The principal literature, now very voluminous, on this subject is contained in the publications of the U.S. Department of Agriculture from 1890 onwards. See especiallyBulletins, Nos. 3, 6, and 9;Farmers' Bulletin, No. 91, 1899; andThe Journal of Mycologyduring the same period. See also Lodeman,The Spraying of Plants, London, 1896. A summary of the principal processes will be found in Massee,Text-Book of Plant Diseases, pp. 31-47.With regard to the history of the subject, which still needs writing, the reader should not overlook Roberts, "On the Therapeutical Action of Sulphur,"St. George's Hospital Reports, date unknown, but subsequent to the following: Berkeley,Introduction to Cryptogamic Botany, 1857, p. 277, with references. These are, I believe, with the references to steeping of wheat in De Bary,Unters. über d. Brandpilze, Berlin, 1853, among the first attempts to utilise such remedies.Further facts will be found in the pages of theGardeners' Chronicle, especially since 1890, and inZeitsch. f. Pflanzen-krankheitensince 1891.
The principal literature, now very voluminous, on this subject is contained in the publications of the U.S. Department of Agriculture from 1890 onwards. See especiallyBulletins, Nos. 3, 6, and 9;Farmers' Bulletin, No. 91, 1899; andThe Journal of Mycologyduring the same period. See also Lodeman,The Spraying of Plants, London, 1896. A summary of the principal processes will be found in Massee,Text-Book of Plant Diseases, pp. 31-47.
With regard to the history of the subject, which still needs writing, the reader should not overlook Roberts, "On the Therapeutical Action of Sulphur,"St. George's Hospital Reports, date unknown, but subsequent to the following: Berkeley,Introduction to Cryptogamic Botany, 1857, p. 277, with references. These are, I believe, with the references to steeping of wheat in De Bary,Unters. über d. Brandpilze, Berlin, 1853, among the first attempts to utilise such remedies.
Further facts will be found in the pages of theGardeners' Chronicle, especially since 1890, and inZeitsch. f. Pflanzen-krankheitensince 1891.
Predisposition and immunity—Pathological conditions vary—Hardy varieties—"Disease-proof" varieties—Disease dodging—Thick skins—Indian wheats, etc. Cell-contents vary—Citrus, Cinchona, Almonds, etc. Double ideals in selection—Cultivation of pest and host-plant—Variations of fungi—Bacteria—Specialised races—Difficulties—Experiment only will solve the problems.
Predisposition and immunity—Pathological conditions vary—Hardy varieties—"Disease-proof" varieties—Disease dodging—Thick skins—Indian wheats, etc. Cell-contents vary—Citrus, Cinchona, Almonds, etc. Double ideals in selection—Cultivation of pest and host-plant—Variations of fungi—Bacteria—Specialised races—Difficulties—Experiment only will solve the problems.
The numerous and often expensive failures in the application of any prophylactic treatment, have proved an acute stimulus to the research for other ways of combating the ravages of plant diseases. It is a matter of every-day experience that particular varieties of cultivated plants may suffer less from a given disease than others in the same district; also that one and the same species may suffer badly in one country and not in another—e.g.the Larch in the lowlands of Europe as contrasted with the same tree in its Alpine home,and the various species of American Vines in Europe.
These matters, in the hands of astute observers, are turning the attention of cultivators and experts to new aspects of the question of plant diseases, namely, the possible existence of immunity, and the breeding of disease-proof varieties; and the existence on the part of the host plant of predispositions to disease which may depend on some factors in the plant or in the environment over which it is possible to exercise control, or which, if known, can be avoided.
The matter is complicated by the recent demonstration of the fact that parasites also vary and can adapt themselves to altered conditions, as is shown by the history of the coffee-leaf disease (Hemileia) in Ceylon, and by Eriksson's results with Wheat-rusts (Puccinia) and various experiments withColeosporiumand other Uredineae; but there are good grounds for concluding that hybridisation, grafting, and selection of varieties may do much towards the establishment of races which will resist particular diseases, as shown by Millardet's experiments with Vines, and the results obtained by Cobb and others with Wheat.
The great difficulty with so-called "disease-proof varieties" is to test them under similar conditions in different countries, and for a sufficient period of time. A particular race of Wheat may behave very differently in Norfolk, Devonshire, and Northumberland, and the recent introduction of the purely experimental method in this connection isa marked advance. However rough the experiments may of necessity have to be, it is only by such means that data can be gradually accumulated.
