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Description of the chief disease-producing Fungi.

A. Phycomycetes.

The fungi included in this order are lowly organisms (somewhat like the Siphonea among Alge), whose mycelium is usually not septated by transverse walls before the reproductive organs are formed. Reproduction takes place either sexually by oospores or zygospores, or asexually by means of conidia, and the spores are often capable of moving in drops of water.

I. Peronosporaceæ.

This family lives endophytically, and is distinguished by forming oospores. The only fungous disease caused on trees by this family is that originated by Phytophthora omnivora.

1. Phytophthora omnivora, the Beech-seedling Fungus.-This disease chiefly attacks Beech-seedlings, the stalks of which become covered with brown patches, the roots turn black, and the cotyledons and primordial leaves become spotted with brown, then wither, die, and rot quickly during rainy weather. Within about a week of the first signs of the disease its full effect is noticeable when May and June happen to be wet months, while in dry seasons it takes a little longer for the seedlings to assume the characteristic scorched, blackened appearance. It occasionally during warm, damp, spring weather does great damage among seedling crops in natural regeneration of the Beech, and also in seed-beds in nurseries. As, besides being wind-borne, the spores are easily conveyed on the boots and clothing of men, and by horses and carts, dogs, game, and vermin, the disease is often noticeable along paths running through Beech - woods undergoing natural regeneration. Next to Beech, it chiefly attacks seedlings of Ash, Maple, and Sycamore; but it is more or less "omnivorous" when once it has asserted its influence. Sometimes the disease only appears at the tip of the seedling, and in such cases the plant may recover from the attack; but when the rootlets are infected and discoloration appears on the stem, the seedling is doomed. Conifer seedlings are often attacked in nurseries, when whole seed-beds may be destroyed before the germinating seedlings even begin to show above ground.

Life-history.-Infection primarily occurs by the germination of oospores that have been resting in the soil. As the mycelium is formed (at first unseptated, but septated subsequently), it mostly grows intercellularly with small buttonshaped haustoria, and spreads throughout the whole stalk as well as within the cotyledons; while numerous conidia-bearing hyphae either pierce the epidermis or else break through the stomata, and on reaching the outside produce a large, terminal, lemon-shaped conidium. Below this the conidiophore usually throws out a short side-process, at the end of which another conidium is formed, which presses the first to one side. After the detachment of the lemon-shaped conidium the receptacles become prolonged and again form conidia; whilst the older ones drop off and either at once germinate, or scatter in all directions the conidia which they contain. The conidia fall off and germinate during damp weather, either germinating at once or else becoming zoosporangia and releasing a number of 2ciliated spores moving in water (rain), which soon come to rest and throw out a tube-like germinating process. If germination happens to take place on any Beech cotyledon not yet protected by a cuticule, this tube-like germinating process pierces the epidermis and quickly develops into a mycelium which soon produces

conidia and sporangia. Numerous oospores are also subsequently produced (by sexual fructification) inside the leaf, which form resting-spores, falling to the ground with the dead foliage in autumn. These either germinate in the following spring or else remain dormant for several years until conditions arise favouring their germination. This fungus multiplies so rapidly during rainy weather, and in close damp situations, that new sporangia-bearers are formed within three or four days of the first signs of infection. When once this disease obtains a foothold in nurseries or areas being naturally regenerated it may do serious damage, owing to the infectious ovispores being easily conveyed by wind and animals, and to the mycelial filaments spreading below ground and piercing the rootlets of neighbouring seedlings.

Prevention and Extermination.-Infected seedlings should be carefully removed and burned while the disease is still only sporadic. Infected seed-beds should for the next two or three years only be used for bedding transplants, and preferably only for a different kind of tree than the seedlings already attacked. Watering the seed-beds with a solution of 4 lb. of bluestone (copper vitriol) and one quart of ammonia in 50 gallons of water prevents the disease spreading.

