Disease In Plants Part 13

_Brown spots_ or flecks, varying in hue from dull slaty brown to deep red browns, are a common symptom of Fungus and Insect diseases, the colour often indicating the death of the tissues, rather than any special peculiarity of the action of the parasite. Good examples are furnished by the Potato-disease, and by _Peronospora viticola_, _Sphaerella vitis_ and other disease-fungi of the Grape Vine. The teleutospore stage of many Uredineae also occurs in deep brown spots.

Black spots and flecks are exceedingly common symptoms of the presence of fungi, _e.g._ _Fusicladium_ on Apples and Pears, and the pycnidial and ascus stages of many Ascomycetes--_e.g._ _Phyllachora graminis_. The teleutospore stages of species of _Puccinia_, _Phragmidium_, etc., are also so deep in colour as to appear almost black.

_Scab_ on Pears is due to the presence of _Fusicladium_, which indurates the outer skin of the fruit causing it to crack under pressure from within, and to dry up, the deep brown to black patches of fungus persisting on the dead surface.

Black spots on gra.s.ses and sedges are caused by Ustilagineae, and are commonest in the grain, the soot-like powdery spores (s.m.u.t) being very characteristic. _Ustilago longissima_ induces black streaks on the leaves. Many of these fungi cause distortions or pustules on leaves and other organs.

Brown and black leaf spots are frequently furnished with concentric contours arranged round a paler or other coloured central point--_e.g._ _Cercospora_ on Beans, _Ascochyta_ on Peas.

Brown spots with bright red margins are formed in young Beans by _Gloeosporium_.

Species of _Fumago_, _Herpotrichia_, etc., may cover the entire surface of the leaf with sooty patches, or even weave the leaves together as if with black spider-webs.

_Mal nero_ of the Vine is a particular case of black spotting and streaking of the leaves for which no satisfactory explanation is as yet to hand. As with Chestnuts, Walnuts, and other plants containing much tannin, the dark spots appear to be due to this substance, but whether the predisposing cause is a lack of some ingredients in the soil, or some temperature reaction, or fungi at the roots, is as yet unknown. The most recent explanation puts the disease down to the action of bacteria, but the results obtained by different workers lead to uncertainty.

The "dying back" of leaves, especially of gra.s.ses, from the tip, is usually accompanied by a succession of colours--yellow, red, brown, to black--and is a common symptom of parching from summer drought; and spots of similar colours, frequently commencing at the margins of leaves, are characteristic symptoms of the injurious action of acid gases in the air.

Brown and blackish spots on Pears are caused by a species of _Thrips_.

In many cases the minute spots of Rust-fungi on one and the same leaf are bright orange yellow (_uredo_), deep brown, or almost purple-black (_teleutospores_), foxy-red brown (older uredospores), or dead slaty black where the old teleutospores have died off--_e.g._ _Uromyces Fabae_ on Beans, _U. Pisi_ on Peas, etc.

_Parti-coloured leaves._--The leaves sometimes start shrivelling with red edges, while yellow, red, and finally brown and black blotches appear on the lamina, from no known cause--_e.g._ Vines. In other cases similar mimicry of the autumnal colouring of leaves results from the action of acid gases.

_Burning_ is a common name for all cases where the leaves turn red or red-brown in hot, dry weather, and many varieties are distinguished in different countries and on different plants, because species react dissimilarly. The primary cause is usually want of water--drought.

_Foxy leaves_ are a common sign of drought on hot soils, and the disease may usually be recognised by the gradual extension of the drying and fox-red colour proceeding from the older to the younger leaves, and from base to apex--_e.g._ Hops.

_Coppery leaves._--The leaves of the Hop, etc., may show yellow spots and gradually turn red-brown--copper-coloured--as they dry; the damage is due to _Tetranychus_, the so-called Red Spider. These cases must of course be carefully distinguished from the normal copper-brown of certain varieties of Beech, Beet, _Coleus_, etc.

_Silver-leaf._--The leaves of Plum, Apple, and other fruit trees often obtain a peculiar silvery appearance in hot summers, the cause of which is unknown.

Discolorations in the form of confluent yellow and orange patches, etc., resembling variegations, are not infrequently due to the ravages of Red Spider and mites--_e.g._ on Kidney Beans.

_Sun-spots._--Yellow spots, which may turn brown or black according to the species of plant affected and the intensity of the action, are often caused by the focussing of the solar rays by lens-like thickenings due to inequalities in the gla.s.s of greenhouses, or by drops of water on them or on other leaves, _e.g._ Palms, _Dracaena_, etc. The action is that of a burning gla.s.s, and extends throughout the leaf-tissues. Young grapes, etc., may also be injured in this way. Water-drops on the gla.s.s can only act long enough to produce such injuries if the atmosphere is saturated. The old idea that a drop on a leaf can thus focus the sun's rays into the tissues beneath is not tenable.

