Fungous diseases of plants

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Preface

It is a noteworthy fact that there has been available to student and reader no general text or reference book of American origin upon fungous diseases of plants. Nevertheless, for thirty years or more there has been active investigation in this field, and during much of this time instruction in plant pathology has been an important part of biological teaching in all colleges where plant industry or country-life interests have been adecjuately represented. In the agricultural colleges the teaching of general mycology has been important, and that of plant pathology is now essential. The presentation should be fundamental, but it should also bear a close relation to the affairs of life. Plant pathological work has been rapidly developed in all countries characterized by a progressive agriculture, and for European conditions the student experiences no great lack of reference works.

Through the agricultural experiment stations and through the extension work in various states a vast amount of information with respect to plant diseases has been published and otherwise disseminated, so that to every intelligent plant producer the opportunity has been extended of becoming more familiar with the crop relations of destructive parasitic fungi. The student and the progressive grower require something further, and it has therefore seemed none too early to put in book form a comprehensive discussion of the chief fungous diseases of cultivated and familiar plants. It is not intended that this book shall be an introduction to systematic mycology ; yet the arrangement of the material in taxonomic sequence with respect to the fungi largely eliminates the necessity of any mycological preparation as a prerequisite.

As far as practicable, in the discussion of each disease, three important considerations have been kept in view: (i) to describe the pathological effects and other relations of host and parasite ;

VI ■ prp:face

(2) to make clear the life history of the causal fungus ; and (3) to indicate the approved or suggested methods of prevention or control. The author fully recognizes that in any complete discussion of a fungous disease there are definite theoretical subdivisions, such as symptoms, pathological morphology, etiology, life cycle of the causal organism, etc. Nevertheless, such a system does not at present recommend itself. In the nomenclature of popular names of diseases uniformity, or special fitness, at a sacrifice of established usage, has been avoided. An extensive host index has been included in order to present in a succinct form all of the diseases discussed upon any host. It is, perhaps, needless to add that the chapters upon culture methods, technique, and physiological relations are designed primarily for reference, and to stimulate the most complete use of the available material. The bibliography is intended to be suggestive, and the titles are made prominent that the suggestion may not be avoided.

Aside from photographs and drawings made by the author, the illustrations have been derived from a variety of sources. Special acknowledgment is made to Mr. F. C. Stewart, of the New York Agricultural Experiment Station, and to Professors H. IT. Whetzel and George V. Atkinson, of Cornell University, for the privilege of using many negatives from their collections. Many others have kindly furnished material for one or more illustrations, as credited in the legends. In the preparation of the drawings much assistance has been given by Mrs. B. M. Duggar. For helpful suggestions respecting the manuscript and for a first draft of the synopsis of species among the Uredinales, the writer is indebted to Professor George P. I. Reed, of the University of Missouri.

