Disease in Plants

Honey-dew.—The leaves, or other organs, of many plants are sticky in hot weather, owing to the excretion of a sweet liquid containing sugar, the consistency and colour of which vary according to circumstances. This honey-dew must not be confounded with the normal viscidity of certain insectivorous plants—e.g. Sundew—or with the sticky secretion on the internodes of species of Lychnis, etc., where it plays the part of a protection against minute creeping things.

Honey-dew is often met with on Lime trees, Roses, Hops, etc. In many of these cases the honey-dew is excreted by Aphides, which suck the juices of the leaves and pour out the saccharine liquid from their bodies. The sweet fluid is in its turn sought after by ants, and also serves as nutritive material for various epiphytic fungi—e.g. sooty mould, Capnodium, Fumago, and Antennaria—which give the leaves and honey-dew a brown or black colour. Certain Coccideae also excrete honey-dew, especially in the tropics.

At least one case is known where honey-dew is formed as the result of the parasitic action of a fungus, namely Claviceps purpurea in its conidial stage on the stigmas of cereals, and this may be compared with the sweet odorous fluid excreted by the spermogonia of certain Aecidia. In both cases the sweet fluid attracts insects which disperse the spores.

Honey-dew may also be formed without the agency of fungi or insects, when hot and dry days are followed by cool nights, with a saturated atmosphere, e.g. Caesalpinia, Calliandra and other trees in the tropics, which are called rain trees owing to the numerous drops of fluid which drip from the leaves under the abnormally turgescent conditions referred to.

Cuckoo-spit.—The leaves of Willows, Meadow grasses and herbs, etc., are often seen with froth on them, in which is a green insect, Aphrophora, which sucks the juices from the tissues and excretes the frothy watery cuckoo-spit from its body.

Slime-flux.—The trunks of trees may sometimes be observed to pour out a slimy fluid from cracks in the bark, or from old wounds, or branch scars. In some cases, e.g. in Oaks, the slime has a beery odour and white colour, and abounds in yeasts and other fungi to the fermentative activity of which the odour and frothiness are due. In other cases the slime is red e.g.—Hornbeam; or brown—e.g. Apple and Elm; or black—e.g. Beech, the colour in such cases being due to the mixture of yeasts, bacteria, and fungi with which these slimes abound. The phenomenon appears to be due to the exudation of large quantities of sap under pressure—root pressure—and is primarily a normal phenomenon comparable to the bleeding of cut trees in spring: the fungi, etc., are doubtless saprophytes, but their activity is concerned with the putrefactive processes going on in the diseased wood, and which may lead to rotting of the timber.

The origin of the wounds in the bark and cortex, and which extend into the wood and other tissues as the putrefactive and fermentative processes increase, appears to be in some cases at least due to lightning.

Resin-flux or Resinosis.—The stems of Pines and other conifers are apt to exude resin from any cut or wound made by insects, or by the gnawing of other animals; but in many cases the flow is due to fungi, e.g. Peridermium, the hyphae of which invade the medullary rays and resin canals and thus open the way to an outflow through cracks in the bark. Agaricus melleus not only invades the resin passages, but stimulates the tree to produce abnormal quantities of resin, which flows down to the collar and roots, and exudes in great abundance at the surface of the soil. Various other plants also exude resin from wounds, and in some cases the flux seems to be increased by degeneration of the tissues, e.g. Copaifera.

Gummosis.—Cherries, Apricots, Acacias, and many other trees are apt to produce abnormal quantities of gum, which flows from any wound or exudes through cracks in the bark. Degeneration of the wood-cells, and especially of the cell-walls of a soft wood formed by abnormal activity of the cambium, points to its origin being due, in some cases at any rate, to a conversion of the cellulose, and fungi are sometimes found in the masses of gum; but beyond the fact that gummosis is a pathological phenomenon we know very little of the disease.

With regard to such gumming, it is significant how frequently pruned trees—Cherries, Oranges, Lemons, Plums, etc.—suffer.

Manna flux.—Certain trees, such as the Manna Ash, species of Tamarisk, etc., yield manna from wounds, and in some cases the latter are due to insects, e.g. Cicada.

