Diseases of greenhouse crops and their control

Front matter

Class S h'U i Book '

CopightN"

CXIPffllGHT DEPOSm

Introduction

The art of forcing vegetables and flowers is not a new one; however, its economic aspect is of modem origin. The Thirteenth Census of the United States estimates that the total area of land under glass in 1909 was 114,655,000 square feet, of which 105,166,000 square feet were in greenhouses and 9,490,000 feet were covered by sashes and frames. About 99% of the value of the plants and flowers in 1909 was produced in 7,444 establishments, the average value of each of these establishments being $4,630.00. As is natural to expect, these establishments were located near large cities. The leading states in value of forced flowers and plants were New York with $5,110,000; Pennsylvania with $3,761,000; Illinois with $3,681,000; New Jersey with $2,839,000; Massachusetts with $2,432,000; Ohio with $2,357,000; California with $1,374,000; Indiana with $1,202,000; Michigan with $1,132,- 000; and Connecticut with $1,042,000. States with less than a million are not here recorded. The total value of forced plants and flowers as estimated for 1909 was $24,930,000.

There are as yet no available figures of the area and the money value of the greenhouse industry. It is, however, reasonable to suppose that the num-

vii viii

Introduction ber of establishments and their money value have lately increased by at least fifty per cent. This, therefore, represents a vast sum of money and an important industry of the United States which cannot be ignored.

Plants under greenhouse culture are far from being subjected to normal conditions. The expression ''tender as a hothouse plant" well expresses the truth. Because of this fact greenhouse plants are naturally more susceptible to diseases indoors than similar plants grown in the open. This at once emphasizes the importance of studying the diseases of greenhouse crops with a view to furnishing the growers such information as may help them to reduce important plant diseases and thereby increase their profits. We have as yet no available figures as to the money losses from diseases of greenhouse crops. A conservative estimate, however, may place these losses at about thirty per cent.

The literature on diseases of greenhouse crops in the United States is rather fragmentary and scattered. The American Plant Pathologists have been too busy in devoting much time to the investigations of the diseases of cereals, fruit and truck crops. Considering that plant pathology is only a new science, the diseases of the greenhouse crops had of necessity to be neglected. It is, therefore, the aim and purpose of the present volume to bring together available information on the subject and to place it at the disposal of the greenhouse men. The author realizes too well the incompleteness of this work;

Introduction ix however, it is felt that no apologies are due and it is hoped that it will meet an important demand for information, and stimulate further research in this line. The book is intended as a guide to practical growers, teachers, students and investigators in plant pathology. It is taken for granted that the practical man who uses this volume will study it with a view to gathering information to serve him as a guide, as no definite hard rules are here laid down which could apply to the problems of each individual case. It is only by combining the information with good common sense that the best results are to be obtained. Constructive criticism is solicited and helpful suggestions will be gratefully received.

Acknowledgments are here due to Dr. and Mrs. D. de Sola Pool of New York City; Dr. 1. Adlerblum, Statistician of the Metropolitan Life Insurance Company, New York City; Professor R. B. Brackett, Professor of English of the Texas A. and M. College; Professor S. C. Hoyle, Editor Extension Service Publication, Texas A. and M. College, for helpful criticisms and for reading the manuscript and proofs; to Professor F. B. Paddock, Entomologist, Texas A. and M. Agricultural Experiment Station, for reading the proofs; Miss Florence Buckman of the Plant Pathology Division, Texas Experiment Station for assistance in preparing the glossary and the index.

Grateful acknowledgments are further due to Dr. C. G. Hopkins for fig. 2a, Dr. T. F. Manns for fig. 2b, Professor L. Green for fig. 5a, Professor L.

X

Introduction

E. Melchers for figs. 8a, b, and c, Dr. Clinton for figs. i6b, 38a and 40b, Professor R. E. Smith for fig. 22, Miss Nelly Brown for fig. 26, Professor A. D. Selby for fig. 28a, Professor Edson for fig. 35, Professor H. E. Stevens for figs. 37a and b, Professor H. S. Jackson for fig. 40a, Professor L. N. Massey for figs. 65b and 66a, Dr. A. B. Stout for fig. 72a, Professor Moznette for figs. 78c and d, The King Construction Company for figs. 25 and 45, The John Moninger Greenhouse Company for fig. 52, Professor F. B. Paddock for figs. 80 and 81.

Credit is also due to the Foley Greenhouse Manufacturing Company for figs. 21, 49, 57a, 58a, 60 and 64. The remainder of the borrowed figures are acknowledged in their proper places. The author also wishes to express his indebtedness to the Texas Agricultural Experiment Station for the original figures which were taken by the Station Photographer for the Division of Plant Pathology and Physiology.