Here again we see that the disease-agencies concerned in producing the symptoms described in this chapter, agree for the most part in so far that the princ.i.p.al effect is generally the disturbance of chlorophyll action in the spots or flecks on the leaves, and the rendering useless of these areas so far as providing further food-supplies is concerned.

The effects may be due merely to the shading action of a parasite--_e.g._ epiphytic fungi--or to actual destruction of the tissues invaded--_e.g._ by endophytic fungi--or the tissues may be burnt, poisoned, etc. In so far the results are again quant.i.tative and c.u.mulative, and the amount of damage depends on the number and size of the spots or other areas affected, and the proportion of foliage involved, as well as the length of time the injurious action is at work.

But, again, it must be remembered that several symptoms may co-exist, and matters may be complicated by the spread of the destructive agent, or its consequences, to other parts, and in some cases we are quite uninformed as to the true nature of the disease.

NOTES TO CHAPTER XX.

Further information regarding these "leaf-diseases" will be found in special works dealing with the fungi and insects which cause them. In addition to works already quoted, the reader may also be referred for Fungi to Ma.s.see, _A Textbook of Plant-diseases caused by Cryptogamic Parasites_, London, 1899; or Prillieux, _Les Maladies des Plantes Agricoles_, 1895. See also Marshall Ward, Coffee-leaf Disease, _Sessional Papers_, XVII., Ceylon, 1881, and _Journ. Linn. Soc._, Vol.

XIX., 1882, p. 299.

The question of "Sun-spots" has been dealt with by Jonnson in _Zeitschr. f. Pflanzenkrankh._, 1892, p. 358.

CHAPTER XXI.

ARTIFICIAL WOUNDS.

_The nature of wounds and of healing processes--Knife wounds-- Simple cuts--Stripping--Cuttings--Branch-stumps and pruning-- Stool-stumps--Ringing--Bruises._

_Wounds._--All the parts of plants are exposed to the danger of wounds, from mechanical causes such as wind, falling stones or trees, hail, etc., or from the bites of animals such as rabbits, worms, and insects, and although such injuries are rarely in themselves dangerous, they open the way to other agencies--water, fungi, etc., which may work great havoc; or the loss of the destroyed or removed tissues is felt in diminished nutrition, restriction of the a.s.similative area, or in some other way.

We have seen that living cells die when cut, bruised, or torn; and that the cells next below in a layer of active tissue are stimulated by the exposure to increased growth and division, and at once produce a layer of cork, the impervious walls of which again protect the living cells beneath. This is found to occur in all cell-tissues provided the cells are still living, and it matters not whether the wound occurs in the mesophyll of a leaf, the storage parenchyma of a Potato-tuber, the cortex of a root or stem, or in the fleshy parts of a young fruit, the normal effect of the wound is in all cases to call forth an elongation of the uninjured cells beneath, in a direction at right angles to the plane of the injured surface, which cells then divide by successive walls across their axis of growth: the layers of cells thus cut off are then converted into cork, by the suberisation of their walls. Further changes may then go on beneath the protective layer of wound-cork thus produced, and these changes vary according to the nature of the cells beneath: the cambium forms new wood, the medullary rays similar rays, cortex new cortex, and so on.

_Knife-wounds._--Artificial cuts in stems are easily recognised and soon heal up unless disturbed. Several cases, differing in complexity, are to be distinguished. The simplest is that of a longitudinal, oblique, or horizontal short cut in which the point of the knife severs all the tissues of the stem down to the wood. The first effect usually observed is that the wound gapes, especially if longitudinal, because the cortex, tightly stretched on the wood cylinder, contracts elastically. This exposes the living cortex, phloem and cambium to the air, and such tissues at once behave as already described above: the cells actually cut die, those next below grow out under the released pressure, and these give rise to cells which become cork. As the growth and cell-division continue in the cells below this thin elastic cork-layer, they form a soft herbaceous cus.h.i.+on or _callus_ looking like a thickened lip to each margin of the cut. Each lip soon meets its opposite neighbour, and the wound is closed over, a slight projection with a median axial depression alone appearing on the surface. The depression contains the trapped-in callus-cork squeezed more and more in the plane of the cut as the two lips of callus press one against the other, and sections across the stem and perpendicular to the axis of the cut show that this thin cork, like a bit of brown paper, alone intervenes between the cambium, phloem and cortex respectively of each lip, as each layer attempts to bridge over the interval. If the healing proceeds normally, these layers, each pressing against the trapped cork-film, and growing more and more in thickness, shear the cork-layer and tear its cells asunder, and very soon we find odd cells of the cambium of one lip meeting cambium cells of the other, phloem meeting phloem, and cortex cortex, and the normal thickening of the now fused layers previously separated by the knife goes on as if nothing had happened, the only external sign of the wound being a slight ridge-like elevation, and, internally, traces of the dead cells and cork trapped here and there beneath the ridge. When the conjoined cambium resumes the development of a continuous layer of xylem and phloem, no further trace of the injury is observable, unless a speck of dead cells remains buried beneath the new wood, and indicates the line where the knife point killed the former cambium and scored the surface of the wood in making the wound.

