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THE AGEICULTUKAL NEWS. 



October 20, 1917. 



PLANT DISEASES. 



The following report of the Mycologist of the Depart- 

 ment, Mr. W. Nowell, D. I. C, on a recent visit to Dominica, 

 has been printed in the Dominica Official Gazette, August 

 27, 1917, and is here reproduced: — 



BRACKET FUNGI OF LIME TREES. 



The fungi found occupying the stems and branches of the 

 trees are to be regarded, according to my observations, as 

 obtaining a foothold in the first place only on dead branches, 

 and then functioning largely as saprophytes, and at most, in 

 a certain type of cases, as weak parasites principally affecting 

 the wood. The bark of dead or dying branches is usually 

 found to be infected with smaller fungi of which Xectria and 

 Siilbum spp. are the most noticeable. 



The damage done, however, arises almost entirely from 

 the destruction of the woody framework of the tree, and not 

 from any direct attack on the active tissues of the bark and 

 young wood. The destruction is probably largely due to the 

 mycelium of the bracket fungi in question. 



They are able to establish themselves on any dead stub 

 left from ihe failure of a branch or from careless pruning, no 

 matter how healthy the tree may be. But the subsequent 

 progress of the infestation depends on the condition of the 

 adjoining parts. In vigorous trees the decay is arrested 

 when the living branch is reached. Even the wood shows 

 considerable powers of resistance. It rots .slowly where it is 

 exposed, and, unless the wound is trimmed so as to permit 

 the bark to grow over it, there is produced in time an 

 unsightly cavity, but the process is no more than goes on in 

 any tree not carefully trimmed, and unless the wound is a 

 particularly large one, it does not produce serious results 

 for a long time. 



The less vigorous the tree, the less resistant is its wood 

 to decay, and the extreme cases of fungus infestation are to 

 be seen in fields where, after the trees have attained to 

 a good development from the original humus content of the 

 soil, or from the practice of a good system of manuring, the 

 fertility of the soil has then been allowed to run out, and 

 the trees are no longer sufficiently nourished. The policy of 

 substituting sulphate of ammonia, without any packing of 

 humus- building material, for pen manuie, mulch, and other 

 organic manures is ckarly seen in some cases to have 

 produced this effect. In other cases it has come about from 

 complete neglect of manuring, combined with careless 

 treatment of the .soil in the way of ignoring drainage, and, 

 in some instances, packing it by the constant pasturing of 

 cattle. As starvation takes effect one or more sections of the 

 trees begin to die back branch by branch, and not only the 

 dead, but the failing branches become infested with fungi. 

 The removal of the dead branches doe.s little to arrest the 

 process; the wood of the next larger branch or stem is 

 usually already infected, and offers little resistance to the 

 progress of decay. In the course of time the base of the 

 tree is reached by way of the stem or stems involved, and 

 permanent restoration is no longer possible. Growth is 

 usually continued by suckers put out around the base, which 

 may attain to a considerable size, and give the tree a renewed 

 lea-'e of life. In time the rotting of the central part of the 

 on wn. due to the downward extension of deov from the 



original stem, destroys the connexion between the suckers, 

 depriving them of mechanical support, and leaving thenL 

 with exposed wood on their inner sides in the neighbourhood 

 of soil level. They are rarely able to establish themselves as 

 permanent stems, and sooner or later, from the weight of the 

 crop or the force of the wind, they fall outward, or are 

 broken off The process of failure thus outlined is a 

 gradual one, commonly extending over some years, and seems 

 in consequence to be usually regarded with indifference, until 

 the end result appears. There is, even in the worst cases of 

 this type seen in Dominica, no question of any quick des- 

 truction of trees, such as is produced by an active parasite 

 like Eosellinia. 



It has been further noted in cases where lime trees have 

 been grown on poorish land, and without manure, and have 

 in consequence grown slowly from the beginning, that 

 although they n"ay develop dead branches in the course of 

 time, their wood does not appear to be nearly so susceptible 

 of decay as is the case with trees which have grown more 

 quickly. The lime will often live, and remain healthy for 

 a long time under hard conditions, but seems to be particu- 

 larly liable to suffer as a result of irregularities in the 

 conditions. This should be kept in mind in constructing 

 a policy for the distribution of the manure available for an 

 estate. 



The remedial and preventive measures which should be 

 adopted are clearly indicated. In fields where die-back is in 

 evidence, or where small and scanty foliage shows that the 

 trees are lacking in vigour, the first essential is to restore the 

 fertility of the soil. If it has become consolidated, or is 

 covered with a firm sod it should be lightly forked; as much 

 organic material as can be obtained in the shape of pen 

 manure, mulch, cotton-seed meal, lime seeds, and sheep 

 manure should be applied; the weed growth, if rank, may be 

 cutlassed and used as mulch towards the end of the wet 

 season Leguminous cover crops may be grown if there is 

 light enough for their development. 



At the same time the work of clearing the trees of dead 

 and sickly branches should begin. Men who have received 

 elementary instruction in this work should be kept to it as 

 much as possible, and it will be necessary that a responsible 

 person who understands the lines of action should direct it. 

 It is proposed to issue shortly from this ( )ffice a pamphlet 

 setting out the main principles of tree surgery, such as are 

 here involved. In outline they are as follows: — 



(a) To cut back to a living branch or stem which seems 

 likely to survive, and to have vigour enough to cover the 

 wound. The most frequent mistake made lies in not cutting 

 back far enough; the upper dead portion of a branch is 

 removed, a sickly lower section being left because it still 

 has green leave.s, or carries a few limes. This in turn dies 

 back, and the process is repeated. If at the first essay the 

 cut is made behind the failing section, there is a prospect 

 of stopping the die-back in one operation. 



(b) To trim the wound so that it is as far as possible 

 Hush with the surface of the branch or stem remaining. 

 Plain saw cuts rarely accomplish this, and should be 

 trimmed with a hatchet, a chisel, or a cutlass into conformity. 

 This usually means a bigger wound, but it is better to expose 

 more surface which will easily cover than to leave a project- 

 ing lip which is out of the circulation of the sap, and will 

 probably decay. 



(c) Protect the exposed wound from ants, woodants, 

 and fungi, until the bark has time to cover it. Asphalt 

 paint, resin petroleum, resin wa.sh, carbolincuin, or ordinary 

 white lead ])aint maj' be used for this purpose. The last 

 mentioned is perhaps the least satisfactory. Coal tar 



