The fungal diseases of the common larch

Front matter

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Preface

Tuts book arose from an investigation which Sir William Schlich asked me to make into the larch canker, a disease on which many articles and papers have been published, though no full and connected account of it has ever been written in English. While studying the canker in the field, I very frequently encountered certain other diseases on the larch, especially the heart-rot and the honey fungus, and it seemed better to include these in the scope of the investigation. For the sake of completeness, all the known diseases on the common larch have also been described, and it is hoped that a book of more general usefulness has been thereby produced. Certain fungi, such as Lophodermium laricis and Botrytis cinerea, which have occasionally been reported as parasitic on the larch, have been omitted, as no reliable evidence of their causing disease has been forthcoming. . :

The arrangement of the diseases bears no relation to the systematic position of the fungi. They are mostly given in the order in which they were studied, which approximates closely to the order of their importance.

The work was made possible by a grant from the Development Commissioners, received through the Board of Agriculture and Fisheries, and latterly through the Interim Forest Authority. Especial acknowledgement is due to these departments, particularly to the last named, which has assisted me in a variety of ways. I also desire to thank Sir William Schlich for constant help and encouragement, and Messrs. A. W. Borthwick and Malcolm Wilson for permission to reproduce Fig. 72 from the Transactions of the Royal Scottish Arboricultural Society.

OxForD, August 1919.

Contents

CHAP, i

Introduction

The relation of a fungus to its host. Life-history of a fungus. The morphology of the larch ; long and dwarf shoots ; internal structure of the stem : ; ;

THE LARCH CANKER.

General. Historical. The mycelium of Dasyscypha calycina and its effect on the tissues. The canker as a pathological structure

THE LARCH CANKER (continued),

The reproductive organs of the fungus. Germination of spores. Pure cultures on nutrient media, Artificial infection with canker

{V. THE LARCH CANKER (concluded).

On the mode of infection in nature. Importance of wounds as a source of canker. Contributory causes of canker. Methods of prevention. The synonymy of Dasyscypha calycina

HEART-ROT. Fomes annosus.

Various fungi which cause heart-rot. Fomes annosus: general ; historical. Secretions induced by Fomes annosus: turpentine and resin ; soluble gum ; insoluble gum. Decomposition of the wood .

VI. HEART-ROT. Fomes annosus (concluded).

Reproductive organs: fructifications; conidiophores. Pure cultures on artificial media. Cultures on natural media. Infection experiments. Mode of attack in nature. The frequency of heart-rot in plantations which form the first rotation on cultivated soil. Methods of prevention

Vil. HEART-ROT CAUSED BY OTHER FUNGI.

Polyporus Schweinitzii, Poria vaporaria, Polyporus sulphureus, Trametes Pint. . ° .

PAGE ii CONTENTS CHAP. PAGE

VII. ARMILLARIA MELLEA, THE HONEY FUNGUS.

General. Microscopic details of the fructification. Rhizomorphs. Effect on the host. The black line and resin flow. The method of infection. Means of prevention . eae fre) eS.

IX. LEAF AND SEEDLING DISEASES.

The larch needle-cast (Sphaerella laricina). Meria laricis. Hypodermella laricis. The larch needle-rusts: Melampsoridium betulinum, Melampsora tremulae, &c. Damping-off diseases :

Phytophthora omnivora ; Fusoma parasiticum : ; . 168 X. GENERAL SUMMARY . ae i apee sake es 67 BIBLIOGRAPELY oo .°oo 02 Saeco ak ae eo