li. M. DUGGAR

Contents

Page

Introduction i

PART I. CULTURE METHODS AND TECHNIQUE Chai'tkk

I. Isolation and Pure-Culture Methods 9

I. The Development and Application of Culture Methods lo

II. Cleaning Glassware '2

III. The Principles and Methods of Sterilization 15

IV. Preparation of Culture Media 23

Liquid Media 23

Solid Media 26

Neutralization of Culture Media 33

V. Present Method of isolating Organisms 34

II. Technique of Fl\in(;, Imbeddinc;, and Staining' ... 41

I. Fixing 4'

II. The Paraffin Process : Infiltration and Imbedding ... 45

HI. Staining 4^^

Part II. Physiological Relations

HI. Germination Studies 55

IV. General Relations to Environmental Factors ... 62

1. Saprophytism and Parasitism 62

II. General Relations to Climatological Factors 66

HI. Special Relations to Environmental Factors 69

V. Artificial Infection 76

VI. The Principles of Disease Control 85

I. Methods of Control 85

II. Preparation of Fungicides 88

PART in. FUNCIOUS DISEASES OF PLANTS

VII. General Classification 93

I. Fungous Diseases and Pathology 93

II. The Classes of Fungi 94

VIII. Mvxomvcetes. Slime Molds 97

I. Phytomyxales (Phytomyxaceae) 97

II. Club Root of Cabbage and Other Crucifers 97 viii ■ CONTENTS

Chapter Page

IX. SCHIZOMYCETES. BACTERIA , . I03

I. Bacteriaceae 106

II. Black Rot of Cabbage 107

III. Wilt of Sweet Corn iii

IV. Crown Gall of Apple, Peach, and Other Plants . . . 114 V. Olive Knot, or Tubercle-Disease of the Olive . . . . 118

VI. Bean Blight 119

VII. Hyacinth Disease 120

VIII. Bundle Blight of Sugar Cane 120

IX. Pseudomonas : Other Species 120

X. Pear Blight 121

XI. Wilt of Cucurbits 129

XII. Soft Rot of Carrot and Other Vegetables 131

XIII. Soft Rot of the Calla 133

XIV. Wilt of Solanaceae ...'.... 134

XV. Bacillus : Other Species 1 34

X. Phycomycetes 135

I. Chytridiales 136

II. Synchytriacese 136

III. Cranberry Gall 138

ly. Pycnochytrium Globosum (Schroet.) Schroet 139

V. Chytridiales: Other Species 139

VI. Saprolegniales 140

VII. A Damping-off Fungus 141

VIII. Brown Rot of the Lemon 144

IX. Peronosporales 147

X. White " Rust " of Crucifers 149

XI. Cystopus : Other Species 152

XII. Downy Mildew of the Grape 152

XIII. Downy Mildew of the Cucumber 158

XIV. Sclerospora 161

XV. Downy Mildew of Crucifers 161

XVI. Onion Mildew 162

XVII. Peronospora : Other Species 164

XVIII. Downy Mildew of the Lettuce 164

XIX. The Late Blight and Rot of the Potato 165

XX. Downy Mildew of Lima Beans 171

XI. ASCOMYCETES 174

I. Exoascaceas i75

II. Peach Leaf Curl 176

III. Plum Pockets 183

IV. Witches' Broom of the Cherry 185

V. Helotiaceae 185

VI. Sclerotinia 186

CONTENTS ix

Chapter Pagf.