Having now obtained some insight into the factors concerned in disease, let us enquire further into the bearing of variation on these. It is evident that pathological conditions may vary; indeed they are themselves symptoms of variation, as we have seen. The history of all our cultivated plants shows abundantly that many of the variations obtained by breeding in our gardens, orchards, fields, etc., involve differences of response on the part of the plant to the very agencies which induce disease. Every year the florists' catalogues offer new "hardy" varieties; but a hardy variety is simply, for our present purpose, one which succumbs less readily to frost, cutting winds, cold damp weather, and so forth. If anyone doubts that hardy varieties have been gradually bred by selection, I refer him to the evidence collected by De Candolle, Darwin, Wallace, Bailey and others. When we come to enquire into the causes of "hardiness," however, difficulties at once beset us. The adaptation may express itself in a difference in the time of flowering or leafing, the exigencies of the season being "dodged," as it were, in a manner which was impossible with the original stock, as appears to have occurred with Peaches in America; or it may be expressed in deeper rooting, as is said to be the case in some Apples, or in the acquirement of a more deciduous habit, or in actually increased resistance to low temperatures.In such cases we cannot trace what alterations have occurred in the cells and tissues concerned, though we may be sure that some changes do occur.
No experienced cultivator doubts that some varieties of Potato, Wheat, Vine, Chrysanthemum, etc., suffer more from epidemic diseases than others, and our yearly catalogues furnish us with plenty of promises of "disease-proof" varieties. Here also we may imagine several ways in which a particular variety may resist or escape the epidemic attacks of fungi which in the same neighbourhood decimate other varieties. If we could breed a variety of the Larch which opened its buds later than the ordinary form in our northern plains, the probability of its escaping the Larch-disease would be increased in proportion to the shortness of the period of tender foliation described onp. 153. It has been claimed for certain varieties of Wheat that increased thickness of the cuticle and fewer stomata per square unit of surface have diminished the risk of infection by Rust fungi, and for certain varieties of Potato, that the thicker periderm of the tuber protects them against fungi in the soil. That certain thick-skinned Apples, Tomatoes, and Plums pack and store better than those with a more tender epidermis seems proved—that is to say, they suffer less from fungi which gain access through bruises and other wounds; but it cannot be said that any convincing proof is yet to hand explaining in detail why some races of wheatresist Rust, or why the roots of American Vines suffer less fromPhylloxerathan others.
One of the most extraordinary cases known to me in this connection is the unconscious selection on the part of native Indian cultivators, perfectly ignorant of the principles involved, of spring and autumn forms of Rice, Wheat, Castor Oil, Sugar Cane, Cotton, and other crops. "It has been estimated that Bengal alone possesses as many as 10,000 recognisable forms of rice." Now there is not the slightest ground for doubt that these have been unconsciously bred from the semi-aquatic native species during the many centuries of Indian agriculture, and nevertheless they have, among other peculiar races, some hill-breeds which they cultivate on dry soils and without direct inundation. That is to say, they possess tropical and temperate races differing far more than our spring and summer wheats.
Something has been gained, then, if we can show that there is nothing absurd or hopeless in the search for disease-proof or resistant races, and I think this can be done. We must not forget that the ideal usually set before himself by a breeder of plants has hitherto been almost exclusively some standard of size, form, colouring, and so forth, of the flower, or of taste and texture of the fruit, tuber, etc., though experiments withCinchona, with brewery yeasts, and other plants remind us that variations in other directions have been attended to also.
Now it is obvious that in breeding sour limesand sweet oranges the cultivator is selecting, and intensifying by selection, very different metabolic processes in the cell: he can test the results of these, and so the selection proceeds.
The question is, Could he select at the same time those variations in cell activity which express themselves in properties of the flower, fruit, foliage, etc., he desires, as well as such variations as aid the cells in repelling fungi, insects, or exigencies of the non-living environment?
That more or less disease-proof varieties could be selected if that object alone were kept in view can hardly be doubted; plenty of examples exist already which show that the necessary variations to work upon exist in just those secretions of protoplasm, etc., which we have seen are concerned in repelling or attracting parasites.