B. Ascomycetes.

The fungi forming this order have from the very first a septated mycelium. In the lower, less highly organised genera, club-shaped or tubular sporangia, called asci, are produced directly, while in the genera with more complex organisation they are produced in sporophores. In these asci (which are generally club-like), after repeated subdivisions of the central part, a certain definite number of cup-like ascospores are generally produced, the number varying according to the species of fungus. The whole plasma of the ascus is not generally used for producing spores. The spores are always at first i-celled, but in many species they afterwards become many-celled by septation. They are sometimes scattered by being shot out of the top of the ascus when it opens. In the fruitreceptacles, which are usually formed asexually, the asci produced, and often intermixed with sterile hypha-ends (paraphyses), form a homogeneous layer (hymenium), wholly or partially enclosed within a closely-woven covering (peridia) of filaments.--In addition to this typical fructification, however, reproduction can also take place by means of many different kinds of conidia and chlamydospores.

Until complete knowledge is obtained of the life-history of what are still unclassified fungi (fungi imperfecti), these are meanwhile included among the Ascomycetes, on account of their fructification taking the form of conidia and pycnidia (see footnote, p. 148).

I. Exoascaceæ.

This family consists entirely of parasitic fungi, and those attacking trees are contained within the comprehensive genus Taphrina (including Exoascus). It has no sporophores, but the asci (usually 8-spored) break out in compact masses and large numbers on the upper side of the part of the plant attacked. The mycelium may either live as an annual directly below the cuticle of the leaves attacked, or it may perenniate in buds, twigs, branches, or leading-shoots. In the latter case, during the annual period of active vegetation it develops subcutaneously in the foliage or the fruit a single layer of interwoven hypha, from each of the cells of which an ascus appears.

1. The genus Taphrina (including Exoascus) produces deformities of various kinds, which may be grouped and classified as follows, but which do no serious damage in woodlands :

(a) Those which form tufted twig-clusters (called "Witches' brooms" in Germany), and develop their asci on the leaves,-

(1) T. turgida, on the common Birch, forming large and very thick-branching twigclusters with pendulous twigs and somewhat crinkled leaves covered with grey bloom on the lower side. (2) T. betulina on the pubescent Birch, forming tufted twig-clusters with

grey bloom on the lower side of the leaves. (3) T. carpini on the Hornbeam, often forming very large, thick-tufted, and thickly-foliaged twig-clusters with crinkled leaves, having a grey bloom on their lower side. (4) T. epiphylla on the white Alder, often forming numerous very hypertrophied but sparsely-branched twig-clusters on one tree, and showing the greyish-white film of asci on both sides of the leaves. (5) T. cerasi and T. insitiæ producing twig-clusters on various species of Cherry-trees, and coating the lower surface of the leaves with a grey or greyish-white ascus-film.

(b) Those which cause malformation of the shoots, though at same time forming "Bladder-growths" and producing their asci on the foliage,

(1) T. ulmi, producing spots on Elm-leaves, and bladder-like swellings. (2) T. Janus, producing pale-red bladders on Birch-leaves, with asci on both upper and lower side of leaf. (3) T. Tosquinetii, common on Alder, thickening the shoots and deforming the leaves by large bladders, and coating the foliage with grey-white ascus-film. cratagi on Hawthorn, producing crinkled leaves with red spots. (5) T. celtis on Celtis australis, producing spots on leaves, and slight swellings.

(4). T.

(c) Those which only produce "Bladder-growths," spots on leaves, or smooth ascus-films, without causing malformation of the shoots,

(1) T. betula on Birch, producing white or yellow spots on leaves. (2) T. carnea on the pubescent Birch, producing flesh-coloured bladder-growths. (3) T. cærulescens on Oaks, producing irregular greyish or bluish spots on leaves. (4) T. acericola on Maple and Sycamore, producing spots on leaves. (5) T. Sadebeckii on Alder, producing round yellowish- or greyish-white spots on leaves. (6) T. aurea on Poplars, producing large bladder-growths on the leaves, with a golden-yellow ascus-film on the concave lower side of the leaf. (7) T. bulbata on Pear-trees, producing bladder-growths on the leaves.

(d) Those which cause deformities of the ovary or other parts of the fruit-producing organs,

(1) T. Johansonii on Aspen, and T. rhizophora on White Poplar, which both produce bladder-plums and turn the ovary yellow. (2) T. alni on Alder (chiefly white Alder), causing the cone-bracts to swell into long, curved, red bladders. (3) T. pruni on Cherry and Plum trees ("bladder-plums "), and T. Rostrupiana on Blackthorn.

Prevention and Extermination in all the above cases can only be effected by cutting off and burning the parts infected.