PND EX oe 8 ee

LIST OF ILLUSTRATIONS

FIG. 1. Section through canker ; Frontispiece 2. Transverse section of one-year-old stem of larch ‘ . page 8 3 33 », three-year-old stem of larch . facing page 8 4 - > 3 abnormal wood hee Of 8 5. Method of formation of periderm . ; : ; . page 10 6. Longitudinal section of phloem Ve 11 7. Transverse section of phloem ‘ é : a en 13 8. Canker healed on both sides . : ' ' . facing page 18 9. Canker which has nearly girdled stem : a os 18 10. Mycelium of Dasyscypha in wood of larch ; = - page 27 11. Radial longitudinal section of abnormal wood . ae a2 12. Stages in the development of a canker ; pers 34 13. Canker on young larch showing swelling at back . facing page 36 14. Canker showing swelling at sides f Ry 36 15. Apothecia of Dasyscypha-calycina on dead tar stem. Dry weather J : : F Watts -, of 16. Apothecia of Dasyscypha calycina on dead larch stem. | Wet weather pce mi 37 17. Section through Sines of Daeachoks Pee aoe ; : page 38 18. Dasyscypha calycina, reproductive organs a 40 19. Ascus which has discharged its spores a 41 20. Spores and germination. a = 43 21. Growth of mycelium in drop cultures Py 45 22. Pure culture of Dasyscypha calycina on nutrient gelatine . : d ’ 4 : : . facing page 48 23. Pure culture on sterilized larch stems showing apothecium es ae .. 48 24, Canker showing origin from branch ‘ A es 56 25. < Us i ay Subsequently healed on one side , ; s : aie re 56 26. Cork layer across base of branch : : . page 60 27. Canker fungus spreading down froma branch . . facing page 61

28. Spread of fungus down the branches

29 92 x LIST OF ILLUSTRATIONS

FIG. 29. Single fructification of Fomes annosus. Upper surface facing page

30. 5 ” $3 45 Lower surface ,, he

31. Fomes annosus. Annular rot with a central unrotted core : : : 5 : } : ee 3 32. Fomes annosus. Figured rot : : ; og pee 33. Black specks in wood of larch caused by Fomesannosus ._ page - 34. Transverse section of black speck in wood of larch . er 35. Longitudinal section near edge of black speck . : Sutra 36. Fomes annosus. Figured arrangement of rot . . facing page Div whe * Base of larch trunk excavated underneath é : : wet be fe eae ae Fructification showing pores in longitudinal section . ; ree 3 3) aes ;; Under surface of fructification showing mouths of pores. eer a

40. Upper surface of fructification showing concentric furrows and white margin . is : 41. Fomesannosus . ye : ; ; ; . page 42. Diagram showing necessity for pores to be vertical ¥s 43. Fomes annosus. Conidiophore and conidia 3 44,45. Cultures of Fomes annosus 2 : : . facing page 46. Polyporus Schweinitzii. Stipitate fructification ee “a 47. 3 3 Bracket-shaped fructification ,, 3 48. Hymenial layer of Polyporus Schweinitzir : : . page

49, 50. Sections of larch-tree showing rot caused by

Polyporus Schweinitzvi : : ‘ . facing page 51. Piece of larch wood rotted by Polyporus Schweinitzii ,, Ff 52. Section of Scots pine stem with rot caused by Poly-

porus Schweinitzit : 4 ; : a 53. Mycelium of Polyporus Schweinitzii in wood of larch . page 54. Section of larch wood after attack by Polyporus Schweinitzir 55. Poria vaporaria

56, 57. Sections of larch wood rotted e ines Pini . facing page 58, 59. Armillaria mellea. Group of toadstools growing

from base of deodar . ous > 60, 61. Ms » Loadstools growing from rhizomorphs in soil : °. 989 ”

85 85

92 92 94 96 97 99

101 102

105 108 127 127 128

128 130

130 13] 132 136 143

145,

LIST OF ILLUSTRATIONS

FIG.

62. Hymenial layer of Armillaria mellea

63. Rhizomorphs of Armillaria mellea between bark and wood of larch stem . facing page 64. Small portion of Fig. 63 enlarged ; + ee A 65. Section of larch stem attacked by Armillaria mellea . ,, a 66. Hollow larch stem rotted by Polyporus sulphureus + }; 67. Armillaria mellea. Radial longitudinal section of larch wood showing ‘ black line’ . page 68. hn ,» Transverse section of larch wood showing ‘ black line ’ os 69, 70. Living larch-trees with honey fungus growing from their roots 3 : . facing page 71. Diagram showing life-cycle of heteroeciousrust . . page 72. Melampsoridium betulinum . facing page

. Sporangiophore of Phytophthora omnivora and conidiophore of Fusoma parasiticum . . page

. page )

xl

Chapter I Introduction

The relation of a fungus to its host. Life-history of a fungus. ‘The morphology of the larch; long and dwarf shoots; internal structure of the stem.

