Hydnum diversidens is a wound-parasite producing ashy-grey stripes in the wood of Oak- and Beech-trees. It has sarcous yellowish-white sporophores about 2 inches broad and 1 inch thick, and either bracket-shaped or else forming incrustations, which are thickly studded on the upper side with prickles about an inch long and covered by the hymenium.-Stereum hirsutum is parasitic on various broad-leaved trees, and produces the peculiar kind of decomposition known as "white- or yellow-piping" in Oak stems, or else sometimes turning it all yellow. After the fungus obtains an entrance at a branch-hole, it spreads peripherally in white zones throughout the stem, so that a transverse section appears dotted over with rows of white points, while the longitudinal section shows numerous white stripes. It produces fawn-brown, rough-haired, leathery sporophores, at first appearing as incrustations and then usually turning cup-shaped, with a sharp yellow edge, and generally a hymenium with orange-red zones.

(d) The genus Agaricus (of the Agaricaceae family, the radial lamella of which are mostly covered with the hymenium).

1. Agaricus melleus, the Common Agaric or Honey Fungus, one of the edible mushrooms very often found growing saprophytically on the dead stools and roots of old trees (especially

Fig. 183.

on Beech), is a common and often very destructive parasite in young coniferous crops (Figs. 183, 184). It especially attacks Scots and Weymouth Pines, but is also found on Spruce, Larch, Douglas Fir, and sometimes also on Austrian and Corsican Pine. It mostly attacks young crops of from 4 to 15 years old, though it may also be found in trees 100 years old. Crowded plantations formed with wisps of three or four seedlings are most liable to bad attacks of the disease. It often breaks out in plantations on deteriorated soil where a coniferous crop has had to succeed a fall of broad-leaved trees, on the roots and stumps of which the mushrooms or "toad-stools " develop saprophytically in large numbers. Such decomposed wood is phosphorescent in the dark, so long as the mycelium is living.

Natural size.

a. Part of a Scots Pine root killed by Agaricus melleus, and showing an external rhizomorph which penetrates the root at a.

b. Flattened internal rhizomorph from between the bark and the dead wood of a Scots Pine.

The first signs of the disease are that the needles of the infected plant, pole, or tree gradually turn yellow, wither, and are shed; then the shoots wither and become stunted, the butt of the stem swells, the bark fissures, resin exudes and flows to the ground, the cambium is killed, and finally the infected poles or trees usually die either in spring (from April till June) or in autumn (October till November). The copious outflow of resin from the diseased roots and base of stem, and the rhizomorphs formed in the rotting wood and pervading the soil round about, show the presence of the disease in an advanced stage, even when no tawny yellow "toad-stools are noticeable.

This disease is characterised by breaking out here and there in patches, and by rapidly killing young plants in full vigour, after they have made good growth during the season. Such damage is easily distinguishable from death. caused by insects, drought, or the like, when the plant first sickens for some time. Young plants attacked usually die during the same year. A microscopic examination will then show that the mycelium pervades the cambial layer and the resin-ducts, which are converted into hollow cavities by disintegration, the starchy cell-matter being transformed into turpentine and causing the morbid resinous outflow. The honey-yellow or dirty yellowishbrown, toughly sarcous receptacles, with darker-coloured brown hairy scales. and yellowish-white lamellæ, which turn flesh-coloured, or on which reddishbrown spots form later-the mushrooms or "toad-stools" (pilei) - appear

Young 8-year-old Scots Pine attacked and killed by the Honey Fungus (Agaricus melleus).

a. Branching subterraneous sterile strands (Rhizomorpha subterranea) thrown out from the mycelium formed under the living bark of the Pine.

b. Abortive sporophores produced at the extremity of a rhizomorphous strand.

C. Normal sporophores produced at the extremity of a rhizomorphous strand.

d. Sporophores produced in a cluster from the bark at the base of the stem of the dead Pine.

The mushrooms

in October, and are most numerous in damp seasons. vary from about 2 to 6 inches in diameter. The pale flesh-coloured stalk of the mushroom shows a yellowish-white ring of skin at the point of rupture below the cap.

Life-history. From the white spores produced during the autumn in large numbers by this fungus when living saprophytically, a fine saprophytic mycelium is formed which produces long, branching, purplish- or brownish-black cordlike strands, called rhizomorpha, which spread singly like roots throughout the soil, as well as in large number between the bark and the wood of the dead stump infected. These rhizomorphous mycelial strands can grow at their points and thus spread the disease below ground, entering the tissues of healthy neighbouring plants or trees wherever any wound-surface is made by the contact of roots, and perhaps also through the unprotected root-hairs and rootlets.

