The Leaf Disease, or Coffee Leaf Disease, of tho coffee plant seems first to have appeared in Ceylon in 1869, and prominently in S. India in 1870. It is the Hernileia vastatrix, a fungus which fixes on the under-sides of the leaves, causing spots or blotches, at first yellow, but subsequently turning black. These blotches are covered with a pale orange-coloured dust, which is easily rubbed off. The blotches gradually increase in size, until at last they have spread over the entire leaf, which then drops off, leaving the trees in a short time perfectly bare. The trees seem to recover in the spring of the year, but the fresh foliage is in its turn attacked, and the crop falls off. The yearly crop in Ceylon is said to have diininished 300,000 cwts. since its appearance, cau.sing a loss of about 1} niillions sterling. The leaf disease is a true endophyte, developing in the tissues of the leaf, and expand ing outwards, as is the case with red rust of corn and grapes. Sir Joseph Hooker, writing to the Colonial Office, 6th August 1874, says the Hetni leia belongs to a class of most minute parasitic plants, which includes the (Atli= of the vine and peronospora of the potato. Recent observations appear to establish the fact that similar plants are also present in the tissues of sheep affected with small-pox, and may probably be found to bo sources of some familiar diseases in man and the lower animals. Hitherto no means have been discovered of extirpating these plants, whose seeds, of inconceivable minuteness, float in inconceivable numbers in the air, and whose growing parts are of such tenuity, that they can penetrate the most delicate living tissues of plants, which they kill by feeding on their substance. The spread of some of the kinds has, however, been effectually checked by the use of flower of sulphur, which it would not be difficult to apply to the coffee plant in Ceylon.
The Ilemilcia vastatrix has been termed the coffee leaf disease, as it confines its depredations to the leaves of the shrub. When suffering from this, the leaves are on their upper surface speckled with brownish spots, while the under surfaces are covered with orange-coloured dust, which consists of the spores of the fungus, or bodies by which it is propagated. The fungus itself consists of fine myceloid filaments, which permeate and live amongst the tissues of the leaf. When the spores are ripe, the shaking of the leaviNs by the wind is sufficient to disperse them, and each spore thus liberated is capable of germinating, and may become a fresh focus of the disease. Dr. Thwaites of Ceylon watched this process of germination. For some time the presence of the leaf. disease caused little or no anxiety in India; but as its disastrous effects on crops became apparent, tho planters began to realize the serious character of the distemper. Nearly all are now in accord in the view that manuring is the best remedy. Confident hopes were at one time entertained that the Liberian plant, which has been recently introduced, would defy the enemy. but uufortu nately this expectation has not been realized. It is very destructive in localities. It comes most suddenly and unaccountably in an estate, and disappears apparently without any reason. The
effects of it are obvious : the leaf of the tree alone is attacked, after a little drops off, leaving the bough bare and unsheltered. The disease does not seem to attack the health and vitality of the tree; but when the shade afforded by the leaf to the berry is removed, the bud, blossom, or fruit is unduly exposed to the scorching sun, or to the wind, and the crop is lost. The disease often affects vvhole plantations shnultaneously. Up to 1872 the Ceylon estates had suffered but little from this fungus. In Wynad, however, scarcely an estat,e escaped this leaf disease, whilst in the fifty-two estates of Travancore only one suffered, and that in a most insignificant. degree.
Mr. G. ..knderson, writing from Munzerabad in 1880, gave the opinion that potash, magnesia, and ammonia, in the form of sulphates, check and destroy Hemileia in coffee. He said he had used it in his experiments with excellent results,--+ to A potash, to magnesia, and to ammonia,—and thinks that the cessation of growth which precedes an attack of leaf disease, is caused by a want of ammonia and other valuable food-constituents in an available form ; that lime exerts a marvellous effect in setting free all the alkalies, and in•con vetting nitrogen into ammonia ;• that potash, magnesia, and lime are required for the produc tion of coffee, and are therefore removed front the soil, and that certain forms of potash and magnesia (especially those coinbined with sulphur) are ininaical to the growth of fungi. He concludes that sulphates of potash, magnesia, and ammonia should be used immediately before the occurrence of disease ; and lime should be employed early in the season to sweeten the soil, and set free its resources. He also recommends that phospho nitrogenous manures, combined with other forms of potash, be applied to sustain and invigorate the trees.
Several learned botanists have suggested reme dies for these plagues.
Baron F. von Mueller says that in 1878 the parasitic fungus growth on coffee plants in Ceylon caused a loss of £2,000,000.
In America the coffee plantations suffered not only from mysiphoid fungi, but also frotn the cendostoma fly.
In the Karen hills, experimental cultivation of tea a.nd coffee was introduced north-east of Tounghoo. Mr. J. Petley says in his report : Towards the end of the rains of '1880 large numbers of the Mole Cricket made their appear ance, and much destruction was done amongst young tca and coffee plants, killing them by nipping off the tops, priucipally amongst the coffee, and boys were employed to catch and kill these destructive insects.' Dr. Thwaites, in his report for 1874, says : Judicious cultivation enables the coffee tree to produce a succession of profitable crops, notwith standing it may suffer from periodical attacks of leaf disease.' Nowhere has the disease committed such havoc as in native plantations,Where cultiva tion is almost entirely neglected. Previously to 1870, planters were divided into two opposite camps,—the party that' advocated manuring, and the party that opposed it, The inroads of leaf disease have led to the matter being speedily and practically settled in favour of high cultivation.