Historical. The history of larch-growing in Britain presents features of peculiar interest. In no other tree have such high hopes been placed, hopes which too often have led to disappointment, and with no other tree has it been so necessary to modify the method of sylvicultural treatment on account of liability to disease.

The story is as follows. The larch, which is a native of the Alps, Carpathians, and a part of southern Russia, was introduced in the early part of the seventeenth century. It was then only grown for decorative purposes, and a hundred years elapsed before the larch was employed as a timberproducing species. From 1730 onwards, however, it was planted extensively in Scotland, particularly by three successive Dukes of Atholl on their estates of Atholl and Dunkeld, and the faith of its ducal sponsors proved to be so well founded that the tree waxed great in popularity, and before the close of the century large plantations might have been found on the south as well as on the north side of the Tweed.

In 1820 a frigate was built for the navy of Scotch larch to test the durability of the wood under conditions of exposure. She was appropriately called the Athol, after the duke who grew the timber, and for the sake of comparison the Nieman, also a ship of war, was built at about the same time of Baltic fir (i.e. Scots pine). According to Laslett, who wrote in 1875, when he was Inspector of

Timber to the Admiralty, the former lasted for a very 1888 B long time without any considerable repairs, whilst the other decayed very rapidly, and from this comparison the superiority of Scotch larch over Scots pine, for durability, was considered to be pretty well established. If Scotland could grow larch timber which was more durable than Baltic fir, there was no reason to despair of British forestry, and the reputation of larch, already great, was still further enhanced.

So foresters continued to plant larch wherever the site was suitable, and in a great many places where it was not. It was used for railway sleepers and other outdoor purposes, _and further experience confirmed the belief in its capacity for bearing exposure ; and it must have been at this time, about 1840 to 1850, that its popularity reached the height from which it subsequently and continuously declined. It was fast-growing, straight, and easy to handle, but it had one great weakness : it became extremely liable to disease. A-book on the larch, written by Mackintosh in 1860, is a sufficient index of the change that had come over the opinion of foresters, and nearly all writings since this date -have partaken of the nature of apologetics for a tree that had failed to fulfil its early promise.

The disease that Mackintosh was most concerned with was variously known as heart-rot, dry-rot, piping, pumping, and internal decay, and was believed to be caused by a fungus which they called Polyporus destructor. Exactly what disease this was is not clear, but it was no doubt one of the heart-rots as we know them to-day, and very likely Fomes annosus was alone responsible for it. In addition to this, larch canker was becoming frequent, and in time proved to be an even more serious trouble than heart-rot, or, at any rate, one that was more visible to the eye and consequently more feared. At the present day there is scarcely a young plantation anywhere in the country which is entirely free from canker, and felling all too frequently discloses the presence of heart-rot as well. So larch is no longer the favourite tree that it was, and the tendency is to use it less often in pure plantations, though it is still very frequently employed in admixture with other trees since canker is less to be feared in mixtures than in pure stands. So far is this true that in Germany larch now scarcely appears at all, except in mixed forest.

The relation of fungus to host. What is the cause of disease ? Why, when the disease has once appeared, does it spread and become more and more general? It is the object of this book to answer these questions ; but before going into the details of various forms of disease, it will add to the clearness of subsequent descriptions to explain a few of the fundamental ideas on which all our knowledge of | tree diseases is based ; and as all the diseases described in this book are caused by fungi, the peculiar nature of fungal nutrition and the parasitism which it often involves should be made clear at the outset.

The normal green plant is self-supporting in a sense which is not applicable to other organisms, even to the higher animals. That is to say, it can obtain all its food without the intervention of any other living being. Water and mineral salts it obtains from the soil through its roots, and carbon-compounds it manufactures from the carbon dioxide of the air which diffuses in through the pores of the leaves. Combining the carbon dioxide with water and giving out part of the oxygen, the plant forms carbohydrates, and subsequently more complex organic compounds. But this process of carbon assimilation can only be performed by those parts of plants which contain the green colouring matter, chlorophyll, and, even so, only in the light. Such plants are called autotrophic, or self-feeding.