The Sweet Almond has lost the power of producing amygdalin and prussic acid in its cells; Cinchona plants vary immensely in the quantity of quinine formed, and in European hot-houses may even form none at all; some varieties of Maize have sugar and dextrine instead of starch in their endosperms, or coloured instead of clear sap in the aleurone layer, and recent researches prove that they can transmit these peculiarities to hybrid offspring; non-poisonous bacteria have frequently been got from poisonous species simply by cultivation under special conditions, and pigmented forms can be bred into non-pigmented races.
But we see that the difficulty of selection isincreased in the case postulated above, because two ideals are to be worked up to, and they may conceivably be incompatible. Not necessarily so, however, for breeders have solved such problems before in obtaining earlyandheavy cropping races of potatoes, wheat, etc., sweetandlarge grapes, strawberries, etc., hardyandbrilliant flowers, and so forth.
There is, however, another aspect of this question of variability in organisms in this connection to be considered. Ever since cultivation began man has probably been cultivating not only the crops he desires, but also the pests which infest them, and if variation of his chosen plants occurs—and no one will deny that—surely variation of the fungi and insects which live on them also takes place. That this is so can be demonstrated, though, since it is not part of my theme to go into the question of peculiarities of species and races of parasites, the subject must here be passed over with a few remarks only.
Recent researches have shown not only that fungi vary immensely in form and morphological characters according to the amount and kind of food-materials put at their disposal, thus bringing the whole question of polymorphism into the domain of experimental physiology, but that their capacities for infection, spore formation, etc., are also capable of variation and are dependent on the quality and quantity of food supplies, water, as well as on the temperature, illumination,and other factors of the environment. This is true of parasites as well as of saprophytes.Botrytisforms conidia only in darkness and in moist air. Klebahn found that aPucciniagrowing onDigraphisinfectedPolygonatumreadily and completely,Convallariaimperfectly, whereas if sown onMajanthemumit only just infected the plant and then remained sterile, while it refused to infectParisat all. Magnus has shown thatPeronospora parasiticacan only infect meristematic tissues, and that when it co-exists withCystopusonCapsella, as is usually the case, it enters the latter plant by infecting the gall-like pustules of hypertrophied tissue induced by that parasite. Numerous parasitic fungi can only penetrate particular parts of plants. For instance, theUstilagoof wheat can only infect the young seedling, and grows for weeks as a barren mycelium, only becoming a dominant fungus in the endosperm. Numerous other examples could be given, but these suffice to show some of the ways in which the nature of the food substratum supplied by the host affects the fungus. It is obvious that if the nature of this food changes, the fungus is also affected, and no doubt this is the principal reason why Rust-fungi, for instance, vary so much in their vigour and reproductive power on different wheats and grasses, though the other factors of the environment must also be of influence on them as well as on the hosts.
But—and this is the second point—modernresearch is also showing that the various species of Rust-fungi have split up into different varieties or specialised races, according to the particular host plants they inhabit. For instance there are special varieties or races of the particular species known asPuccinia graminis, the wheat rust, each of which grows well on various kinds of grain and grasses but refuses to infect others. Thus, the variety which infects Wheat refuses to infect Barley or Oats, while that variety which grows on Rye will not take on Wheat and so forth. Now it is important to notice that these specialised races are indistinguishable one from another by their visible microscopic characters: they are all botanically of the speciesPuccinia graminiswhich forms its æcida on the Barberry. We must therefore conclude that we have here the same phenomenon as that met with in culture-races of bacteria which, having been fed for several generations on media rich in proteids, refuse to grow on media rich in carbohydrates, or when attenuated races are developed by culture under special conditions.
Now since such physiological races as I have described are by no means confined toPucciniabut are also known inMelampsora,Gymnosporangiumand other fungi, we must conclude from this and from what we know of variation in plants and animals generally, that variation and adaptation are common among parasites, insects as well as fungi.
These considerations will serve to show moreoverthat the question of breeding disease-proof varieties of our cultivated plants is complicated by the danger of our breeding at the same time adapted races of their pests. It appears at first sight extremely improbable that we should escape the danger by breeding from those specimens of our plants which have best survived a fungus epidemic. Still, it must not be forgotten that "hardy varieties," and races adapted to other exigencies of the non-living environment, have been bred by selection—and nevertheless this variable non-living environment is always with us. The matter is therefore simply and solely one of experiment, and the retort that a disease-resisting variety of any particular plant has not yet been raised is no more valid than the objection that a true blue primrose has not yet been obtained: whether the same remark can be made with regard to any hope of adisease-proofplant may be another matter, but in any case it must be made more cautiously in the light of our present experience.