XI. AscoMVCETES {Cotitinued)

VII. Brown Rot of Stone Fruits 187

VIII. Gray Mold, or Botrytis Disease 196

IX. Lettuce Drop 198

X. Stem Rot of Clover 201

XI. Larch Canker 202

XII. Mollisiaceae 203

XIII. Alfalfa Leaf Spot 203

XIV. Anthracnose of Currants . : 204

XV. Phacidiaceae 207

XVI. The Black Spot of Maple 208

XVII. Perisporiales 209

XVIII. Perisporiaceas 209

XIX. Root Rot of Tobacco, Violets, Peas, Lupines, etc. . 210

XX. Sooty Mold of Orange 213

XXI. Erysiphaceas 215

XXII. The Gooseberry Mildew 221

XXIII. Mildew of Peach. Rose Mildew 224

XXIV. Mildew of Apple and Cherry . . 226

XXV. Powdery Mildew of Peas 227

XXVI. Mildew of Composites and Other Plants .... 228

XXVII. Mildew of Woody Plants 228

XXVIII. Powdery Mildew of (irape 229

XXIX. Powdery Mildew cf Willow and Poplar .... 230

XXX. Common Mildew of Trees 23 1

XXXI. Hypocreaceae 232

XXXII. Wilt Disease of Cotton, Cowpea, and Watermelon . 233

XXXIII. A Canker of Woody Plants 239

XXXIV. European Apple Canker 242

XXXV. Stem Rot of Sweet Potato and Eggplant .... 243

XXXVI. Ergot 244

XXXVII. Dothidiace^ 248

XXXVIII. Black Knot of Plums and Cherries 248

XXXIX. SphjEriales 253

XL. Black Rot of Grapes 254

XLI. Cranberry Scald 259

XLII. Leaf Spot of Strawberry , 261

XLIII. Leaf-Spitting Blight of Sugar Cane 263

XLIV. Apple Scab and Pear Scab 264

XLV. Bitter Rot of the Apple and Other Fruits . . . . 271

XLVI. Anthracnose of Sycamore 278

XLVII. A Disease of Young Oaks 280

XLVIII. Bark Disease of Chestnut 281

XLIX. Blister Canker of Apple 282

X CONTENTS

Chaftf.r Page

XII. FuNCii Imperfect! 285

I. Hyphomycetes 286

II. Melanconiales 288

III. Sphasropsidalcs 289

IV. Potato Scab 290

V. Bud Rot of Carnations 293

VI. A Pink Rot following Apple Scab 295

VII. Ramularia 296

VIII. Cercosporella 297

IX. Rice Blast 297

X. Polythrincium 298

XI. Peach and Apricot Scab 299

XII. Cladosporium : Other Species 300

XIII. Early Blight of the Potato 301

XIV. Onion Mold 304

XV. Macrosporium : Other Species 304

XVI. Blight of Ginseng 305

XVII. Leaf Spot of Beets 309

XVIII. Early Blight of Celery 312

XIX. Leaf Blight of Cotton 313

XX. Cercospora : Other Species 314

XXI. Spongy Dry Rot Fungus of Apple 316

XXII. Dry Rot of Potatoes 317

XXIII. Flax Wilt 319

XXIV. Fusarium : Other Species 320

XXV. Root Rot of the Vine 321

XXVI. Anthracnose of Bean 322

XXVII. Anthracnose of Cotton 325

XXVIII. Wither-Tip and Spot of Citrus Fruits .... 327

XXIX. Anthracnose of Clover and Alfalfa 328

XXX. Anthracnose of Snapdragon 329

XXXI. Colletotrichum : Other Species 330

XXXII. Gloeosporium 330

XXXIII. Anthracnose of Grape 332

XXXIV. Anthracnose of Raspberry and Blackberry . . . 334 XXXV. Gloeosporium: Other Species 335

XXXVI. Marsonia 336

XXXVII. Peach Blight 336

XXXVIII. Leaf Blight of Cranberry 338

XXXIX. Shot-Hole Disease of Plum and Cherry .... 339

XL. Fruit Spot of Apple 341

XLI. Heart Rot and Blight of Beets 343

XLII. Dry Rot of Sweet Potato 344

XLIII. Seedling Stem Blight of Eggplant 345

CON'l'KN'rS xi

Chai'trk Page

XLIV. I'hyllosticta 345

XLV. Black Rot of Sweet Potato 348

XLVl. Black Rot and Canker of Pomaceous Fruits . . . 350

XLVII. Raspberry Cane Blight 354

XLVIII. Rose Leaf Blotch 357

XLIX. Leaf Spot of the Pear 358

L. Late Blight of Celery 361

LI. Septoria : Other Species 362

LI I. Currant Cane Blight 364

LI 1 1. Leaf Blight of Pear and Quince 365

LIV. Sooty Blotch and Fly Speck of the Apple and Other

Plants 367

XIII. Hemihasidiomvcetes 370

I. Ustilaginales 370

II. Loose Smut of Oats 372

III. Loose Smut of Wheat . . . 375

IV. Smut of Corn 376

V. Smut of Blue-Stem Grass 378

VI. Tolyposporium bullatum (Schroet.) Schroet. '. . . 378

VII. Bunt, or Stinking Smut of Wheat 379

VIII. Tilletia : Other Species 380

IX. Entyloma 380

X. Onion Smut 381

XIV. Protobasidiomycetes 384

I. Rust Fungi 384

II. Families and Genera 388

III. Synopsis of Species 391

IV. Clover Rust 395

V. Rust of Beans 397

VI. Rust of Vetch and Garden Pea 398

VII. Beet Rust 399

VIII. Carnation Rust 399