"They can penetrate the roots of the most different kinds of healthy conifers, although this certainly takes place under conditions still requiring closer investigation,

because this fungus is one of the commonest saprophytes on old stumps and roots. According to Hartig, it also infects broad-leaved trees, and especially Maple and Sycamore, as a wound-parasite under certain conditions, and can kill Oak-stools in coppicewoods before they flush their new shoots" (Klein, op. cit., p. 410).

When a subterraneous rhizomorph has obtained entrance to the root of any conifer, numerous mycelial filaments are produced from its apex, and these spread quickly upwards in the woody tissue, especially in the resin-ducts, and kill the wood-parenchym. The mycelium which happens to penetrate under the bark of living roots and stems grows more slowly, and forms thickish, white, ribbon-like strands, producing most of the well-known pilei or mushrooms in autumn, while the characteristic outflow of resin takes place at the base of the stem, and often in young trees causes the formation of hard masses of earth round the roots. Afterwards the mycelium penetrates the sap-conducting tissues, and produces a kind of "white-rot." When once the mycelium has spread from the place of infection on the roots upwards to the base of the stem, and has from there been able to extend throughout the other healthy roots (as also does Fomes annosus), the poles or trees soon wither and die, and the process of decomposition is arrested before the mycelium has time to reach the harder heartwood.

Prevention and Extermination. The best way of preventing this fungus from spreading is to pull up infected plants with all their roots and burn them. Infected spots may also be isolated by completely encircling them with small trenches about 1-1 ft. deep, so as to hinder the subterraneous extension of the rhizomorphs. The saprophytic mushrooms on old stumps should also be collected. When broad-leaved crops are replaced by conifer plantations, the stumps should, if possible, be grubbed up. Where blanks have been made in young conifer crops by this disease, it is best to beat them up with broad-leaved species, as conifers are more liable to be again infected.

2. Agaricus adiposus is (like Polyporus Hartigii) a common wound-parasite on canker-spots of the Silver Fir. It has golden-yellow mushrooms about 2 to 3 inches in diameter, and covered with concentric rows of dark flocky scales, which afterwards disappear. Its mycelial strands penetrate and soon decompose the wood, which turns yellow or honey-coloured and scales off in its annual layers, while they also extend horizontally and radially throughout the wood.

Dry-Rot in Timber used in buildings is occasioned by the "dry-rot" fungus (Merulius lacrymans). Despite modern improvements in ventilation, &c., it appears to be gradually increasing, owing chiefly to the use of immature and imperfectly-seasoned wood, and to the rapidity with which modern houses are built.

Infection with the dry-rot fungus sometimes takes place in the forest, when felled timber remains stored there for some time. This is first indicated by the presence of red stripes in the sawn wood; but if such wood be thoroughly seasoned, the mycelium present in the red stripes is killed. If seasoning be imperfect, the latent mycelium recommences active growth when the wood is used in any part of a building where it is exposed to dampness-e.g., as when the ends of joists are built into a wall.

But the fungus is also often found in old beams and boards in woodyards, &c., and it is mainly from such sources that spores, or portions of the spreading mycelium, are introduced into buildings.

Thorough ventilation is of primary importance; to try and exclude dry-rot by hermetically closing all communication with the outer air in the spaces between flooringboards and joists, and similar places, has proved an utter failure. The best preventive is painting with antinonnin (see p. 539), or with a solution of corrosive sublimate in methylated spirit (6 oz. to 1 gallon). In timber infected, the spreading mycelium can be checked by applying carbolic acid; and when once its presence is detected, all wood-work that can be reached should be thoroughly saturated with this. For further details, see Agricultural Leaflet No. 113 (Dry-Rot).

CHAPTER VI.

PROTECTION AGAINST INJURIOUS INFLUENCES IN SOIL

OR ATMOSPHERE.

I. Protection against Non-Parasitic Diseases of Trees.

Any abnormal disturbance of the organism in a plant which may cause any portion of it to die prematurely is a diseased condition. But this term, of course, does not apply to the normal disturbance in growth due to want of sufficient nourishment, light, or moisture-e.g., as in the suppression of unnecessary branches, by maintaining close canopy overhead.