The reader will find more on this subject in Bailey'sSurvival of the Unlikeand the literature quoted in thenotes to Chapter VIII.For varieties of Indian Wheats, etc., see Watt,Agricultural Ledger, Calcutta, 1895.For a discussion on so-called "Disease-proof Wheats" consult Eriksson & Henning,Die Getreideroste.Magnus' paper is in theBerichte der Deutschen bot. Gesellsch., 1894, p. 39.Concerning physiological races and adapted varieties ofPuccinia, etc., see Eriksson, "A General View of the Principal Results of Swedish Research into Grain Rust,"Botanical Gazette, vol. 25, 1898, p. 26.For an account of Wheat-rust see Marshall Ward, "Illustrations of the Structure and Life-history ofPuccinia graminis, etc.,"Ann. of Bot., 1888, Vol. II., p. 215.
The reader will find more on this subject in Bailey'sSurvival of the Unlikeand the literature quoted in thenotes to Chapter VIII.
For varieties of Indian Wheats, etc., see Watt,Agricultural Ledger, Calcutta, 1895.
For a discussion on so-called "Disease-proof Wheats" consult Eriksson & Henning,Die Getreideroste.
Magnus' paper is in theBerichte der Deutschen bot. Gesellsch., 1894, p. 39.
Concerning physiological races and adapted varieties ofPuccinia, etc., see Eriksson, "A General View of the Principal Results of Swedish Research into Grain Rust,"Botanical Gazette, vol. 25, 1898, p. 26.
For an account of Wheat-rust see Marshall Ward, "Illustrations of the Structure and Life-history ofPuccinia graminis, etc.,"Ann. of Bot., 1888, Vol. II., p. 215.
Discolorations—Pallor—Etiolation—Laying of Wheat—Chlorosis—Yellowing—Albinism—Variegation—Uprooting, Exposure and Wilting of seedlings.
Discolorations—Pallor—Etiolation—Laying of Wheat—Chlorosis—Yellowing—Albinism—Variegation—Uprooting, Exposure and Wilting of seedlings.
Everybody knows in a general way when the geraniums in the window pots are drooping from want of water, or when the young Wheat is sickly, or the Pear-trees "blighted," and we have now to see how far we can systematise the knowledge that has been gained in course of time regarding the signs which sick plants exhibit.
Pallor.—Under this heading, which includes all cases where the normal healthy green colour is replaced by a general sickly yellow or pale hue, ultimately resulting in death of the parts if not arrested, we have several totally distinct diseases of the chlorophyll apparatus, each recognised by the co-existence of other subordinate symptoms. The principal varieties of pallor usually met with are the following:
Etiolationis due to insufficient intensity of light, the pale sickly yellow organs being unusually watery and deficient in vascular tissue, the internodes abnormally long and thin, and the leaves generally reduced in size, or, in some plants also "drawn."
Forced Endive, Rhubarb, Asparagus, and earthed Celery afford examples of etiolation purposely induced. The want of light causes the true chlorophyll colouring matter to remain in abeyance, and consequently the plant as a whole suffers from carbohydrate starvation.
Layingof Wheat and other cereals is a particular case of etiolation. The seeds having been sown too thickly, the bases of the haulms, owing to the etiolation and consequent lack of carbohydrates, suffer from want of stiffening tissues, and the top-heavy plants fall over.
False etiolationdepends on a similar abeyance of the chlorophyll, but in this case due to too low a temperature. It is often seen in Wheat and other monocotyledons when the young leaves unfold in cold weather in spring. The symptoms of "drawing" and tenderness are however absent.
Pallor due to too intense illumination must be kept sharply distinct from etiolation, the pale green or yellow hue being here due to the destruction of the chlorophyll by insolation, and the accessory symptoms of "drawing" are wanting.
Chlorosisis a form of pallor where the chlorophyll grains themselves are fully developed, but their green pigment remains in abeyance owing to a deficiency of iron in the soil, and can often becured by adding traces of a ferrous salt. The distinction betweenIcterus, where the organs are only yellow, andChlorosisproper, where they are nearly white cannot always be maintained. In the typical case only those organs whose cells are still young can become green on adding iron.
