364 



THE AGRICULTURAL NEWS. 



XliVEMEEK 



1914. 



FUNGUS NOTES. 



BLACK ROOT DISEASE OF LIMES. 



The ^lycologist tc tlif Imperial Department of Agri- 

 culture lias recently returned from a visit to Dominica, under- 

 taken for the purpose of ascertaining the distribution of root 

 diseases of lime trees, and of making a trial of possible reme- 

 dies. The following general account of the observations made is 

 published for the information of planters, in anticipation of 

 tletailed reports to follow. 



There are several more or less well-detined root diseases 

 attacking lime trees in different parts of the island, Ijut the 

 only one which was found to be of really serious importance 

 is the lilack root disease, cau.sed by the fungus Rosellinia 

 bunodes. 



This disease was investigated by ^Mr. ¥. W. South, late 

 ^Mycologist to the Department, in 1911-12, and a de.scription, 

 with sugge.stions for i)reventive treatment, was published in 

 the Report on the Agricultural Department, Dominica, for 

 1912-13. 



As affecting lime trees, the disease is only known on 

 estates with fairly recent forest clearings. The effects of 

 a (losely similar disease have long been known on cacao, 

 which is usually affected as a .sequel to the death of an 

 avocado pear, or breadfruit tree. There is evidence which 

 .seems to indicate that the form which usually occurs on cacao 

 is not identical with that which is common on limes. 



The losses occurring on lime estates of the character 

 above mentioned are serious, and appear to be quite general. 

 The most discouraging feature of the situation to the planter 

 lies in the fact that the more the soil and situation favour 

 the rapid development of the trees, the more susceptible they 

 are to the disease. There are many flat tracts in the 

 interior of the island where a somewhat shallow soil overlies 

 :i continuous sheet of thick hardpan. On such flats the 

 drainage is bad, and the trees in general grow slowly, and 

 bear little fruit. Such trees, however sickly they might 

 look, were not found to have contracted the black root disease. 

 t)n immediately adjacent slopes, where no hardpan has 

 formed, and the trees thrive much better, the disease is 

 conmion, as it is on valley .slopes, however steep, and on flats 

 with j)ervious .subsoil. The u.sual idea with regard to fungus 

 diseases, that they attack only unthrifty plants, is directly 

 negatived in this case. 



The reason for this anomaly can only be conjectured. 

 Among the more obvious possibilities are (1) that the fungus 

 can only exist in certain conditions of soil — a test of the 

 relative acidity of the soils would be interesting in this 

 coimexion; (2) that the forest trees whose roots are suscepti- 

 ble to the disease occur only on good soils; (.3) that the roots 

 of slowly growing lime trees are harder, and thus resist the 

 disease. The fact that new roots put out above the wound 

 oil the collar often remain healthy, for a time at least, and 

 lea<l to partial recovery, may prove significant in this con- 

 nexion; but until such roots have been kept under observa- 

 tion for a longer period, it cannot be .said whether they 

 ultimately become affected. 



The course of the di.sease is often ea.sy to trace. The 

 fungus can exist upon woody matter, roots or branches, 

 buried in the soil. Should a lime root come in contact with 

 sudi infected material, it is liable to become diseased by the 

 tlu'eads of the fungus growing into it. Roots as thick as 

 a finger may be thus attacked. The fungus then works 

 along the root, passes from one root to others in contact 

 with it, and eventually reaches and infects the collar of the 

 ti-'ee. Around this it gradually .spreads, involving other roots 



as it comes to them, until the tree is completely girdled. 

 The spread of the disease up the stem is strictly limited by 

 the degree of dampness of the bark. Where the stem is 

 open to the air and well ventilated, the bark remains sound 

 almost to the soil-level, and a callus is formed along the edge 

 of the wound, from which roots are sent out in the attempt 

 to replace those which have been killed. If on the other 

 hand the stem is' shut in by weeds and low branches, or 

 ventilation is prevented in any other way, the disease may 

 extend for 2 or 3 feet up the stem. As seen under 

 the conditions prevailing during the recent visit, the foliage of 

 the infected trees usually showed no sign of ill health until 

 some time after the girdling of the tree was completed. "\'igor- 

 ous looking trees, with dark green foliage and al.)undance of 

 fruit, were often found to have the bark of the collar com- 

 pletely infested. Suddenly the leaves of such a tree turn 

 yellow, and soon afterwards fall, leaving the branches bare 

 save for the withered immature fruit. At this stage the 

 tree may linger for a time. The loss of the leaves eases the 

 strain on any small connexions which still exist at the 

 collar or have been established by roots springing from 

 the lower margin of the healthy bark. The latter are 

 usually mere matted tufts, but sometimes a few of them get 

 established, thicken up, and give rise to a rudimentary root 

 system. New shoots then appear on the branches, but they 

 are small and scattered, and though such trees have been 

 met with which have apparently lingered for months, it seems 

 extremely doubtful that they are ever of any use. 



As soon as the fungus, in its course along the roots,, 

 reaches bark which is exposed above the soil, whether on 

 a projecting knuckle of a root or on the collar, it proceeds to- 

 form its spores. The first form of fructification consists of 

 short l)lack stalks, about one twelfth of an inch in length, 

 surmounted by whitish tufts which are heavily powdered 

 with the minute spores. Large surfaces of bark, and occasion- 

 ally adjoining dead twigs and leaves, are closel3- covered with 

 the.se structures. At a .somewhat later stage they are mixed 

 with, or replaced l)y, the second form of fructification, which 

 consists of round black bodies, about one twelfth of an inch 

 in diameter, closely set with irregular projecting scales. 

 These bodies are hollow, and within them are formed large 

 numbers of a second, and probably nuich more resistent, form 

 of spore. Dead trees, whether standing, or dug up and thrown 

 aside, were often fouijd completely coated for a foot or more 

 at their base with these fructifications. They are also quite 

 common on the dead portions of the bark of still living trees. 



There is little doubt that the disease is usually communi- 

 cated to the lime trees in the first instance from wood left m 

 the soil when the clearing is made, but the examination of 

 a large number of stumps and of much decaying wood gave 

 negative results, and it is possilile that the number of sucli 

 original sources of infection is quite s)nall. By far the greater 

 number of cases now occurring appear to arise from contact 

 mth, or infection friim, earlier cases of the disease on lime 

 trees. It is true that many isolated cases still occur, but the 

 seriousness of the present situation is largely due to the occur- 

 rence of groups of diseased trees in which very commonly 

 a definite .sequence cau be traced from a more or less central 

 source. Were sporadic cases promptly and vigorously dealt 

 with, the losses would be very greatly reduced. Further, it 

 must be pointed out that the chance of the occurrence of 

 sporadic cases is greatly increased by allowing dying and dead 

 trees to .stand; for upon each of these are producctl myriads of 

 the spores of the fungus. Whether such spores infect the 

 trees direct, or whether, as is much more probable, they first 

 attack dead material, the result is the same in the end. Dead 

 stumps and rotting wood as such have no power to give rise to 



