost certain, it is the same 

 as that described previously, the list of host plants is as 

 follows: cacao, lime, breadfruit, breadnut, mango, pomme 

 rose, pois-doux, avocado pear, immortel and pigeon pea. 

 There is also a decided possibility that it is the same as the 

 fungus attacking the roots of coffee and pois doux in Guade- 

 loupe. In many. instances, the fungus originates on decaying 

 forest stumps left in the plantation; from these it can spread 

 to cacao or limes. In other instances, the attack maj- origin- 

 ate in shade belts of any of the above-mentioned trees, and 

 from them spread to cacao. In the case of limes, the attacks 

 have up to the present been almost entirely confined to isol- 

 ated trees, occurring here and there, more especially on newly 

 cleared estates. In such cases, it is usually- found that there is 

 a decaying stump in the neighbourhood of the tree attacked, 

 from which the fungus has been enabled to spread to the lime 

 tree. The general experience has been that if such trees are 

 carefully removed and burned, and the soil around them treated 

 with lime, no further trouble occurs, and supplies can safely 

 be put in, six months after the destruction of the tree. In 

 the case of cacao, the sequence of events is often somewhat 

 different. The^ disease may appear suddenly on thn-e or four 

 trees at once; in this case it is usually found that the trees 

 are arranged approximately in a circle around some 

 source of infection. LTnder such circumstances, the diseased 

 trees should be isolated liy a trench, and all those badly 

 attacked .should be dug up and burned, the soil being treated 

 with lime as is mentioned above. Similar measures may be 

 employed for preventing the spread of the disease along 

 a wind-break. 



KACTOK'^ iXFLUENCixc THE iiisEASE. In many instance.s, 

 the primary cause of the disease is, as has been stated already, 

 the presence of decaying forest stumps in the soil; but this is 

 not always the case. It would seem that, occasional!}-, 

 the fungus is able to attack shade trees directly; though it is 

 somewhat difficult to understand by what method it is 

 dispersed, as it never appears to form spores. The only, 

 alternative mode of infection is by means of mycelium spread- 

 ing through the soil and living on small pieces of decaj'ing 

 vegetation that happen to be present. AVhen such a mycelium 

 encountertj the living roots of shade trees, more particularly 

 those of breadfruit and avocado pear, it must be able to 

 attack them directly and ultimately cause the death of the 

 trees. There is a very great probability that before this 

 happens, other trees in the neighbourhood have already- 

 become infected, and this accounts for the .subsequent death 

 of several more trees in the infected area. Consequently, 

 care should be taken to remove decaying stumps as far as is 

 practicable, and to choose hardy trees for wind-lielts, or 

 for shade purpo.ses. Another very important consideration 

 is that of drainage. When trees are growing in a water-logged 

 soil, or one which is subject to occasional inundations, some 

 of their roots may be actually drowned, since the water pre- 

 vents them from obtaining the necessary supply of oxygen. 

 Such dead roots then serve as sources of food to the fungus, 

 if it is present, and enable it to develop the additional vigour 

 necessary for its spread to the living tissues. Trees growing 

 in unsuitalile soil, or in wind-swept situations, are also weak- 

 ened in vigour, and con.sequently are less able to resist the 

 fungus. It has been found, on certain estates in Dominica, 

 that soil which is unsuitable to cacao can be used for the 

 cultivation of limes. The latter are planted between 

 the cacao trees, and as they grow up, the original crop is 

 removed, and the limes are left alone. This course would 