The Mildew Fungi (Erysiphacea) and the Soot Fungi (Perisporiacea) are two families which both belong to the order Ascomycetes, and are characterised by forming fruit-receptacles (Carpoasci). They frequently occur on the foliage of trees, but the damage they do in woodlands is hardly perceptible.

The Mildew-fungi (Erysiphaceœ) all live epiphytically on leaves and young twigs, and extend their haustoria into the cells of the epidermis. Numerous hypha of the mycelial film develop into oval spores (oidia), and the infected parts then look as if covered with mealy dust. The covering of the small sporophores scattered over the mycelium is closed on every side, and often grows into characteristic thread-like appendages. The ascospores are freed when the covering of the fruit-receptacles decomposes and rots.-There are three genera distinguishable according to their fruits, and but few species of any special interest to the forester. Of these three genera (Podosphæra, Uncinula, and Phyllactinia) the chief species are:

(1) Podosphæra oxyacantha, which chiefly attacks Apple-trees, and also Hawthorn and Mountain-Ash.

(2) Uncinula aceris, producing white spots on Maple and Sycamore leaves. Leaves become dwarfed and shrivel up if infected while still developing. 2. U. salicis, producing either white spots or thick films on Willow, Poplar, and Birch leaves. 3. U. Tulasnei on foliage of Willow, Poplar, and Birch, but forming a more regular fungous film over the leaves. 4. U. clandestina on Wych Elm foliage. 5. U. Prunastri on Blackthorn foliage.

(3) Phyllactinia suffulta, producing white spots and films on Beech, Oak, and most other broad-leaved trees, and sometimes causing premature drying of the foliage in Beechwoods.

The Soot-fungus (belonging to the Perisporiaceae), common on the leaves of many trees and shrubs, is caused by the much-septated, thick-walled aerial mycelium of the genus Apiosporium (syn. Capnodium, Fumago). Attacks of black-smut fungi are generally worst when preceded by any marked increase in plant-lice (Aphides), as the fungi then live saprophytically on the "honey-dew" exuded by the lice. Although covered above by the black smut, all the lower side of the leaf remains green; but if the fungus is in large numbers the leaves gradually die. Only two species are to be found on woodland trees,-(1) Apiosporium salicinum on Willows and Poplars; and (2) A. pinophilum on Silver Fir, often covering with black smut the needles and twigs of whole branches.

The fungi remaining as yet unclassified (see p. 148), which are in the meantime included among the Ascomycetes, include several genera and species destructive in nurseries, young plantations, and older woods. These may thus be summarised, following the enumeration given in the table on p. 148:

2. Phoma abietina causes a cankerous disease on Silver Fir twigs and branches. Among the young undergrowth in Silver Fir-woods, branches, twigs, and leading-shoots with dead foliage adhering to them are often found above a broad constricted ring where the bark is burst, and where numerous small black pycnidia appear, whose spores are scattered in August and September. Next spring the infected 1-year-old twigs die without any constriction of the bark, which is only effected in the case of larger branches when growth in girth is continued for one or more years above and below (but chiefly above) the dead ring of bark reaching down to the cambium.

3. Phoma pithya occasions a cankerous disease on twigs and branches of Douglas Fir and Scots Pine, not unlike that produced on Larch by Peziza. It is now, unfortunately, found attacking Douglas Fir plantations in many parts of Britain. The fungus destroys the bark and produces constriction right round the stem, while above and below this contracted part an effort is made to heal the wound by cicatrisation. This failing, death ensues. When the fungus only partly destroys the bark and does not completely ring the stem, a callus is formed round the cankered part. As is also usual in the case of the Larch-canker, the infection usually takes place near where branches grow from the stem. At St Quinox, in Ayrshire, fully 10 per cent of a large number of Douglas Fir recently planted had to be cleared (1897) within three years because of this disease.

4. Pestalozzia Hartigii causes a similar constriction of the periderm on the twigs of almost all kinds of young plants. But this fungus chiefly attacks Spruce and Silver Fir in nursery-lines and young plantations. It also kills them in patches on seed-beds, bending them down just above the ground. Among the broad-leaved trees it is more frequently found on Beech, Ash, Maple, and Sycamore than on the other kinds. The young stem infected can be seen to be more or less constricted close to the ground, and this first of all makes the foliage wither, and then causes the plant to wilt and die. On examining the part where the periderm is constricted a cushion-like stroma will be found with depressions where conidia are emitted. These conidia issue from the epidermis like small black cones.