IX. LIromyces : Other Species 402

X. Asparagus Rust 403

XI. Violet Rust 407

XII. Mint Rust 407

XIII. Black Rust of Grain 408

XIV. Rust of Maize 414

XV. Timothy Rust 415

XVI. Brown Rust of Wheat and Rye 416

XVII. Rust of Stone Fruits 417

XVIII. Hollyhock Rust 419 xil CONTENTS

Chafter Page

XIV. PROTOBASiDiOiMYCETES {Continued)

XIX. Puccinia : Other Species 420

XX. Gymnosporangium 422

XXI. Cedar Apples and Apple Rust 425

XXII. Gymnosporangium: Other Species 426

XXIII. Orange Rust of Raspberry and Blackberry . . . 427

XXIV. Rust of Roses 430

XXV. Rust of Rhododendron and Norway Spruce . . . 432

XXVI. The European Currant Rust 433

XXVII. Orange Rust of Aster and Golden-rod 435

XXVIII. Rust of Poplar 437

XV. AUTOBASIDIOMYCETES 439

I. Exobasidiales 439

II. Gall of Heaths 440

III. Hymenomycetales 441

IV. A Root and Stem Rot Fungus 444

V. Heart Rot of Sugar Maple 452

VI. White Rot of Deciduous Trees 453

VII. Decay, or Brown Rot, of Trees 457

VIII. Polyporus : Other Species 462

IX. Fomes 464

X. A Brown Rot of Conifers 467

XI. Root Disease of Sugar Cane 469

XII. Root Rot of Fruit Trees 471

XIII. The Honey Agaric 473

XIV. European Root Disease of Alfalfa and Other Plants 477 XV. Root Rot of Cotton and Alfalfa 479

Host Index 483

Downy Mildew on Niagara Grapes

Introduction

A proper study of the funij^ous diseases of plants is at once scientific and practical. The fungi were carefully studied, however, long before their importance as disease-producing organisms was recognized. A history of our knowledge of the fungi in general, therefore, takes us through periods when the scientific and the practical attitudes were not associated ; yet a brief historical survey must develop important and interesting facts bearing upon the relations of scientific work to practical affairs.

Systematic mycology. A careful study of the fungi as independent groups of plants was begim in the latter part of the eighteenth century, and if we examine the results of the work beginning at that time and continuing into the early half of the nineteenth century, it will be found that this period was one of most accurate and painstaking endeavor in systematic mycology. Much credit is therefore due to the more prominent students of that time, such as Bulliard, Persoon, Nees von Esenbeck, Schweinitz, Leveille, Fries, and Berkeley. The work so well begim was continued into the second half of the same century, and among the names particularly associated with that period are those of Fuckel, Karsten, the Tulasne brothers, Corda, and many others. This systematic study has, of course, continued to the present time, although the nature of the work produced has undergone important changes. Two phases in the modern development of this systematic work are well shown by the appearance, on the one hand, of Saccardo's monumental " Sylloge," and, on the other hand, of such complete morphological monographs as Thaxter's " Laboulbeniaceae."

Physiology and morphology. The progress in systematic mycology has made possible for more than half a century a comprehensive study of the diseases of plants ; yet systematic study alone is. not responsible for the rapid progress subsequently achieved in plant pathology. A number of causes might be suggested as of importance in the development of the latter field. It should not be overlooked that advances in general plant physiology were also manifest at about the beginning of the nineteenth century, and that this phase of botany had undergone unusual development toward the middle of the century, under the influence of Sachs and other experimentalists of his time. Again, a more intensive method in the study of morphology had been introduced, and in mycology the efforts of such men as the Tulasne brothers had shown what could be done in carefully following out the life histories or development of the fungi. Beginning about the middle of the nineteenth century, another distinctive epoch is entered upon, and the developments of this period are due chiefly to Anton de Bary and his contemporaries.