In the course of evolution some plants have taken to living on carbon compounds which have already been elaborated ; that is to say, these plants have become either parasitic on other living plants or animals, or else saprophytic on the remains of such plants or animals. Such plants always tend to lose their chlorophyll, and are called heterotrophic. Some of the higher plants are heterotrophic, such as the bird’s-nest orchis, but the phenomenon is most commonly seen in the large group of lower plants known as the Fungi. In no member of this group has chlorophyll!

B 2 been found, and the green colour seen in some fungi is due to another pigment. Thus all fungi have to live either parasitically or saprophytically, and their mode of life has become entirely specialized to assist them in this kind of existence. Though the distinction between parasites and saprophytes is not always easy to draw, there is a very marked difference between the two modes of life, at any rate in the extreme cases, and in pathology, which is the science of disease, we shall naturally be chiefly interested in parasites. But, as will be shown later, saprophytes must also claim our attention for the reason that they can sometimes live on the dead parts of a living tree, such as the heart-wood of the trunk, and thereby not only destroy the timber but weaken the tree and render it more easily blown by the wind.

Parasites, again, are not all of the same kind. Some can thrive only on living organisms, and are called obligate parasites, and others can grow either on living organisms or on dead ones, and are therefore called facultative parasites. An example of the former is any one of the rusts such as Peridermium and Coeoma on larch needles, and of the latter the canker fungus or the heart-rot fungus which are to be described. The common mushroom will serve as an instance of a saprophyte, as it lives on the organic remains in the soil.

Life-history of a fungus. The part of the fungus which absorbs food from whatever it is growing on is called the mycelium, and is composed of a more or less felted mass of fine threads or hyphae. Each hypha grows at its extremity and may also branch, giving rise to numerous other hyphae as shown in fig. 21. It contains protoplasm and nuclei, and certain spaces in the protoplasm, known as vacuoles, filled with an aqueous solution, besides drops of oil and other food reserves. The mycelium is formed of these hyphae growing and becoming intermingled, and, though a single hypha is too fine to be seen with the naked eye, the mycelium as a whole may be very conspicuous, and is often seen as a white felt-like mass on pulling away the bark of a rotten

Ed stump. The mycelium may itself become reproductive in various ways, but, in principle, it is the vegetative part of the fungus. The true reproductive part is generally very specialized, and in nearly all the fungi considered in this book it has a marked form peculiar to each species of fungus, and is (somewhat loosely) called the fructification. This fructification bears a large number of spores or single, generally very small, cells which are distributed by wind or other agencies ; and each spore, if it falls on a suitable feeding ground, is capable of producing a new mycelium, which in its turn produces a fresh fructification. A familiar fructification is a mushroom or any toadstool, which is solely a reproducing organ, and is nourished entirely by the mycelium, which is out of sight and buried in the soil. The large bracket fungi found on trees are also fructifications, only these fungi differ from a mushroom in the fact that their mycelium lives and grows in the trees instead of in the ground.

The fructifications of the fungi which grow on trees are often very large, and.when it is remembered that all the food necessary to produce them is derived from the tree, it will be clear that the tree must suffer accordingly. Frequently the tree is killed, but if the fungus is a facultative and not an obligate parasite, its growth will not immediately be checked by the death of the tree, and not uncommonly dead trees are a dangerous source of infection to living ones. The tree or other plant on which a parasite grows is somewhat unkindly called its host.

When investigating the life-history of a fungus it is often found that there is another form of reproduction besides the fructification. In some species almost any part of the mycelium may give rise to specialized hyphae, which bear cells which are not unlike spores and have the same faculty of reproducing the plant. These cells are given the distinguishing name of conidia, and the hypha that bears them is called a conidiophore. The distinction between spores and conidia has given rise to much controversy, and is now based chiefly on certain nuclear phenomena. which precede their formation ; but as nuclear phenomena will not be dealt with in this book, the application of the terms will have to be taken on trust. At the same time the word conidium is very useful as a term for an extra reproductive cell which is generally not an essential part of the life-cycle of the fungus: the qualification is used advisedly, for in a large number of fungi no typical fructifications have ever been found, so that they must reproduce themselves solely by conidia. The discovery of the formation of conidia in the life-history of any fungus is manifestly of the first importance to the pathologist, for infection may be caused by conidia just as well as by spores. For instance, a whole plantation of larch may be heart-rotted by Fomes annosus without a single fructification being produced, and this is probably due solely to infection by conidia. (For conidia of this fungus, see fig. 43.)

