366 



THE AGRICULTURAL NEWS. 



November 11, 1911. 



FUNGUS NOTES. 



OBSERVATIONS ON ROOT DISEASES IN 



THE WEST INDIES. 



PART I. 



In the following article information is given on the sub- 

 ject of root diseases of cacao, limes and other host plants 

 found in certain of the West Indian islands. This is of 

 a preliminary nature, and is based on a report recently sub- 

 mitted to the Imperial Commissioner of Agriculture by 

 Mr, F. W. South, B.A., Mycologist on the Staff of this 

 Department. The report .summarizes the results of work 

 conducted by that Otiicer during a recent visit to Dominica; 

 while this article contains in addition reference to investiga- 

 tions made on material consisting of diseased plants of leveral 

 kinds forwarded from certain of the other islands during the 

 course of the last eighteen months. 



As a result of the work referred to above, three different 

 forms of root disease have been recognized in Dominica. 

 Two of these have been found on lime trees only, up to the 

 pre^rent, while the third is common to several hosts, among 

 which are included cacao, pois doux {Inga laurina) and prob- 

 ably several other plants of economic importance. In addition 

 to these it would appear likely that a disease of different 

 origin occurs on limes in Montserrat and Antigua, while yet 

 another is to be found on cacao and nutmtgs in Grenada. 



liLACK ROOT disea.se. Sincc it is probibly the most 

 important and most widely distributed, this disease, the third 

 of those referred to in the preceding paragraph, will be dealt 

 with first. In the island of Dominica, it is the most common 

 form of sporadic disease, to be met with particularly on newly 

 cleared estates in the interior. It is frequently accountable 

 for the death of pois doux trees planted as a wind-break; 

 from these it may spread to cacao or lime trees in the vicin- 

 ity. It must, however, be pointed out that its original 

 attack is by no means confined to pois doux, but it has been 

 found to spread from dead mahoe coclion (Sterailia cariljaea) 

 to lime trees, while it has also killed Hibiscus and Acalypha 

 planted as hedges. Moreover, instances are on record of its 

 appearance on lime and cacao trees that were not near any 

 of the plants mentioned above. In these cases it is doubtful 

 whether the causative fungus actually commenced its growth 

 as a parasite on the trees mentioned, or spread to them from 

 some decaying v/ood, such as a tree stump, buried in the soil. 



Fungi with almost identical mycelial characters have 

 been found in several of the other islands, but conclusive 

 evidence of the identity of these with the Dominica sjjecies 

 is wanting, since in all the former cases no fructifications 

 have been found. One of these fungi occurs in St. Lucia on 

 cacao, Castilloa rubber and pigeon peas, and probably also 

 on immortel, limes and oranges. Another has been found 

 in Grenada on young Castilloa trees, while yet another 

 was seen in St. Vincent on cacao. Possibly, also, the 

 disease known as 'burning' of arrowroot, in the same island, 

 may be due to this fungus; though the evidence nn this point 

 is by no means conclusive. Another disease, possibly of the 

 same origin as the black root disease, is that occurring on 

 pois doux and coffee in Guadeloupe, and attributed by 

 Delacroix to a species of Posellinia or Dematophora, whose 

 effect may be combined with the injurious action of eelworms. 

 For the sake of sim[)lifying the position, it may be added 

 that the root disease of cacao, and probably of nutmegs and 

 mangoes, in Grenada, described l>y 1 Inward in the We.tl Indian 

 Bulletin, Vol. II, p. 207, and ascribed by him to a hasidomy- 



cetous fungus, is probably different from the black root 

 disease. 



In many instances, the first symptom of infection on 

 limes, cacao and pois doux is a thinning of the foliage. This, 

 however, does not always occur, particularly where lime trees 

 are concerned : while, in some cases, it is overlooked, or attri- 

 buted to other causes. The next symptom is the sudden 

 wilting of all the foliage, succeeded two or three days later 

 by the complete death of the tree. The suddenness of this 

 occurrence, followed as it often is by the death of other trees 

 near that first killed, frequently causes some alarm, as it 

 naturally gives rise to the idea that a serious epidemic is 

 about to destroy the whole field. Such is, however, far from 

 being the case, and careful attention to the comp.aratively 

 simple remedial measures will result in completely stayinc^ 

 the spread of the disease. 



On carefully examining the collar and roots of a diseased 

 tree, the first thing noted, in advanced cases, is the presence 

 of a dark, olive green mycelium, possessing a grey border 

 along its .advancing edge, and encircling the entire basal 6 or 

 12 inches of the stem. Where this is absent, a dark-brown 

 mycelium, often mixed with earth, is to be found on the 

 larger roots, and the portion of the collar below- ground. 

 Beneath this the bark is usually destroyed, and may contain 

 a thin, black, brittle crust of fungus, from which narrow, 

 black streaks run horizontally into the wood. Similar streaks 

 also occur, running vertically in the wood. On removing the 

 bark, white fan shaped masses of mycelium are to be found 

 on the surface of the wood, particularly in the case of cacao 

 and pois doux. On younger roots the brown mycelium is 

 present on the bark, while the presence of fungus in the wood 

 is indicated by a grey discoloration. Finally, very thin 

 plates of black fungoid tissue, appearing as lines in horizo.ital 

 or vertical sections, may in some cases separate the badly infect- 

 ed wood from that less seriouslj" damaged. 



The mycelium of the causative fungus— a species of 

 Kosellinia— is white when young, but dark brown when old 

 or exposed to the air for any length of time. Numerous 

 partition walls occur in the hyphae, while pear-sh.aped 

 swellings, characteristic of several members of this genus, 

 are formed on one side of many of these cross walls. The 

 hyphae vary immensely in size; in some cases they form 

 into strands consisting of colourless hyphae in the centre, 

 surrounded on the outside by several layers of dark-brown 

 hyphae. The streaks mentioned as occurring in the wood 

 are about ?, mm. wide and are made up of small polygonal 

 black cells on the outside, with colourless hyphae running 

 longitudinally in the centie. 



The fungus reproduces itself by means of two f^rms of 

 siiore, both borne on the mycelium surrounding the liase of 

 the stem above ground. The first is a conidia! form, which 

 is apparently evanescent in character and has not been 

 satisfactorily made out. It would appear to be produced on 

 crowded, short, simple or branched black stalks, projecting 

 at right angles to the surface mycelium and having the 

 effect of the pile of a carpet. This type is borne on trees 

 that have just died. The second is a spindle-shaped black 

 spore with a long straight or curved appendage at either end. 

 Fight of the.se are produced in sacs or asci contained in black 

 perithecia. The perithecia are superficial, crowded together, 

 black, brittle and about 25 mm. in external diameter. On 

 the outside, they are crowded with short club-shaped a[)|ien- 

 dages. The spores are extruded in a black tendril through 

 a small, scarcely prominent opening at the apex of the T)eri- 

 theciuni. This form of reproductive organ only occurs on 

 trees that have been dead at least three months. The 

 fungus may spread either by means of its spores or l>y means 