The rise of plant pathology. De Bary became the conspicuous leader in this field, establishing in an incontrovertible manner the connection between the polymorphic stages of certain parasitic species, and the possibility of following, under well-controlled conditions, the development of little-known groups. His work was, furthermore, particularly significant in that he so thoroughly appreciated the nature of parasitism, the epidemic character of fungous diseases of plants, and the practical value of methods of inoculation and infection. To him more than to any one else we owe the influence which directed future work along the lines of the most profitable research. This period witnessed also the advances made by Pasteur and others in the study of fermentation and disease, and it wa^ closely followed by those perfections in the development of pure culture methods which have finally resulted in the possibility of cultivating practically all bacteria and a very great majority of the fungi. In the study of the fungi as the cause of plant diseases, at this time, valuable service was also done by Kiihn, who in his early career devoted himself particularly to a study of the fungous parasites of cultivated plants. The last decades of the century yield work of such diversity and importance that it is impossible here to

INTRODUCTION 3

do more than make briefest reference to it. The work of Brefeld is perhaps most distinctive, and while his theoretical views have not had a host of followers, his fundamental studies in the general field of mycology, and particularly, in this connection, his wide range of experiments in the artificial cultivation of organisms, are invaluable. Among many others who contributed special service in some phase of pathological or general mycological work of that time may be mentioned also Frank, Hartig, Schroeter, Sorauer, and Winter in Germany; Oudemans in Holland ; Cornu, Millardet, and Prillieux in France ; Comes in Italy ; Woronin in Russia ; Eriksson in Scandinavia ; Plowright and Ward in England ; Farlow, Burrill, and many others in the United States. The work has continued vigorously, investigations and problems have multiplied, and with adequate conservatism the outlook is most encouraging.

There were some indications of a plant pathology in existence from the time of the first studies in systematic mycology, but an examination of the books which purport to be discussions of plant pathology shows that they were in large part an attempt to classify and suggest ideas having to do with plant diseases, after the manner of the older attempts which were made in human medicine. In most cases the life history of the organism which caused the disease was entirely unknown ; and, in fact, there is no plant pathology which deserves the name affixed to it prior to the appearance of Kiihn's "Die Krankheiten der Kulturgewachse, " Berlin, 1858, Between that time and 1900 an extensive literature developed. The status of the morphological work is well shown by De Bary's " Morphologic und Biologic der Pilze," etc., and in addition to many special life-history or monographic studies we have, from the pathological point of view, such comprehensive reference books as those of Comes, Frank, Hartig, Prillieux, Sorauer, Tubeuf, Ward, and others.

Practical pathology and disease control. An important epoch in the general study of fungous diseases had its beginning in the prevalence of grape diseases in France, which condition led to the discovery of Bordeaux mixture by Millardet in France about 1883. After the severe tests to which the copper mixtures were subjected, it became evident that there was a bright prospect for controlling many of the fungous diseases of plants, and there developed therefore an immediate need for plant pathologists, or students of fungous

4 FUNGOUS DISEASES OF TLANl'S

diseases of plants, — persons- who should be, at the same time, appreciative of the problems of disease control. Incidentally it may be noted that plant diseases were, for the most part, understood to be of fungous origin. In the United States this was more or less coincident with the organization of a section of Plant Pathology in what was then the Division of Botany at Washington, and with the development of plant pathological work in many of the state experiment stations. In a very short time there was unusual activity in this study throughout the country. There was also much stimulus to the further development of the work in Europe, and the outcome was that the foundations were laid for a more careful study of the fungi from a phytopathological point of view. In this country the work was directed more especially toward immediately practical ends, and that which was accomplished within a brief period of time through the efforts initiated by Scribner and Galloway was remarkable. In more recent times the work has also been put upon a higher plane, and investigations along broader and more fundamental lines have gone forward rapidly at many points throughout the country, so that to-day the extent of the organization and equipment for research in this field is better than may be found anywhere else in the world. It is perhaps fortunate that this work in the United States has developed in conjunction with the agricultural experiment stations, although, when the equipment in men, books, and apparatus was new, many mistakes were made. This association of the work insures that the direction of it will be at least more practical than if confined more or less to investigations carried on in botanical gardens or herbaria. It is perhaps to be regretted that there cannot be more unity of action, or cooperation, in the study and control of epidemic diseases. This, however, may be brought about in the course of time.