‘Fhe morphology of the lareh. The study of disease thus resolves itself into a study of the relationship of the parasite and host. The complete life-history of the parasite must be known as well as the structure and mode of growth of the host. In this book many different parasites are dealt with, but the host is always the same ; so a description of those parts of the host with which we shall be chiefly concerned will be given at the outset. The account will be made as simple as is consistent with accuracy, and only those parts will be described which are necessary to an understanding of the more important diseases.

Long and dwarf shoots. When growing under forest conditions the leading shoot of a-larch grows rapidly and maintains its vertical direction until maturity, while the lateral branches are mainly horizontal. The apical bud of each shoot, and those other buds which develop into branches, elongate in the spring and become what are called long shoots, i.e. shoots in which the internodes, or portions of the stem between the leaf insertions, are lengthened. The young stems are at first green, but soon put on a layer of bark and then appear yellowish brown. The leaf bases are decurrent, so that any transverse section shows five swellings which reach down from the five leaves next above. The leaves are deciduous and bear buds in their axils which develop in the following year. The majority of these buds, however, do not develop into long shoots, but remain stunted, and are called dwarf shoots. In these shoots the internodes do not lengthen, so that all the leaves borne by them are inserted very close together, and appear in a rosette. These dwarf shoots generally bear leaves for a number of years, and their age can be determined by counting the whorls of dark-brown bud-scales which remain attached at their bases. The dwarf shoots may, under favourable circumstances, grow out into long shoots, but they more commonly get left behind by the growth of the tree, and when they become too much shaded by other branches the shoots die and remain on the branches as tufts of -brown bud-scales, until they are ultimately cast off the tree. :

Internal structure of the stem. This will be described in detail, as the stem forms the feeding ground for the mycelia of all the most important disease-causing fungi. As it will be necessary to introduce a number of botanical terms which could not be explained except at great length, readers who are uhacquainted with the ‘general features of a plant’s anatomy are recommended to omit this section and pass on to the general account of larch canker in the next chapter.

If a first-year shoot, about 1:5 mm. in diameter, be cut in winter and a transverse section examined under the low power of a microscope, the tissues will be found to be arranged as in fig. 2. On the outside are five protrusions, the decurrent leaf-bases or ‘ leaf-cushions ’, between which are narrow furrows. ‘The cushions are nearly | mm. in breadth and about 0°2 mm. in thickness. ‘These cushions contain rather large resin ducts, one near each edge, which pass down from the leaves and end blindly in the cortex. Their length is one to four times that of an internode, so that any section may disclose their presence in one to four cushions. Not infrequently one resin duct in a cushion is longer than the other, so that only one of a pair is cut across in a transverse section of some cushions. Immediately inside the cushions is a continuous cork layer, which kills the outer cortex and epidermis, and inside this again is the inner cortex composed of live cells, with intercellular spaces, somewhat thick cellulose walls, and living contents with chloroplasts. A narrow continuous layer of phloem surrounds the wood cylinder, and immediately outside the phloem are a few nearly spherical resin cysts, which look lke resin ducts in section, but are not extended longitudinally. The wood cylinder may be 4 or 5mm. in breadth, and contains a few resin ducts; and in the centre is a star-shaped medulla.

Sections of older lia. 2.—Diagram of a transverse section stems show progressive of a one-year-old stem of larch. ca., cam- stages of thickening. bium; co., cork; 7%.c., inner cortex; /(r., The leaf-cushionsgradufoliar resin duct; m., medulla or pith ; m.7r., medullary ray; o.c., outer cortex; p., ally disintegrate and fall peal yee cyst; r.d., resin duct off, and are usually indistinguishable afterthe fifth year. New cork layers are formed immediately under the first one. These layers are not always complete, and, in the second year especially, partial layers are frequently formed stretching under the furrows. In general a new cork layer is formed each year. The inner cortex remains living for an indefinite time, but loses its chlorophyll after a while. The phloem becomes thicker, and the outer firstmade elements are so stretched tangentially as to leave numerous large intercellular spaces. A fresh ring of wood is put on each year.