_Stripping._--Now suppose that, instead of a mere slit with the knife-point, a strip of bark is removed down to the wood. Exactly the same processes of corking and lip-like callus formation at the edges of the wound occur, but of course the occlusion of the bared wood-surface by the meeting of the lips occupies a longer time. Moreover, the living cells of the medullary rays exposed by the wound on the wood-surface also grow out under the released pressure, and form protruding callus pads on their own account. In course of time the wood is again completely covered by the coming together over its face of these various strips of callus, but two important points of difference are found, as contrasted with the simpler healing of the slit-wound. In the first place the exposed wood dries and turns brown, or it may even begin to decay if moisture and putrefactive organisms act on it while exposed to the air; and, in the second place, the normal annual layer of wood--or layers, as the case may be--formed by the cambium only extends over that part of the stem where the cambium is still intact, and is entirely wanting over the exposed area. Thus, if it takes two years for the cambium to extend across the wound, a layer of wood will be formed all round the intact part of the stem, from lip to lip of the cut tissues during the first year; then a second annual layer outside this will be formed during the second year, but extending further over the edges of the wound, and nearly complete, because the cambium has now crept further across the wounded surface to meet the opposite lip of cambium; and during the third year, when the cambium has once more become continuous over the face of the wound, the annual wood layer will be complete. But, of course, this last layer covers in the edges of the two previously developed incomplete wood-layers as well as the exposed and brown, dry, or rotten dead face of the wood. It also covers up the trapped-in brown cork and any debris that acc.u.mulated in the wound, and this "blemish," though buried deeper and deeper in the wood during succeeding annual deposits of wood-layers, always remains to remind us of the existence of the wound, the date of which can be fixed at any future time by counting the annual rings developed subsequently to its formation. Obviously, also, the deficiency of wood at this place makes itself visible on the outside by a depression.

_Cuttings._--When a cutting of _Pelargonium_, Willow, or other plant is made, we have a typical knife-wound, the behaviour of which is very instructive in ill.u.s.tration of plant-surgery, and may be most easily seen by keeping it in damp air instead of plunging it into sand or soil.

All the living cells actually cut or bruised turn brown and die as before; those beneath--_e.g._ the living pith, medullary rays, cambium, phloem, and cortex, grow out under the released pressure and form a callus, the outermost layer of which becomes cork, while those below, abundantly supplied with food-materials, proceed to spread, as if flowing over the surface of the cut wood, and rapidly occlude the wound.

Meanwhile new roots are formed advent.i.tiously from the cambium just above the plane of section, and push out through the cortex into the damp air, and if the cutting had been in soil it would now be capable of independent existence. It is important to keep cuttings upright, as the roots only spring from the lower end. Such cuttings can be obtained not only from stems, but also from roots and even leaves.

Callus-formation is not confined to the basal end of a cutting; it has nothing to do with position, but is a reaction to the wound stimuli, independent of light, gravitation, etc. As time goes on, however, the internal organisation of the erect cutting usually reacts on the callus at either end, and roots only rise from the lower one, while shoot-buds may form in the upper one, though it is possible to bring about the formation of buds from the lower end also.

_Branch stumps._--A more complex example is furnished by a branch cut off short some distance--say a foot--from the base, where it springs from the trunk. As before, the immediate effect of the section is the formation of a callus from the cambium, phloem and cortex, which begins to rise as a circular occluding rim round the wood. The transpiration current in the trunk, however, is not deflected into the 12 inches or so of amputated branch, because there are no leaves to draw the water up it, and so the stump dries up and the cortex and cambium die back to the base, leaving the dead wood covered with shrivelled cortical tissues only. This dead stump gradually rots under the action of wet, fungi, and bacteria, and since the pith and heart-wood afford a ready pa.s.sage of the rot-organisms and their products into the heart of the trunk, we find in a few years a mere stump of touch-wood and decayed bark, which falls out at the insertion like a decayed tooth, leaving a rotten hole in the side of the trunk.

