Part 13 (1/2)

_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.