The Potato-disease is best known by the pale whitish fringe, giving an almost mealy appearance to the margins of the brown to black patches in damp weather. In dry weather the brown patches shrivel and dry, and as they are apt to be at the edges and tips of the leaflets, these curl up. The young disease spots are yellowish, and the leaves of badly affected plants are apt to be sickly yellow throughout.

This Potato-disease due to Phytophthora must be distinguished from the curling and puckering, with wilting and browning of the leaves and yellow glassy look of the stems, due to the invasion of the vessels by a fungus which lurks in the tubers, and gains access thence to the shoots.

In the disease traceable to Phytophthora the stock remains green and the leaves plump and plane, and only the brown patches slough out in wet or shrivel in dry weather, and are bordered by the pale whitish zone of conidiophores.

In the leaf-curl the yellow and flaccid appearance of all the leaves of a stalk, or even of the plant, is the striking symptom, and the stem soon droops and blackens just above the soil, a white mould appearing also at the black spots. Subsequently black spots appear higher up, and bacteria gain an entrance. The stolons rot, and eventually the roots and the leaves wither. The tubers appear sound, but are small; they are apt to rot in the store, the vascular zones turning brown.

This leaf-curl has been ascribed to Pleospora, Polydesmus, Verticillium, and other parasites, as well as to excessive manuring and other agencies, but it still needs explanation.

Rot of Potato tubers in the soil, or in store, may be brought about by very different agents.

If Phytophthora has obtained access, the fungus hyphae spread between the cells, starting from the haulm, and cause the flesh to turn yellowish and then brown in patches. On the exterior are discoloured patches, depressed, with the flesh beneath brown and soft. The mycelium spreads mostly in the outer layers, which though they turn deep brown remain firm.

Wet rot of potatoes may be due to various fungi, and, in excess of water, to putrefactive bacteria (e.g. Clostridium), which destroy the cell-walls. The flesh becomes soft, then soup-like, and finally putrefies to a liquid mass with a vile smell of butyric acid, etc., in which the starch grains may be seen floating.

Tubers are often found with the cork burst and peeling in shreds, the flesh more or less converted into a putrid and stinking pulp, with a spotted brown boundary of partly destroyed but firmer tissue between the dark utterly rotten and the white and still firm healthy flesh. The principal agent in the destruction of the tissues is Clostridium, an anaerobic bacillus which consumes the cell-walls but leaves the starch intact. Hence a thoroughly decomposed tuber consists of a cork bag full of starch and foetid liquid. In the dried condition the flesh shows a brown marbling; this passes into a soft soupy starchy part, and here and there may be violet grey cavities lined with Spicaria, Hypomyces, etc., the white stromata of the latter often appearing externally. The excavations are filled with loose starch grains, and bounded by cork and cambium formed in the peripheral cells. The cell-walls eventually undergo slimy decomposition.

Spicaria, Fusisporium, various moulds, and bacteria may all be associated with wet-rot.

Dry-rot of Potatoes is also due to various fungi and bacteria, but the destructive action goes on slowly, owing to there being no more moisture than the tissues afford. The flesh becomes excavated here and there, owing to the slow destruction of the cell-walls by Clostridium: the destroyed tissues are brown, and the uninjured starch grains powder them all over. Finally the whole shrunken mass has a crumbly consistency.

When the flesh remains white, but assumes a powdery consistency and dry-rot, with the cork destroyed here and there, Frank refers the damage to Phellomyces. Where the dry-rot is due to Fusarium the chalk-white stromata may often be detected breaking through the periderm; but it must be remembered that the soil-contaminated, broken skin of a potato-tuber is a favourable lurking spot for many fungi, and Periola, Acrostalagmus, and others have been detected therein.

Brown spots, depressed into the flesh, sometimes result from the ravages of Tylenchus, the minute worms being found in the diseased tissues.

In some cases the flesh turns watery and soft, grey, almost glass-like, starting at the haulm end, and this may be owing to the invasion of Rhizoctonia.

