Vol. XIII. No. 327. 



THE AGRICULTURAL XEWS. 



36.5 



the di.sea.se. They may uiul do exist in tons around a tree 

 without the slightest harm ensuing. Only when they have first 

 themselves become infected do they become dangerous, and 

 such infection must in the first place be due to the spores of 

 the fungus. One diseased lime tree is thus capable of causing 

 a whole clearing to be infested, and the fact that the disease 

 may occur in scattered individuals or discontinuous patches 

 does not necessarily show that more than one original centre 

 of infection existed. 



The disease is stated by planters to have become much 

 more serious during the present year; and a period of 

 e.xtremely wet weather is considered to be responsible for the 

 increase. It is however highly probable that it is due in 

 great part to the obvitjusly cumulative nature of the disease, 

 which is further aided in its spread as the growth of the lime 

 roots brings the trees more into contact with each other. The 

 fact must be faced that the disease is capable of making 

 a clean sweep of the trees in the situations which favour its 

 <levelopment. Its occurrence is no mere passing incident: 

 it may be compared to a slowly smouldering tire which will 

 spread as long as it has material to feed upon, while the 

 .spores, like sparks, scatter and cause fresh outbreaks. 



Very careful attention is being paid to possible addi- 

 tional methods of prevention or cure. .Certain areas have 

 been surveyed tree by tree, and will be treated with carbon 

 bisulphide emulsion according to the method advocated l.>y 

 !M. Jkii'daz in Martinique. It is difficult to understand how 

 this a'.;ent can act successfully, but cases are not at all 

 uncommon of remedies proving successful without apparent 

 reason. Nevertheless plantei'S would be wise to avoid disajj- 

 pointment and lo.ss by not depending on the treatment until 

 its efficacy has been confirmed. 



The arrest of the rot, when bark not kept constantly damp 

 is reached, suggests that thorough ventilation of the collar 

 and the main roots near their attachment should consider- 

 ably delay, if not stop, the progress of the disease. At any 

 rate, since there is no known objection to such a course, 

 and there is possibility of considei'able benefit, its effects 

 are being tested. The earth is removed from the collar an<l 

 the crown of roots Ijy hand, to avoid damage by implements, 

 the lower branches ard surrounding vegetation are removed to 

 give the air free access, and the bark is kept clean of spongy 

 moss by the use of lime sulphur wash. There are two 

 ■weaknesses in this method: (1) it is not possible to put the 

 tree entirely on stilts — the central roots' and the under sides 

 of the lateral roots still have a connexion with the collar kept 

 moist by the soil; (2) the roots are so interlaced that the 

 fungus advancing akmg one root can cross over to the next 

 by lateral branches, and infect the whole root system without 

 having to pass round at the collar. 



The detection of diseased trees at an early stage is much 

 facilitated by keeping the collar clear as described. Only by 

 •close inspection of this region can such trees be found. As 

 already explained, by the time the affection shows in the 

 leaves, the destruction is usually almost or quite complete. 

 A diseased root when found may be removed with its 

 branches as completely as possible, but this is in practice 

 • a very diflicult operation, owing to the interlacing of adjacent 

 roots, some of which are u.sually already infected, or easily 

 become so when they are cut or broken in the necessary 

 digging. Several trees have however been treated in this 

 maimer, and the results are being watched. 



In this connexion it is important to be able to recognize 

 the disease on the roots. When the bark is lifted the surface 

 of the wood is seen to be jnarked with dots and with closely 

 a<lhering short black threads, which may be straight, curved, 

 or angular. The dots, which represent the threads cut across, 

 may be seen on close examination to have white cen- 



tres. If a cut is made into the wood, it is seen 

 to be completely penetrated by continuations of these 

 threads, which appear as dots, and as short straight 

 black lines running lengthwise and crosswise. If care 

 is taken to distinguish these from the long continuous zig-zag 

 black lines made liy another fungus common in dead wood, 

 there is little possibility of nustake. Frequently another 

 form of growth Vielonging to the black root disease is seen 

 in the shape of a thin white substance spreading in fan-like 

 patches under the bark. The smallest piece of such infested 

 material is capalile of infecting living roots by contact. 



From the description of the nature of the disease which 

 has now been given it should lie clear that the main pre- 

 cautions recommended in the past are absolutely necessary, 

 if the disease is to be dealt with, difficulties arising from the 

 nature of the ground, and the scarcity of labour notwith- 

 standing. They are, first, the prompt destruction by fire 

 of the dead and dying trees, together with as much cf the 

 root system as it is possible to remove from the soil: second, 

 the isolation of diseased areas by means of trenches, which 

 may be narrow, liut must be deep enough to get below the 

 lateral roots. It is obviously not , sufficient to isolate simply 

 the dead or dying trees. By the time the}" reach that stage 

 there is every probability of the roots of adjacent, apparently 

 healthy trees having alread}- become infected. Either such 

 roots should be laid bare and removed well back into their 

 healthy ]j'_'rtions, which gives a chance of saving the trees; or 

 the trench should lie carried outside of the suspected trees. If 

 both measures can be carried out, so much the better. The 

 nature of the ground often renders such operations very diffi- 

 cult, but they should be carried out as far as possible. 

 The first prrcautiou, the destruction of the trees by fire, is 

 rendered easy by the inflammable nature of lime wood, and 

 should never be neglected. 



No case of the disease on the sour orange has yet been 

 found, although the trees of two orange plantations in forest 

 clearings are on stocks of this nature. Should this apparent 

 immunity be i;onfinned, the experiments at present being 

 carried out with limes budded on such stocks may be of great 

 importance. 



In conclusion, it must be said that there is no desire to 

 create alarm with respect to this disease. Its dangerous nature 

 has been emphasized only in order to impress upon planters 

 the al;>solute necessity for taking elementary precautions 

 against its spread. It is believed that it is by reason of the 

 lack of the.se, and for that reason only, that any serious degree 

 of loss is being experienced. 



SUM.MAKY. 



1 . The black (0(jt disease is due to a specific fungus, 

 which can exist on wood buried in the soil, but is not by any 

 means general on such material. 



2. The fungus spreads by contact from this infected 

 material to the roots of healthy lime trees, and from one root 

 to another. 



.S. The rate of the spread of the fungus along the roots 

 is not known, but probably a very considerable time is taken 

 before the ad\anced stage is reached at which a tree shows 

 signs of sickness. 



-t. On reaching the light the fungus produces spore.s, 

 which are carrieil to a distance, and are capable of producing 

 fresh infections. 



•5. 8uch spores are produced on diseased spots long before 

 the»-tree dies, and also after it is dead. 



6. It has not lieen proved that any curative treatment 

 is practicable. 



7. Preventive ra-jasures consist in the destruction by 

 fire of all diseased material, and the prevention by trenching 

 of the spread of the disease underground. 