Any tree may become predisposed to disease, perhaps only temporarily, and not necessarily involving injurious after-effects; but such natural or abnormal periods of predisposition are those when it is most exposed to the danger of diseases due either to parasitic (see chap. v.) or non-parasitic causes. Such predisposition to disease may result from unsuitable soil or situation, from old age, from early or late flushing of foliage in spring, from smoothness of the bark (sunburn), from atmospheric impurities, &c., or from wounds and injuries (leading also to parasitic rusts, rot, and cankers).

In plantations, where the plants are often not indigenous, are more or less all of one age, and are all at nearly equal distances apart from each other, trees grow under conditions less favourable to normal healthy development than are offered in natural woods, where young crops spring up as the old trees die of senile decay. If the soil and situation be such as to satisfy the natural requirements of the trees forming the crop, disease is infrequent. But if, on the contrary, the plants have not been well chosen for the given locality, then the crop, or any individual species forming part of it, may be more or less predisposed to disease in various forms and degrees, according as the soil and situation may vary from the conditions best suited to the kind of tree in question.

Predisposition to disease in woodland trees is more often caused by unsuitability of soil than by anything else. But in Britain, where the woods often consist of many trees from different countries, predisposition to disease also arises from unsuitability of situation and local climate, and becomes more or less pronounced according to the degree of fitness or unfitness of the situation for the given timber-crop. For example, if any tree whose natural habit is to grow in damp sheltered valleys be planted on a dry exposed situation (the soil being similar in both cases), it will soon show symptoms of disturbance more or less pronounced according to the degree of exposure and other conditions; or take any tree that naturally grows at lofty elevations, with pure dry air (e.g., Larch), and

plant it in a low-lying, sheltered, and humid situation (the soil being similar in both cases), and it will probably soon begin to show more or less marked symptoms of predisposition to disease. The best means of preventing disturbances of the organism amounting to disease, or to predisposition towards disease, among woodland crops is therefore to select for each species of tree only such conditions of soil and situation as are known to be favourable to its healthy or normal development, and to imitate nature by forming mixed crops in place of pure woods.

1. Diseases due to unsuitable Soil and Situation are principally noticeable in the stunted growth of pole-crops, or in the crown becoming dry and "stag-headed," or the roots becoming spongy or "dosed," and the stem "pumped" or hollow.

(1) Stunted growth in Young Crops. When the mistake made in planting has been so great that even the young pole-thickets look stunted and backward, there is no remedy but to wait and clear them whenever a suitable opportunity offers, and then replant with other trees more likely to thrive on the given soil and situation.

(2) Stag-headedness, or decay or death of a part or the whole of the crown, may be due to old age or to want of water and nourishment. It is then merely the first stage in the gradual death of the tree. It is the commonest sign of senile decay, and is especially noticeable in park-trees (e.g., the old Oaks in Richmond Park at present). Stag-headedness is also common when Oaks and other trees that have long stood in close canopy are given a larger growing-space and free enjoyment of light, warmth, and air, or when coppicestandards are heavily pruned of lower branches, because, in either case, this stimulates the development of shoots from the adventitious or dormant buds on the stem.

Oaks

In his Text-book of the Diseases of Trees, p. 14, R. Hartig remarks that which have grown up in close canopy along with Beech, and have only a slightly-developed crown of foliage, acquire a predisposition towards a drying-up of the top of the crown (stag-headedness) when they become fully exposed to light and air; while, under similar conditions, trees with well-developed crowns do not suffer from this disease.”

Sudden lowering of the water-level in the soil through drainage is almost certain to interfere with the health of any timber-crop, and in most cases leads to stag-headedness. Beech-woods often become stag-headed long before the trees are mature if the fallen leaves are removed from the soil (to be used as litter) for several years in succession. This affection is easily distinguished from any other by the top and upper branches of the tree becoming quite bare of leaves and young twigs, and assuming the look of stag's horns. Willows and Poplars, which grow best in a rather damp soil, generally become stag-headed when grown on dry soil. Decay of the top of the crown of conifers is almost always soon followed by the death of the tree; but stag-headed Oaks and other broad-leaved trees usually remain alive for many years before finally succumbing.

Prevention.-Any drainage required should be done before planting, and not. after the crop is formed. In old woods nothing should be done to interfere with the soil-moisture, e.g., draining or removing (for litter) the layer of dead foliage on the ground. In open woods of Oak or other light-demanding trees, the soil should