Part 12 (2/2)

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 s.h.i.+eld-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.

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