Some aspects of modern plant pathology. It is very evident from the nature of the study, as well as from the historical notes which have been presented, that an analysis of the modern work in plant diseases indicates several important aspects, which may be grouped in the following category : (i) mycological relations ; (2) anatomical effects ; (3) physiological relations ; (4) control measures.

Mycological relations. The mycological aspect will be concerned more particularly with a minute investigation of the fungi from

IN'I'RODUCTION 5

systematic, morphological, and physioloi^ical standpoints. Too often, in the early work, the chief object of the study has been to identify the fungus associated with a given disease and to describe its fruiting stages. An investigation of the fungus, however, should include an account of its complete life history wherever possible, the relations of the fungus to conditions under which it is injurious, the character of the growth ]3roduced upon various culture media (when the organism is culturable), the conditions under which fruiting stages may be developed, etc. In the course of time, therefore, it will be necessary to repeat much of the work of earlier date, which has seemed to be more or less complete.

Anatomical effects. The anatomical study, in the sen^e in which it is here used, will be concerned with the relations of the host to the parasite, in so far as the former may be modified in growth or minute structure. All lesions, hypertrophies, or other structural changes produced in the host plant are worthy of the closest attention. These changes in the host are most diverse, varying, on the one hand, from minute modifications of a single cell, or of a small group of cells, to those changes of form which involve an immense increase in the size of the host organism, often giving rise to relatively enormous deformities, such as may be noted in the case of the club root of cabbage, plum pockets, cankers, and smut of corn. Again, the deformities may result in the pushing into growth of an abnormal number of buds, in many instances accompanied by decreased size of the branches and changes in the trophic relations, such as to develop the various forms of witches' brooms. The anatomical changes^ in the host are those most commonly termed pathological changes. Unfortunately these are often discussed as if they were the only pathological effects. They are, at any rate, the chief evident pathological effects in many cases, and for that reason they constitute in fhe popular view that which is properly designated " plant pathology."

Physiological relations. In close connection with the anatomical changes produced in the host, a study should be made of the physiological relations of host and parasite, particularly of the

^ Kuster (Pathologische Pflanzenanatomie, 312 pp., 121 figs., 1903) has attempted a general classification of anatomical modifications induced by diverse stimuli.

physiological disturbances in the host itself .^ The normal physiology of th^ host requires attention in order that a proper comparative study may be made. The conditions which predispose the host plant to attack, or, in other words, the conditions favorable to the penetration of the fungus and its development within the host are most fundamental from the standpoint of pathology, and also in order that control measures may be properly developed. It is a direction in which future work promises most profitable returns. Very little of lasting value has been done towards determining the exact conditions under which the host plant is most susceptible to attack. It is well known in the case of certain forced plants that the undue suffusion of the plant with water, whether due to lack of ventilation or to a combination of causes, is a certain factor in inducing disease. Under such conditions many fungi are able to gain entrance and become the cause of epidemics, whereas, under more normal conditions, they may remain as harmless inhabitants of dead materials.

Every season shows differences in the prevalence of the more injurious fungous diseases. One season the brown rot of the peach may affect only extremely sensitive varieties, and the following season it may cause the loss of those most resistant. Again, some varieties of the host may be, under most conditions, but slightly predisposed to attack, notable instances being those of the very slight predisposition of the Kieffer pear to the blight, or in the resistance of certain American varieties of grapes to the downy mildew. Such cases might be multiplied indefinitely, and, in fact, there is scarcely a known fungous disease of the variable cultivated crops with reference to which all varieties of the host plant are equally susceptible. This important fact has led to the selection and to the production through hybridization of varieties which shall at once possess both the qualities desired from the standpoint of their own fruits or other products, and which shall, at the same time, carry with them the high resistance necessary to enable them to compete with the fungous pests.