Detailed structure of tissues : epidermis. The cells measure

Fig. 3. —Microphotograph of transverse section of three-year-old stem of larch.

Fig. 4—Microphotograph of transverse section of abnormal wood.

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10-20» tangentially, 17 radially, and are longitudinally extended to about 400. They are not only covered by a thick cuticle, but the outer and radial walls are strongly cuticularized, and a cellulose wall is superposed on this cuticularized wall, leaving a lumen with a diameter about half that of the original cell. Owing to the formation of the cork layer, this and the outer cortical layers are killed before the lapse of the first year.

Outer cortex. Next the epidermis are one to three layers of collenchyma, the walls of which give both the chlor-zinc-iodine reaction for cellulose and the phloroglucol and hydrochloric acid reaction for lignified walls. The walls are thick, with well-marked secondary and tertiary layers, and there are small intercellular spaces at the corners.

The parenchyma inside this is composed of large cells 30-100 » in diameter, and nearly isodiametric in all three planes. The resin ducts of the cushions are formed in this tissue, and may either touch the collenchyma or be separated from it by one or two layers of parenchyma. They are up to 170, in breadth, and are lined by an epithelium protected by small parenchymatous cells on the outside. There is no further mechanical tissue to prevent their being crushed. These, with all the parenchyma in the cushions, are cut off by the primary cork layer, and are thus functional for less than one year.

Periderm. The phellogen is first formed in the layer of parenchyma immediately inside the collenchyma at the furrows, and forms a circle by cutting across the parenchyma of the leaf cushions (see fig. 2). The phellogen forms 1-3 layers of cork cells on the outside, and:as many layers of phelloderm cells on the inside. |

A new cork layer is formed each year in the following way. ‘The innermost phelloderm layer becomes meristematic, and forms a new phellogen. The outer phelloderm cells and the phellogen of the previous year develop thick walls, become lignified, and take on the appearance of stone cells. In this way layers of stone cells, laterally welded together, are formed, and the walls become so thick that it is often nearly impossible to find the lumina or to distinguish one cell from another; the layer may be two cells in thickness, or it may be only half a cell in thickness, only one side of the cell having taken on a thick wall. The new phellogen then forms a few layers of cork cells and a fresh phelloderm. .

This process is repeated year by year, so that a large number of cork layers are formed, all in the same radial rows with the rest, except where phellogen cells have

divided tangentially, and ee ee

: CS = without encroaching on the cea RS | Si inner cortex, which remains —— Roamer = a Ley u intact .

a Bi en In special ian = — a a n special cases, howa a ——— =e, ~~ Pl ever, e. g. where the cortex Rae i has become injured, a fresh A AN Ree ; phellogen may be formed ike ‘ through the cortex or outer fis, 5Diagrem showing method phJoem, asin ordinary scale

¢,, first-year cork; ¢, second-year bark trees. cork ey third yar cork; lumifed A Jongitudinal section shows at the top of each leaf cushion a leaf scar where the abscission layer has cut off the leaf. A poorly developed cork layer is formed under the abscission layer, and the cells beneath this become suberized. The cork layer is joined on to the primary cork layer of the stem on all sides. These leaf scars would appear to be weak points in the defensive armour of the stem. At the same time they may serve for the interchange of gases, since no lenticels are present in the cork until the second year. Inner cortex. 'The parenchyma inside the cork layer remains alive, and the cells contain chloroplasts. The cells are at first nearly circular in all sections, but later become extended tangentially to keep pace with the increasing girth of the stem. These cells usually divide by radial walls, and each elongated cell is replaced by two or three circular ones. This continued vitality of the cortex is rendered necessary by the peculiar form of periderm formation, which leaves the inner cortex intact even in old stems. |

Two kinds of specialized elements occur in this layer. The first are small cells set aside for the purpose of containing crystals of calcium oxalate. These cells are only found in older stems, and are very frequent in the neighbourhood of cankers. The second are irregularly branched sclerenchymatous cells with

. 0 | { very thick walls (fig. 6, s.e.). y, These elements grow longi- fi gfe} 5 . : A A /oo \ tudinally, forcing their way py Broo \ \\\