CHAPTER XIX.

SYMPTOMS OF DISEASE.

_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 princ.i.p.al varieties of pallor usually met with are the following:

_Etiolation_ is 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.

_Laying_ of 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 etiolation_ depends 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.

_Chlorosis_ is 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 be cured by adding traces of a ferrous salt. The distinction between _Icterus_, where the organs are only yellow, and _Chlorosis_ proper, 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.

_Yellowing_ or _False Chlorosis_ may 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 acc.u.mulating dissolved substances which poison the plant. Trees frequently thus suffer from "wet feet" when their roots have penetrated down to a sodden impervious subsoil.

_Yellowing_ accompanied by _Wilting_ is 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 acc.u.mulations 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, sulphurous acid, 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 acc.u.mulation of foul gases. In other cases Fungi, and even Bacteria, have been found to have made their way into the princ.i.p.al 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, and _Cecidomya_, the Hessian Fly, which attack young winter wheat within the sheaths and cause the plants to turn yellow and wilt.

_Albinism_ and _Variegation_ are apparently due to causes totally different from any yet mentioned. Church's a.n.a.lyses have shown that albino leaves contain more water and less organic matter than green ones of the same plants, but not necessarily less ash const.i.tuents. 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 certain _variegated_ plants--_e.g._ Ribbon gra.s.s, _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 rest.i.tution 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 thawing during spring when there is no snow to protect the plants. The soil freezes during the night, and during the thaw next day water acc.u.mulates 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 repet.i.tion of these processes results in such complete exposure of the roots to the full sun that the plantlet falls over and wilts.

_Exposure of roots_ is 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 princ.i.p.al 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 quant.i.tative and c.u.mulative: 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 often impossible, 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.

NOTES TO CHAPTER XIX.

The princ.i.p.al 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's _Manual 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), in _Verhandl. d. Kon. Akad. v. Wetensch, te Amsterdam_, 1898.

Koning's paper is in _Zeitschr. f. Pflanzenkrank._, Vol. IX., 1899, p. 65. See also _Nature_, Oct. 11, 1900, p. 576.

CHAPTER XX.

SYMPTOMS OF DISEASE (_Continued_).

_Spotted leaves--The colours of spots--White, yellow, brown, and black spots on leaves--Parti-coloured spots--The browning, etc., of leaves._

_Discoloured spots_ or patches on the herbaceous parts of plants, especially leaves, furnish the prominent symptoms in a large cla.s.s 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 on _albinism_, 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 cla.s.ses, though artificial like all such cla.s.sifications, 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. Many fungi, 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.

_Scale_ is 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 Jose 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 pa.s.sing--_e.g._ _Bombyx neustria_ on Apple twigs, _Aporia Crataegi_.

_White_ or _greyish spots_ are the common symptom marking the presence of many Peronosporeae and Erysipheae in or on leaves, _e.g._ _Peronospora Trifoliorum_, _P. parasitica_ on Crucifers, etc., and _Sphaerotheca_ on Hops; also _Septoria piricola_, _Cystopus_, _Entyloma Ranunculi_, etc.

White spots are also caused by insects such as _Tetranychus_ (red spider) on Clover and other plants.

_Yellow_, or _Orange-coloured Spots_. In cases where these occur on leaves, and in the case of gra.s.ses, etc., on the leaf sheaths as well, they commonly indicate the presence of Uredineae, and sections under the microscope will show the mycelium in the tissues beneath. Species of _Uromyces_, _Puccinia_, etc., in the Uredo state have the spots powdery with spores; _Aecidia_ show the characteristic "cl.u.s.ter 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._ _Phyllosticta_ on Beans, _Exoascus_ on Poplars, _Clasterosporium_ on Apricot leaves, _Synchytrium Succisae_ on _Centaurea_, 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 to _Phylloxera_ punctures.

Yellow patches are formed on the large leaves of _Arisarum_ by 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 of _Farfugium grande_ (_Senecio Kaempferi_) are so like those of _Petasites_ attacked with _Aecidium_ in its early stages, that an expert might be deceived until the microscopic a.n.a.lysis was completed.

_Red spots_, varying from rusty or foxy red to bright crimson, are the symptomatic accompaniment of 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 by _Gloeosporium Fragariae_ on Strawberry leaves, _Polystigma rubrum_ on Plums.

Crimson spots on Apple and Pear leaves are also due to _Phytoptus_: they turn brown later.