The effect of the fungus upon the host may be, further, merely to modify the quality of the product, such as the sugar or starch content, without seriously affecting the appearance of the economic product. In fact, the different means whereby the effect of the

1 See Ward, H. M., Disease in Plants, 1901.

INTRODUCTION 7

fungus may show itself in slight physiological disturbances of the host are too numerous for special consideration.

Control measures. Control measures for the prevention of fungous diseases should be a part of every study which is undertaken. Reference has already been made to the very rapid development of systematic methods of control. Control may be developed along one or more very different lines. In the first place, it may concern itself more particularly with a maintenance of the host plant in a thoroughly sanitary environment, or in one which renders it more resistant to the attacks of fungi. It may again concern itself with the application of deleterious substances (fungicides) to the host, in order that the germination and growth of the fungous spores may be prevented. This use of fungicides may take the form of disinfection of the seed or of propagative parts, the application of reagents to the soil in order to prevent the growth of the fungus in the vicinity of the host plant, or the application of fungicides to the aerial vegetative portions of the host, which is commonly acconlplished by the operation of spraying. This latter operation has been practiced to a considerable extent for a long period of time, but the really substantial development of the work began with the discovery of Bordeaux mixture, to which reference has already been made. At the present time a great variety of spraying mixtures are employed, a large number of which contain copper compounds, or copper combined with lime, subsequently discussed in detail. There are, however, a great many directions in which the development of desirable fungicides may yet go forward. At the present time the use of lime-sulfur washes and sprays is rapidly taking an important place. It is particularly in connection with control measures, or facts concerning the life history of the organism suggesting such measures, therefore, that the study of fungous diseases of plants makes for itself a place among practical sciences. The amount of injury annually suffered by the various crops, due to fungous diseases, may be more or less definitely ascertained, and this represents the possibilities to which control work may be pushed. On the other hand, the relation of the crop in unsprayed regions to that in regions where spraying is used may permit one more or less roughly to determine the actual saving through the present imperfect and rather haphazard practice of control measures. Estimates which have been placed upon the damage caused by prevalent l)lant diseases during a single season amount frequently to a very considerable per cent of the total value of the crops. In the United States alone the destruction wrought by fungous diseases is sometimes not far from half a billion dollars.

The diseases of plants induced by other biological, physical, chemical, or mechanical agencies are not included. The lack of plant nutrients, or the presence of particular nutrients in quantities sufficient to cause injury, the phenomena commonly termed sunscalds, wind effects, abrasions due to contact, etc. are all disturbances which demand attention, but they may have no definite relation to epidemic fungous diseases, and would therefore be fundamentally considered only in a general treatise on plant pathology. On the other hand, it is felt that in connection with any account of the fungous diseases of plants it is desirable to place within easy reach of the student certain related information. In isolated chapters, therefore, there is presented a brief review of culture methods, histological technique, and such facts of physiological significance as seem requisite.

Culture methods are here concerned with the essential steps in preparing important nutrient media and means for the isolation and study of fungi in artificial cultures. Such cultures are important in morphological and physiological study, and they afford in the majority of cases the only proper source of spores or mycelium for inoculation purposes.

Histological technique is requisite not merely to insure a proper morphological study of a fungus and its distribution in the host, but also in order to make possible a more comprehensive analysis of the tissue modifications in the host.

A discussion of special biological or physiological relations has been limited to a few topics. The germination of spores is from the outset one of the investigational or routine duties of the pathologist ; the relations of the fungi to chief environmental factors cannot be disregarded; artificial infection is required in determining the causal organism ; and the principles of disease control are concerned with the immediate application of pathological study to economic purposes.

PART I

Chapter I

LOEFFLER, Fr. Voilesungeii iiber die geschichtliche Entwickelung der Lehre von den Bakterien 1 : 252 pp. 3 pis. 37 figs. 1887. Leipzig.