If, however, instead of allowing the basal part of the amputated branch to protrude as a stump, we cut it off close to the stem, and shave the section flush with the normal surface of the latter, the callus formed by the cambium, etc., rapidly grows over the surface, and soon forms a layer of cambium continuous with that of the rest of the stem. The wound heals, in fact, much as if it were a strip-wound, and beyond a slight prominence for a year or two no signs are visible from the outside after the occlusion. Of course these matters depend on the relative thickness of branch and stem, and if much wood is exposed the dangers of rot and a resulting hollow in the stem are increased. It is interesting to note how much thicker the callus lips are at the sides of the wound than above and below, owing to differences in the distribution of the nutrient materials.

_Stool-stumps._--When a tree is felled, the stump may, if the section is close to the ground and kept moist, begin to form a thick rim-like callus round the wood, in which advent.i.tious buds soon make their appearance, and grow out into so-called _Stool-shoots_. The products of a.s.similation of these, and the stores acc.u.mulated in the stump, often suffice to feed the callus sufficiently to enable it to grow over and completely occlude the wound, if the wood surface is not too large, or so long exposed that rotting processes have meanwhile set in.

_Ringing._--If the strip of cortical tissues and cambium is removed all round the stem, exposing the wood in a form of a ring, complications may ensue owing to the following circ.u.mstances. A well-marked callus appears at the upper edge of the wound, because, the transpiration current up the young wood not being stopped, plenty of water and salts from the soil can reach the leaves; but the nutritive materials supplied by the latter are acc.u.mulated at the upper lip of the wound owing to the stoppage there of their descent in the phloem, cortex, etc. No such callus-lip appears at the lower margin of the wound owing to want of these supplies. Consequently the occlusion and healing of the ring-wound only takes place from above downwards, and if the ring of cortical tissues removed is a broad one, the healing may be a long process, or may even be indefinitely delayed, a thicker and thicker callus projecting over from above. For similar reasons no annual wood layers are formed below, but only above the wound, and thus the branch or tree may die. The latter contingency is the more likely the further up the tree the ringing takes place, owing to the risk of drying up which threatens the exposed wood, and to the consequent interruption of the transpiration current, and the likelihood that lateral shoots below the wound may divert the water to their own leaves. If the ringing occurs low down on a stem, and the environment remains damp, the upper thick callus may put out new roots; the part above the wound then behaves like a cutting. If the ringing is done on a young and vigorous branch of an old tree, the lower lip may receive supplies from the leaves of branches below the wound, or from shoots which spring from advent.i.tious buds close to it, and the wound may heal over normally. Such healing may be rendered more certain by keeping the wounded surface moist--_e.g._ by means of damp moss, and so encouraging the formation of callus-bridges from the medullary rays.

If on ringing a tree or a branch the young wood is removed as well as the cambium and cortical layers, the death of the parts above the wound is almost certain, owing to the stoppage of the transpiration current: the exceptions to this rule depend simply on the existence of other channels of communication, such as internal phloems, very thick sap-wood, and so forth.

_Bruises._--If a branch or woody stem is struck sharply, with a hammer, for instance, the bruised cortex, phloem and cambium are killed by the blow, and the general effect is as if these tissues had been removed at that spot by the knife, but with the following complications. The bruised cortical tissues rapidly dry as they perish, and may adhere to the wood below. Consequently the still sound parts bordering on the wound are not released from pressure, but, on the contrary, have to advance towards each other over the surface of the wood under still greater pressures, in part due to the tightening of the whole cortex as the dead parts dry and contract, and in part due to the above-mentioned adherence of the latter to the wood. It results from this that such wounds heal very slowly and badly, and when the killed patch at last ruptures, wound-fungi, insects, and other injurious agencies may get in and do irreparable damage, as has been found to occur in cases where such wounds have been made in striking trees to shake down insects, fruit, etc.

NOTES TO CHAPTER XXI.

The essential facts regarding wounds and healing by occlusion are given in Marshall Ward, _Timber and some of its Diseases_, 1889, chapters viii. and ix., and in Laslett, _Timber and Timber Trees_, 1894, chapters iv. and v. More detailed treatment will be found in Frank, _Krankh. d. Pflanzen_, B. 1.

cap. 2, where the special literature is collected. The reader may also consult Hartig, _Diseases of Trees_, Engl. ed. 1894, pp. 225-269.

CHAPTER XXII.

NATURAL WOUNDS.

_Burrows and excavations. Bark-boring--Wood-boring--Wood fungi--Leaf-miners--Pith flecks--Erosions. Skeleton leaves-- Irregular erosions--Shot holes. Frost cracks--Strangulations-- Spiral grooving._