195 



THE AGRICULTURAL NEWS, 



Junk S, 1912. 



FUNGUS NOTES. 



THE GENERAL TREATMENT OF ROOT 

 DISEASES OF PERMANENT CROPS. 



L)uring recent months, a considerable amount of atten- 

 tion has been called to the damage inflicted upon permanent 

 cultivations, such as cacao and limes, liy fungi, Avhich attack 

 tlieir roots, and are capable of pa,ssing from one tree to 

 another wherever a diseased and a healthy root come into 

 contact with one :inother beneath the surface of the soil. 

 These diseases occur throughout practically all of those West 

 Indian islands in which permanent crops form the staple 

 cultivation, and their existence has long been known and 

 recognized by scientific workers, and to some extent by 

 planters. More recently, however, there has been growing 

 up among those interested in agriculture a wider appreciation 

 of their true nature, and of the definite, though limited, pro- 

 portion of damage they inflict. The increased attention and 

 more general observation that have been bestowed upon them 

 have resulted in some instances, as is natural, in a tendency 

 to regard the damage as greater than it is actually. In spite 

 of this, it cannot be denied that the harm inflicted is, in 

 many instances, of sufficient importance to warrant the expen- 

 diture of some time and money on adequate means for pre- 

 venting the spread of the fungus in infected areas, and it is 

 thought that an account of the measures likely to prove suc- 

 cessful in attaining this object may be acceptable to those who 

 have to contend with the problem. 



It is not intended here to enter into a description of 

 the fungi responsible for the diseases under consideration, as 

 this may be found in the Agricultuial Xeivs, Vol. X, 

 pp. 3G(J and 382. Suffice it to state here that, in a majority 

 of cases at any rate, the causative fungus is only capable of 

 spreading from a diseased to a healthy tree when the infected 

 ■roots of the one come into immediate contact with the pre- 

 viously healthy roots of the other; that is, the fungus is 

 incapalile of spreading by independent strands of mycelium 

 running through the soil. If left to itself, the di-sease pro- 

 gresses gradually in an ever-widening circle, and, theoretically, 

 vould only be arrested by the presence of natural barriers, 

 such as ravines, streams, or ditches, which form a break in 

 the continuity of the soil. The first object, therefore, is eftec- 

 tually to isolate the diseased area. When this has been 

 done, all infected material must be removed from the soil 

 and burned, and the soil itself must be well aerated, and 

 thoroughly treated with a fungicide. 



The isolation of the diseased area is usually effected by 

 means of a trench; this must be not less tlian 3 feet deep, 

 and should be about 18 inches wide. It must be kept 

 open to its full depth until the area it isolates has been freed 

 from the presence of the root fungus. Shallow trenches are 

 useless for isolation purposes, as are also trenches that are 

 allowed to silt up before the fungus has been entirely eradi- 

 cated. Another matter of great importance in successfully 

 isolating the disease is the location of the trench in such 

 a position that all infested roots are included within it. 

 Many of these diseases commence on the stumps of forest 

 trees or on living avocado pear or bread fruit trees whose 

 roots extend for a considerable distance and for a considerable 

 depth in the soil. The root fungi are liable to travel along 

 all such roots to iheir furthest ramifications, and therefore all 

 of them must be included within the area limited by the 

 trench At the same time, it should be remembered that the 

 ri'ots of the outermost infected cultivated tiees in the area 



may possibly extend beyond the healthy trees next to them,, 

 and that these healthy trees may in reality be themselves 

 slightly aftected, though no symptoms of ill health are visible 

 on Iheir aerial portions. All this indicates clearly that the 

 trench must be run in such a way as to include a margin of 

 at least two or three healthy trees in a belt round the infected 

 area. Furthermore, all roots cut in the process of digging 

 the trench should be carefully examined, and if signs of the 

 disease be found upon them on the side remote from the- 

 infected area, the position of the trench must be altered to- 

 include them within its boundary. It need hardly be added 

 that the soil taken from the trench should be thrown on to the 

 infected area. These details have been emphasized here as 

 there is reasori to believe that, occasionally, failure to consider 

 them has resulted in a corresponding failure to control suc- 

 cessfully the progress of the disease. 



Once the infected area has been thoroughly isolated, the 

 next step is to destroy as carefully as possible all infected 

 material in the soil and thus with the aid of di.'^infectants to 

 eradicate the fungus and render it once more safe for the 

 planting of supplies. The dead trees must first be felled, 

 the soil then cleared away from the stumps, and the 

 roots very carefully followed up as far as possible; the 

 whole tree, top, stump and roots, should then be cut up 

 and burned in the hole from which the stump has been 

 removed. It is important that dead trees, especially the 

 fungus-infested portions below the soil, should not be dragged 

 through the healthy portions of the cultivation. When it is 

 observed that fructifications of the fungus have been formed 

 at the base of the stem above the ground, dry leaves or 

 other inflammable material should be piled up round the tree 

 and lighted, so that the spores are burned. This prevents the 

 scattering of the spores into the air when the tree is dis- 

 turbed. When the diseased roots of dead trees are followed up, 

 it may be found that they have come into contact with, and 

 infected, those of apparentlj- healthy trees near the margin 

 of the quarantined area. Such roots should be carefully 

 followed up in turn and cut off at a point some 3 or 4 inches 

 above the limit of the visibly diseased tissues; the cut ends 

 should be tarred, and the soil in their neighbourhood 

 treated with lime or some other fungicide, as is described 

 below. This process may be expected to save the lives of 

 the healthy trees left on the margin of the diseased area. 



Having freed the soil as far as possible from material 

 carrying the fungus, the next step is to apply a heavy dress- 

 ing of lime at the rate of about 6 lb. per scjuare yard, and 

 to mix it thoroughly with the soil by forking. Any small 

 pieces of root turned up in this process should be collected 

 and burned. The dressing of lime recommended here is 

 heavy, but is not likely to be excessive on lieav clay soils 

 or in situations where the soil is liable to be acid. Certain 

 fungicides such as sulphate of copper and of iron have been 

 recommended for soil disinfection, but it is not certain that 

 they will prove satisfactory in all cases, and their use cannot 

 be encouraged until it has been thoroughly tested by experi- 

 ment The e.xposed soil inside the isolated area should be 

 . forked over sufficiently frequently to maintain it free from any 

 strong growth of weeds. This will serve at the same time to 

 keep it well aurated and to expose fresh surfaces to the sutt. 

 It is possible that where this treatment is carefully carried 

 out, replanting may be commenced after an interval of six 

 months, though it might be preferable to wait twelve months. 

 Experiments have been arranged to determine how long a time 

 should elajise before replanting, so that absolute safety may 

 be ensured. Before leaving this part of the subject it may 

 perhaps be advisable to point put that during all forking- 

 operations, care shojjld be titlcen to prevent the breaking^ 