YellowingorFalse Chlorosismay be experimentally induced by too much carbon-dioxide in the atmosphere. It also often ensues when the roots of plants in the open are waterlogged, owing to the stagnant water not only driving air from the root-hairs but accumulating dissolved substances which poison the plant. Trees frequently thus suffer from "wet feet" when their roots have penetrated down to a sodden impervious subsoil.
Yellowingaccompanied byWiltingis a predominant symptom in most cases where transpiration is more active than root-absorption beyond a certain limit, as is well known in cases of prolonged drought. It may also be caused in evergreens by the foliage transpiring actively in bright January weather, for instance, while the ground is frozen and the chilled root-hairs cannot absorb.
In other cases similar appearances are traceable to insects devouring the roots,e.g.wireworms, and the malady is sometimes enhanced by their accumulations so fouling the wet soil that the roots die off, owing to want of oxygen and to the excess of carbon-dioxide and poisonous matters.
Yellowing may also result from the presence of poisonous or acid gases in the atmosphere or soil, such as chlorine, hydrochloric acid, sulphurousacid, etc., in the neighbourhood of chemical works, or from the escape of coal-gas in streets, etc., points of importance in connection with the use of fungicides and insecticides.
Yellowness is the prevailing symptom in many cases of fungus attack of the roots or collar of the plant, the resulting stoppage of transpiration being also sometimes supplemented by rotting of the roots, and the consequent deprival of oxygen and accumulation of foul gases. In other cases Fungi, and even Bacteria, have been found to have made their way into the principal vessels, the lumina of which they stop up, thus reducing the transpiration current.
Certain insects may also induce a general yellowing and wilting of plants by entering or destroying the tissues concerned in the transpiration—e.g.Oniscus, the Frit Fly, andCecidomya, the Hessian Fly, which attack young winter wheat within the sheaths and cause the plants to turn yellow and wilt.
AlbinismandVariegationare apparently due to causes totally different from any yet mentioned. Church's analyses have shown that albino leaves contain more water and less organic matter than green ones of the same plants, but not necessarily less ash constituents. The composition of the ash points to there being more potash and less lime in the white organs than in the green ones, and, speaking generally, the former are related to the latter much as young leaves are related to mature ones.
The whole matter is complicated by the behaviour of certainvariegatedplants—e.g.Ribbon grass,Calla,Abutilon, which are usually regarded as partial albinos.
Meyen showed long ago that such variegated plants, if grafted on green ones, may induce the development of variegated leaves on both scion and stock, and Morren and others have not only confirmed this but have also shown that variegation may be inherited through the seed. Nevertheless some care has to be taken with many of these variegations lest rich soil, bright light, and other favourable treatment favour the restitution of the green colour. These facts may be interpreted in various ways. Some disturbance of physiological functions of the roots, due to unfavourable conditions of soil, may be the cause; but Beijerinck has lately published some results which show that some of these albino diseases can be induced by inoculating normal plants with the juice of spotted ones even though such juice has been filtered through porcelain, and concludes that a "contagium fluidum vivum" of the nature of a transmissible enzyme is the agent which disturbs the physiology of the infected cells.
Koning, while confirming these results in the main, refers them to a micro-organism so small that it traverses the porcelain filter.
Upheaval of seedlings.—This is a common form of injury, resulting in death by drought and exposure, especially in seedling pines, wheat, etc., in soils exposed to alternate freezing and thawingduring spring when there is no snow to protect the plants. The soil freezes during the night, and during the thaw next day water accumulates just below the surface. The freezing is then repeated, and, partly owing to the expansion of the forming ice and partly to the mechanical effect of the ice-crystals in the interstices, the surface of the soil is lifted and draws the roots with it. During the succeeding thaw the soil particles fall away from the lifted root-fibres, and frequent repetition of these processes results in such complete exposure of the roots to the full sun that the plantlet falls over and wilts.
Exposure of rootsis also sometimes effected by winds displacing sandy soils liable to shifting in dry weather, and the resulting wilting of the plants thus exposed at their roots may be supplemented by damage due to the repeated impact of the wind-driven sharp grains of sand, which act like a sand-blast and erode the tissues.