5. Septoria parasitica is a disease which often causes the death of the leadingshoots of the common Spruce and the Menzies Spruce in seedlings, small plants, and poles up to thirty years old. The young shoots infected at their base begin to bend and hang down in a sickly condition about the end of May or early in June, and soon afterwards wither and die. During the summer the small black pycnidia break through the bark of the dead twigs, being usually most numerous on the needle-cushions, the base of the twig, and the needles at the extreme tip of the shoot. Shoots infected in spring die within one week to a fortnight.

6. Cercospora acerina causes a disease among Maple and Sycamore seedlings, which is often widespread in wet seasons, and much resembles that occasioned by Phytophthora omnivora. The cotyledons, primordial leaves, and stalks become spotted or wholly blackened, then wither. At the infected spots masses of short conidia-bearers appear and produce tufts of long many-celled conidia at their topends, while the intercellular mycelium swells and forms short rows of brown, thread-like, dormant mycelium (the simplest form of sclerotium), which makes the disease break out again in the following spring.

7. Cercospora microsora produces small black patches on the leaves of the Lime, and often occasions premature shedding of the foliage.

8. Other fungi of the above unclassified group include (1) Fusomi pini, which in May and June causes a disease in Pine and Spruce seed-beds, and also among Birch and Alder seedlings, scarcely distinguishable from that due to Phytophthora omnivora. Wet weather greatly favours its spread, as the mycelium then extends far beyond the seedling attacked and attacks neighbouring plants. (2) Allescheria laricis, also in May and June, and especially during damp weather, causes disease in the foliage of young Larch in seed-beds and nursery-lines. The needles become covered with brown spots and die. Thick, short conidia-bearers grow out of the pores, and septate into three or four cells, each of which produces a 1-celled, biscuit-shaped conidia. (3) Brunchorstia pini attacks the shoots of the Corsican Pine, the infection originating in the bark, and by summer spreading to the base of the needles, where the small pycnidia are developed, hidden under the leaf sheath. (4) Glæosporium nervisequium causes an epidemic disease on the leaves, and sometimes also on the young shoots, of Plane-trees. During a wet spring it destroys many of the young leaves just while they are expanding, so that about the middle of May they become brownspotted along the nerves, and later on dry and wither. (5) Fusicladium tremulæ kills the leaves and dries the shoots of the Aspen in spring; and the new shoots in summer are also often attacked. (6) F. dendriticum infects the leaves, twigs, and fruit of Appletrees and Mountain-Ash; and (7) F. pirinum produces brown patches on those of Peartrees, sometimes doing much damage.

C. Pyrenomycetes.

The Pyrenomycetes or fungi having a perithecium (hence the name) have spherical ascus - fruits, and are therefore really only a sub-class of the Ascomycetes. Their hymenium fills the interior of round or bottle-shaped perithecia having a narrow opening at the top; and it is from the base of these that the asci are produced. The ascus fruits may consist of one single perithecium, but several are usually contained in a sporophore of characteristic shape, or in a flat or cushion-like layer (stroma).

I. Hypocreaceæ.

This family is characterised by soft, coloured perithecia united in one stroma. The only genus of importance to the forester is the very destructive canker-fungus Nectria, which produces yellow or red perithecia developing (usually in large patches) on a stroma of the same colour. The asci contain eight 2-celled spores.

1. Nectria ditissima, the Canker of broad-leaved trees, is very common in Britain, and attacks almost all kinds of broad-leaved trees (Oak, Alder, Maple, Sycamore, Lime, Hornbeam, Hazel, also Apple, Pear, and Plum in orchards), but principally Beech and Ash. When these are attacked, the mycelium spreads quickly and enters the wood; and many trees are covered with canker-spots all over the stem and branches (Fig. 172). It is this fungus which causes Ash to become "black in the heart" and valueless as timber. When once the disease gets a firm footing in any Ash plantation it soon becomes epidemic, and whole plantations are sometimes totally ruined, especially when growing on wet undrained land. This fungous disease is dis

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