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 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 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 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. 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 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 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 grasses, 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 "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._ _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 analysis 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.

_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 grasses and sedges are caused by Ustilagineae, and are commonest in the grain, the soot-like powdery spores (Smut) 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 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 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 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 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 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.

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 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, and _Journ. Linn. Soc._, Vol. XIX., 1882, p. 299.

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