Notes to Chapter XXV

The rotting of bulbs, roots, etc., has been much discussed during the last few years in the pages of the Gardeners' Chronicle, Zeitschrift für Pflanzenkh., and elsewhere. The principal references to Bacteriosis—the rot in which bacteria are stated to be the primary agent causing these and similar diseases—may be found in Massee, Diseases of Plants, pp. 338-342, and more fully in Russell, Bacteria in their Relation to Vegetable Tissue, Baltimore, 1892; and in Migula, Kritische Uebersicht derjenigen Pflanzen-krankheiten, welche Angeblich durch Bakterien verursacht werden, Semarang, 1892.

The most convincing accounts, however, are since that date; see Smith, "Pseudomonas Campestris," Cent. f. Bakt., B. III., 1897, p. 284, and Arthur and Bolley, Bacteriosis of Carnations, Perdue University Agr. Expt. Station, 1896, Vol. VII., p. 17. Woods has lately shown that this disease is due to Aphides only, the bacteria having nothing to do with the disease primarily, Stigmonose, Bull. 19, U.S. Dept. Agr., 1900; but it is necessary to bear in mind that actual penetration of the cell-walls from without must be proved, as De Bary proved it for germ-tubes of fungi, before the evidence that Bacteria are truly parasitic in living plants can be called decisive. This is a difficult matter, but until it is settled we do not know whether these organisms are really parasitic in the sense that Phytophthora is, or merely gain access by other means—I have traced them through dead fungus-hyphae—to the vessels, dead cell-walls, etc. The proof of infection via water pores and vessels is given for one species by Harding, "Die Schwarze Faulnis der Kohls," etc., Cent. f. Bakt., Abh. II., B. VI., 1900, p. 305, with literature.

Concerning the "Damping off" of seedlings, see Marshall Ward, "Observations on the Genus Pythium," Quart. Journ. Microsc. Soc., Vol. XXIII., 1883, p. 485, and Atkinson, Bull. 94 of Cornell University Agric. Expt. Station, 1895, p. 233.

On Bacteriosis in Turnips, see Potter, Proc. R. S. 1901, Vol. LXVII., p. 442.

CHAPTER XXVI.

NECROTIC DISEASES

Patches—Frost-patches—Bruising due to hail, shot, etc.—Fire—Sun-burn or scorching—Sun-cracks. Dying-back—Frost—Fungi—Wound fungi—Defoliation by insects—Defoliation by hand—Staghead.

Necrosis.—This is a general term for cases where the tissues gradually turn brown or black in patches which die and dry up, the dead area sometimes spreading slowly and invading the usually sharply demarcated healthy tissues around. It is a common phenomenon on the more slender stems or branches of trees, especially those with a thin cortex, and the terms Brand or Scorching sometimes applied signify the recognised resemblance between burnt patches and these dead areas of necrotic tissue.

Necrosis is often due to frost, which kills the cortex of Pears, Beech, etc., in patches of this kind. The dead cortex and cambium stick to the wood beneath and contract as they dry. The living cambium and cortex around them then begin to push in callus towards the centre of the necrotic area; but since this callus is formed under the pressure of the cortical tissues it does not form a thick lip or margin to the healing wound, as it does in a Canker, but insinuates itself with thinned-off edges between the wood and the dead tissue, or at most traps a little of the latter in the final closing up of the wound. It is easy to see how such an area of Necrosis may become a Canker if the dead tissues split or slough off, and fungi or insects obtain access to the callus at the margins of the area, setting up the disturbances described on p. 222. As matter of fact many Cankers—e.g. those of the Larch disease, and those due to Nectria, or Aphides, etc.—often begin as flattened or depressed areas of Necrosis started by frost, and many small necrotic patches would eventually become Cankers if not healed up by the callus.

Necrosis may also be due to the bruising of the tissues by large hailstones, to gun-shot wounds, or to any form of contusion which kills the living cells of cortex and cambium.

Necrosis is a natural and common result of fire, and it frequently happens after forest-fires which have run rapidly through the dry underwood, fanned by steady winds, that the lower parts of the boles are scorched on one side only. The killed cambium and cortex then dry up in black necrotic patches, which may eventually heal up by intrusion of callus from the uninjured parts.