In many of the cases given above the principal result is the weakening or destruction of the chlorophyll action. This means a loss of carbohydrates—sugars, starches, etc.—and in so far a starvation of the plant. The injurious effects are quantitative and cumulative: if large areas of foliage are concerned, or if the effect lasts a long time, the plant suffers from loss of food, and may die. In those cases where the effect is due to the cutting off of supplies at the roots, and where the yellowing is a secondary symptom, the disease is more general in character, and recovery is oftenimpossible, because the loss of water cannot be compensated, and the results may be further complicated by the gradual penetration of poisonous matter into the cells. It is frequently necessary, though sometimes very difficult, to decide which is the primary and which secondary (or tertiary, etc.) symptoms in the order of their importance, and the diagnosis may be complicated by a number of accessory factors which it is impossible to treat generally.
The principal cases here described are dealt with in works on plant physiology, and in the works of Sorauer and Frank already referred to.As regards damage due to uprooting of seedlings by frost, see Fisher, "Forest Protection" (Engl. ed. of Hess'Forstchutz), in Schlich'sManual of Forestry, Vol. IV., 1895, pp. 439-442.On Albinism, see Church, "A Chemical Study of Vegetable Albinism,"Journ. Chem. Soc., 1879, 1880, 1886.Beijerinck's results are contained in his paper, "Ueber ein Contagium vivum fluidum," etc. (with English abstract), inVerhandl. d. Kon. Akad. v. Wetensch, te Amsterdam, 1898. Koning's paper is inZeitschr. f. Pflanzenkrank., Vol. IX., 1899, p. 65. See alsoNature, Oct. 11, 1900, p. 576.
The principal cases here described are dealt with in works on plant physiology, and in the works of Sorauer and Frank already referred to.
As regards damage due to uprooting of seedlings by frost, see Fisher, "Forest Protection" (Engl. ed. of Hess'Forstchutz), in Schlich'sManual of Forestry, Vol. IV., 1895, pp. 439-442.
On Albinism, see Church, "A Chemical Study of Vegetable Albinism,"Journ. Chem. Soc., 1879, 1880, 1886.
Beijerinck's results are contained in his paper, "Ueber ein Contagium vivum fluidum," etc. (with English abstract), inVerhandl. d. Kon. Akad. v. Wetensch, te Amsterdam, 1898. Koning's paper is inZeitschr. f. Pflanzenkrank., Vol. IX., 1899, p. 65. See alsoNature, Oct. 11, 1900, p. 576.
Spotted leaves—The colours of spots—White, yellow, brown, and black spots on leaves—Parti-coloured spots—The browning, etc., of leaves.
Spotted leaves—The colours of spots—White, yellow, brown, and black spots on leaves—Parti-coloured spots—The browning, etc., of leaves.
Discoloured spotsor patches on the herbaceous parts of plants, especially leaves, furnish the prominent symptoms in a large class of diseases, due to many different causes, and although we cannot maintain this group of symptoms sharply apart from the last, as seen from the considerations onalbinism, it is often well marked and of great diagnostic value. By far the greater number of spot-diseases are due to fungi, but this is by no means always the case. The most generally useful method of subdividing the classes, though artificial like all such classifications, will be according to the colour of the spots or flecks, which, moreover, are usually found on the leaves. It is necessary to note, however, that various conditions may modify the colour of spots on leaves. Manyfungi, for instance, induce different coloured spots according to the age of the leaf or other organ attacked, or according to the species of host, the weather, etc. Moreover the spots due to these parasites are frequently yellow when young and some other colour, especially brown or black, when older.
Scaleis the name given to the characteristic shield-like insects (Mytilaspis,Aspidiotus, etc.) which attach themselves to branches of Apples, Pears, Oranges, Camellias, and numerous other plants, and suck the juices. It is the female insect which has the body broadened out into the "scale," under which the young are brought up. Enormous damage has been done by some forms—e.g.the San José scale in the United States.
The superficial resemblances of the patches of eggs of some Lepidoptera to Aecidia and other fungi may be noted in passing—e.g.Bombyx neustriaon Apple twigs,Aporia Crataegi.
Whiteorgreyish spotsare the common symptom marking the presence of many Peronosporeae and Erysipheae in or on leaves,e.g.Peronospora Trifoliorum,P. parasiticaon Crucifers, etc., andSphaerothecaon Hops; alsoSeptoria piricola,Cystopus,Entyloma Ranunculi, etc.
White spots are also caused by insects such asTetranychus(red spider) on Clover and other plants.
