Disease In Plants Part 17

_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 acc.u.mulated 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 princ.i.p.al 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 ill.u.s.tration 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.

The wilting and shrivelling of Clover is sometimes due to _Sclerotinia_, the mycelium of which pervades the roots and stock, on which the sclerotia may be found. Lucerne is similarly killed in Europe by the barren mycelium of _Leptosphaeria_, which may be found as a purple mat on the roots.

_Dwarfing_ consists in partial atrophy of all the organs, and is a common result of starvation in poor, dry, shallow soils, as may often be seen in the case of weeds on walls or in stony places. Dwarfs which are thus developed in consequence of perennial drought are not, however, necessarily diseased, in the more specific sense of the word; their organs are reduced in size proportionally throughout in adaptation to the conditions, and simply carry out their functions on a smaller scale.

Dwarfing is frequently a consequence of the lack of food materials, or of some particular ingredient in the soil, and in such cases is a diseased condition of some danger; similar results may ensue in soils containing the necessary chemical elements, but in unavailable forms.

Dwarfing may also be brought about by repeated maiming, nipping off the buds, pruning, etc., as in the miniature trees of the j.a.panese; and the case of trees continually browsed down by cattle, or of moor plants perennially dwarfed by cutting winds, are further ill.u.s.trations in the same category, as are also those of certain alpine and moraine plants, whose only chance of survival depends on their adapting themselves to the repeated prunings suffered by every young shoot which rises into the cutting winds, since there is no question of lack of food-materials in these cases.

The practice of the j.a.panese is to pinch out the growing tips of the shoots wherever they wish to prune back, and it is by the judicious use of this heading in, and suitable pot-culture, that the dwarfs are made, 6-20 inches high at from 30-80 years old.

Dwarfing is often brought about by grafting on a slow-growing stock, and this method is employed in practice, as are also heading in, pruning of roots, and confinement in pots.

Dwarfing may also be due to poor or shrivelled--partially atrophied--seeds or such as have had their endosperms or embryos injured by insects or fungi, and although it is possible to nurse such dwarfs into normal and vigorous plants with good culture, they do not usually recover under natural conditions in compet.i.tion with more vigorous plants.

_Distortions_ or _Malformations_ may be defined as abnormalities in the form of organs which concern all, or nearly all the parts, and do not refer merely to swellings or excrescences on them or excavations, etc., in them.

_Fasciation._--Shoots of Asparagus, Pine, Ash, and many other plants are occasionally expanded into broad ribbon-like structures often studded with more than the normal number of buds or leaves, etc., such as would be found on the usual cylindrical shoots. Such _fasciations_ are due to several buds fusing laterally under compression when young and the whole ma.s.s growing up in common, or, in a few cases, to the unilateral overgrowth of one side of the terminal bud. Fasciations appear to depend on excessive nutrition in rich soils. They may spread out above in a fan-like manner, exaggerating the abnormality, or they may revert to the original form. Some cases are more or less fixed by heredity--_e.g._ _Celosia_. Fasciated stems are frequently curved like a crozier, owing to one edge growing more rapidly than the other.

Cauliflowers are really cultivated monstrosities. Fasciated Dandelions, _Crepis_, monstrous Chrysanthemums, peloric _Linaria_, five-leaved Clovers, spiral Teazels, etc., may all, if grown with care, be kept more or less constant in the monstrous state. That is to say, the particular kinds of variation here manifested can be maintained in proportion as the external conditions controlling the variation are maintained. Such conditions are chiefly rich supplies of food-stuffs, plenty of water and air, suitable temperature and lighting, etc. Mutilations, favouring the development of abnormal buds may also induce fasciations.

_Torsions_ or spiral twistings of stems also frequently arise among plants grown in rich soils, and are often combined with fasciations--_e.g._ Asparagus, _Dipsacus_; and De Vries has shown that the peculiarity is not only transmissible by seed, but may be more or less fixed by appropriate culture.

_Contortions_ of stems are often due to the unequal growth on different sides of the stems owing to the presence of fungi--_e.g._ _Caeoma_ on Pines, _Aecidium_ on Nettles, also _Puccinia_ on petioles of Mallow, _Cystopus_ on inflorescences of _Capsella_, etc.

_Distortions_ of roots may be brought about in various ways by the hindrances afforded by stones.