Sun-burn or Scorching.—If thin-barked trees, such as Hornbeam, Beech, Firs, etc., which have been growing in partial shade owing to dense planting, are suddenly isolated by thinning, the impingement of the sun's rays on the south-west side during the hottest part of summer days may kill the cambium, and produce necrosis of the cortical tissues, and such necrotic patches heal very slowly or not at all, because the dead tissues have contracted so tightly on to the wood below that the callus cannot readily creep between.

Sun-cracks are due to intense insolation on the south side of trees in clear weather in early spring, causing the drying and contraction of the wood and its coverings down that side of the tree: the contracted tissues consequently split, as in the case of frost-cracks, the healing up of which is very similar.

Dying-back.—All that is true of the necrosis of cortical tissues in small patches also applies to cases where the whole of the outer tissues of thin twigs and branches die of inanition owing to a premature fall of leaves—e.g. after a severe attack of some insect or fungus pest. The consequent arrest of the transpiration current and the proper supply of nutriment to the cambium and cortex explain the phenomena. The younger branches of Coffee trees suffering from severe attacks of leaf-disease are often denuded of leaves and die back from the causes mentioned, the whole of the outer tissues becoming necrotic, and drying up tight on to the wood, because other branches with functionally active leaves on them divert the transpiration current, and drought and inanition supervene.

Dying-back is frequently also a direct effect of early frosts, which kill the thin twigs before the "wood is ripened," as gardeners say.

Dying-back is also a frequent result of direct frost action on thin watery shoots or "unripe wood," and is apt to occur every year in certain varieties of Roses, for instance, in particular situations, such as "frost-beds," or aspects exposed to cutting winds, and so forth. The necrosis which results may affect all the tissues, or only the cortex and cambium, and the frequent accompaniment of all kinds of saprophytic Ascomycetes and moulds or other fungi is in no way causal to the phenomenon.

Dying-back may also be caused by fungi, and not necessarily parasites, for cases are often observed where saprophytes only are to be found in the necrotic tissues of the cortex, having made their way in through minute cracks, lenticels, etc.

A simple case is often seen in Chrysanthemums, Roses, etc., chilled and wetted to danger point, but not frozen, during the nights of autumn. The lowered resistance of the chilled tissues enables fungi like Botrytis cinerea to gain a hold, and the peduncles die-back with all the symptoms of Necrosis, the fungus gaining power more and more as its mycelium spreads in the dead tissues.

Many other cases are known where wound-fungi, such as Nectria, Cucurbitaria, Phoma, etc., in themselves incapable of true parasitism, gain a hold on the necrotic tissue of a wounded twig, and having laboriously accumulated a vigorous mycelium saprophytically, extend into other parts. In many of these cases the dying-back of the twigs is expedited owing to the mycelium invading the medullary rays and wood vessels, and so obstructing the transpiration current. The much more rapid spread of the hyphae up into the parts thus killed sufficiently indicates the fundamentally saprophytic character of such fungi.

Dying-back in all its forms is a common result of defoliation by insects, e.g. caterpillars, especially if it occurs when the wood is depleted of reserve materials, and thus cannot supply the auxiliary buds and enable the twigs to clothe themselves with a new flush of foliage, a common danger in Conifers.

Any form of defoliation—e.g. excessive plucking of tea and mulberry leaves, browsing of animals, etc.—exposes the twigs to the dangers of dying-back, the accessory phenomena being similar to those already described.

Stag-head.—Old trees, though vigorous and in full foliage throughout the crown generally, frequently lose the power of bearing leaves on their topmost branches and twigs, which stand out bare and brown, and fancifully resemble the antlers of a stag: hence the forester's name "stag-head." This "top-dry" condition is frequently due to the removal of litter, or to excessive draining, or to the roots having gradually penetrated into unsuitable soil. The consequence is that some dry summer the drought causes the breakage of the water columns above, and the twigs die back.

Tropical trees may also become stag-headed owing to the attacks of Loranthus and other parasites, the portions above the point of attachment dying back from inanition.

Cases also occur in the tropics where the stag-head condition is due to the persistent roosting of frugiferous bats—"flying foxes"—which tear the bark and foliage with their claws, and befoul the twigs generally.