Yellow, orOrange-coloured Spots. In cases where these occur on leaves, and in the case of grasses, etc., on the leaf sheaths as well, theycommonly indicate the presence of Uredineae, and sections under the microscope will show the mycelium in the tissues beneath. Species ofUromyces,Puccinia, etc., in the Uredo state have the spots powdery with spores;Aecidiashow the characteristic "cluster cups," and so forth. These spots are often slightly pustular, and in some cases markedly so.
Other fungi also induce yellow spots on leaves—e.g.Phyllostictaon Beans,Exoascuson Poplars,Clasterosporiumon Apricot leaves,Synchytrium SuccisaeonCentaurea, etc.
Yellow spots are also a frequent symptom of the presence of Aphides, of Red Spider, etc. Thus the minute golden yellow spots sometimes crowded on Oak leaves are due toPhylloxerapunctures.
Yellow patches are formed on the large leaves ofArisarumby a species of parasitic Alga,Phyllosiphon, which lives in the mesophyll. Many tropical leaves are spotted yellow by epiphytic Algae—e.g.Cephaleuros.
It must be noticed that many fungi produce yellow spots or flecks in the earlier stages, which turn brown or black as the fructifications appear,e.g.Dilophia graminis,Rhytisma acerinum.
The yellow-spotted leaves ofFarfugium grande(Senecio Kaempferi) are so like those ofPetasitesattacked withAecidiumin its early stages, that an expert might be deceived until the microscopic analysis was completed.
Red spots, varying from rusty or foxy red to bright crimson, are the symptomatic accompanimentof several fungi, the former often characterising the teleutospore or aecidium stage of Uredineae—e.g.Aecidium Grossulariae—the latter sometimes indicating the presence of Chytridiaceae.
Red spots are also caused byGloeosporium Fragariaeon Strawberry leaves,Polystigma rubrumon Plums.
Crimson spots on Apple and Pear leaves are also due toPhytoptus: they turn brown later.
Brown spotsor 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 byPeronospora viticola,Sphaerella vitisand 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.Fusicladiumon Apples and Pears, and the pycnidial and ascus stages of many Ascomycetes—e.g.Phyllachora graminis. The teleutospore stages of species ofPuccinia,Phragmidium, etc., are also so deep in colour as to appear almost black.
Scabon Pears is due to the presence ofFusicladium, 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 grasses and sedges are causedby Ustilagineae, and are commonest in the grain, the soot-like powdery spores (Smut) being very characteristic.Ustilago longissimainduces 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.Cercosporaon Beans,Ascochytaon Peas.
Brown spots with bright red margins are formed in young Beans byGloeosporium.
Species ofFumago,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 neroof 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 grasses, 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 similarcolours, 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 ofThrips.
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 Fabaeon Beans,U. Pision 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.
Burningis 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 leavesare 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 turnred-brown—copper-coloured—as they dry; the damage is due toTetranychus, 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 glass 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 glass, and extends throughout the leaf-tissues. Young grapes, etc., may also be injured in this way. Water-drops on the glass 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 principal effect is generally the disturbance of chlorophyll action in the spots or flecks onthe 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 quantitative and cumulative, 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.
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 Massee,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, andJourn. Linn. Soc., Vol. XIX., 1882, p. 299.The question of "Sun-spots" has been dealt with by Jönnson inZeitschr. f. Pflanzenkrankh., 1892, p. 358.
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 Massee,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, andJourn. Linn. Soc., Vol. XIX., 1882, p. 299.
The question of "Sun-spots" has been dealt with by Jönnson inZeitschr. f. Pflanzenkrankh., 1892, p. 358.
The nature of wounds and of healing processes—Knife wounds—Simple cuts—Stripping—Cuttings—Branch-stumps and pruning—Stool-stumps—Ringing—Bruises.
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 assimilative 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 onceproduce 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 cushion orcalluslooking 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 developmentof 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 extendacross 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 débris that accumulated 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 ofPelargonium, Willow, or other plant is made, we have a typical knife-wound, the behaviour of which is very instructive in illustration 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 adventitiously 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 thesection 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 passage 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 thesides 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 adventitious buds soon make their appearance, and grow out into so-calledStool-shoots. The products of assimilation of these, and the stores accumulated 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 circumstances. 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 accumulated 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 beindefinitely 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 adventitious 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.