Notes to Chapter XXVI

The principal literature as regards frost is given in the works of Frank, Sorauer, and Hartig already referred to. An excellent summary will be found in Hartig's Diseases of Trees, p. 282, and in Fisher "Forest Protection," Vol. IV. or Schlich's Manual, p. 423.

CHAPTER XXVII.

MONSTROSITIES AND MALFORMATIONS

Monstrosities—Teratology—Atrophy of organs—Shanking of grapes—Barren fruit trees—Dwarfing—Distortions and malformations—Fasciations—Flattened roots—Torsions—Curling and puckering—Leaf rolling—So-called "spontaneous" teratological changes.

Monstrosities.—In a wide sense this term is applicable to many cases here treated under other headings, and signifies any departure from the normal standard of size, form, arrangement, or number of parts, and so forth, due to arrest of growth, excessive growth of parts, or of the whole organs, etc.

Such teratological conditions are however by no means always pathological: that is to say, they may be variations which do not threaten the existence of the plant. In some cases they are clearly due to exuberant nutrition, and although they may occasionally predispose to disease, in others they show no evidence of doing so. The whole practice of horticulture and agriculture abounds in examples of teratological sports or varieties which are transmissible by seeds, budding and grafting, and other means—e.g. double flowers, hypertrophied floral organs (cauliflowers), seedless grapes and oranges, crested ferns, etc.; and even when such varieties could not live as such in a state of nature, there is evidence to show that many of them readily revert to the original seed-bearing or single condition, and adapt themselves to the altered environment.

Every part of the plant may exhibit teratological changes, and I shall for the most part select cases in illustration which indicate approach to pathological states, and group with them cases known to be pathological in origin.

Atrophy is a common phenomenon denoting dwindling or reductions in size of organs due to insufficient nutrition, or arrest of growth from various causes.

Atrophy of leaves is a common result of the attacks of parasitic fungi, even when the latter induce local hypertrophy—i.e. excessive growth of particular parts, e.g. Synchytrium on Dandelions and Anemones. Puccinia suaveolens causes partial atrophy of the leaves of Thistles, Aecidium Euphorbiae of those of Euphorbia.

The carpels of Anemone are atrophied in plants attacked by Aecidium, and the whole flower is suppressed in Cherries infested with Exoascus Cerasi, while other fungi—e.g. Cystopus, Exoasci, etc.—cause atrophy of the seeds, and numerous instances of atrophied grain occur in plants infested with Ustilagineae.

Atrophy of the grains of cereals is sometimes due to the direct attack of animals, e.g. eel-worms (Tylenchus) eat out the grains of Corn; weevils and other beetles (Curculio, Bruchus, etc.) similarly devour the contents of grain and nuts, the flowers of Peas and Apples, and so forth, inducing atrophy of the parts left. Still more striking cases are afforded by small insects which bore into the halms of cereals, and cause atrophy of the whole ear—e.g. Cephus in Wheat and Rye. Barley occasionally withers after flowering, the grain atrophying from no known cause, terms like consumption given to the disease conveying no information.

Atrophy of young fruits is commonly due to the flowers not setting—i.e. some agent has interfered with the normal transference of the pollen to the stigma. This may be due to excessive rain washing out the pollen (e.g. Vine), to a lack of the necessary insects which effect pollination, often seen in greenhouse plants; to the stamens being barren—e.g. certain varieties of Vine—or to the premature destruction of the stigmas by frost, as in Cherries, Pears, etc., or by insects, as in Apples, or fungi, e.g. the infection of bilberries with Sclerotinia; or even by poisonous gases, as is sometimes seen in Wheat, etc., growing near alkali works. Drought is also a common cause of atrophy of young Plums.

Shanking of Grapes is a particular case of atrophy and drooping of the immature fruits, due to the supplies being cut off by some agency. It may arise from very various causes which bring about disease in the leaves or roots, and should always be looked upon as a sign of weakness in the Vine, the structure of which is affected, e.g. poor wood—or the functions interfered with, e.g. water supplies deficient owing to paucity of roots.

Barren Apple, Pear, Plum, and other flowers are often found to have been bored through the petals while in bud, and the whole "heart" of the flower eaten out by the grubs of Anthonomus, leaving the unopened buds brown and dead, as if killed by frost or drought, and often erroneously supposed to be so.