J. J. Taubenhaus

College Station, Texas May 1, 1919

Part I Chapter Page

I. The Healthy Soil 3

11. Sick Soils i6

III. Treatment of Sick Soils .... 28

Part II

TV. Light in Its Relation to Greenhouse

Culture 53

V. Moisture and Water Requirements 73

VI. Breaking the Rest Period of Plants 86

PART III diseases of greenhouse vegetables

VII. Nature of Plant Diseases ... 93

VIII. Germination Troubles 104

IX. Beet Diseases 114

X. Cauliflower Diseases 122

XI. Cucumber Diseases 133

XII. Lettuce Diseases 145

XIII. Mushroom Diseases 159

XIV. Parsley Diseases 166

XV. Radish Diseases 173

XVI. Tomato Diseases 180

Contents

Part IV Diseases Of Ornamentals

CHAPTER PAGE

XVII. Alternanthera Diseases .... 200

XVIII. Aster Diseases 207

XIX. Carnation Diseases 217

XX. Chrysanthemum Diseases .... 235

XXI. Cyclamen Diseases 248

XXII. Hyacinth Diseases . . . . . . 266

XXIII. Lilac Diseases . 275

XXIV. Orchid Diseases ....... 290

XXV. Primrose Diseases 315

XXVI. Sweet Pea Diseases 334

XXVII. Tulip Diseases . 348

PART V greenhouse pests

XXVIII. Plant Pests . 364

XXIX. Greenhouse Thrips 369

PART VI methods of control

XXX. Methods of Control 381

LIST OF ILLUSTRATIONS

FIGURE FACING PAGE

1. Effect of Fertilizers and Watering on the Yield

of Lettuce 5

2. Effect of a Balanced Fertilizer on Corn and

Clover 9

3. Conidiophore and Conidia of Pythium . . 19

4. Nematode 25

5. Leaf Blight Nematode 27 '

6. Steam Sterilization Apparatus 37 v

7. Effect of Soil Sterilization 39

8. Melcher's Soil Sterilizer 43

9. Action of Different Light Rays on Coleus and

on Lettuce 63

10. Action of Different Light Rays on the Color of

Plants 65

11. Sling Psychrometer 77

12. Effect of Subirrigation on Lettuce .... 85

13. Effect of Etherization on Plants .... 87'

14. Effect of Etherization on Hybiscus syriacus . 89 '

15. Bean Diseases 111/

16. Beet Diseases 115

17. Cauliflower Disease . 123

18. Celery Disease 131

19. Celery Disease 133

20. Young Healthy Greenhouse Cucumber Plants 135

22. Cucumber Root Knot 141

23. Egg Plant Diseases 143

24. Egg Plant Diseases 145

xiii xiv List of Illustrations

FIGURE FACING PAGE

25. Type of Lettuce House 147

29. Typical Muskmelon House I5S

30. Muskmelon Diseases 157''

31. Root Knot of Muskmelon 159

32. Healthy Mushroom Bed i6iv

33. Sclerotinia Rot on Pea Pods 167 ^

34. Pepper Diseases 169 1

35. Rheosporangium Rot on Radish .... 175

39. Effect of Fumigation on Tomatoes .... 197

40. Anthirrhinum Diseases 203

41. Anthirrhinum Diseases 205-

42. Aster Diseases 209 '

43. Begonia Root Knot 213

44. Calceolaria Leaf Blight 215

45. Type of Carnation House 217

49. Type of Chrysanthemum House . . . .235

50. Chrysanthemum Diseases 237

51. Broom Rope of Coleus 245

52. Type of Cyclamen House 249

53. Dracena Diseases 251

54. Fern Yellows 255

55. Fern Diseases 257

56. Geranium Diseases 261

57. Hyacinth Diseases 267

List of Illustrations xv

FIGURE PACING PAGE

60. Type of Palm House 307

61. Palm Diseases . 309

62. Pansy Diseases 311

63. Primrose Diseases 315

64. Rose House . 319

68. Rubber Plant and Schizanthus Diseases . .331

72. Tulip Diseases 349

76. Red Spider 365

77. Mite Injury of Geranium and Cyclamen . . 367

78. Snapdragon Mite Injury 369

79. Snapdragon and Geranium Mite Injury . . 370

80. Plant Aphids 371

81. Bucket Spray Pump 407

82. Violet Cuttings 409

Chapter 1

An intelligent understanding of the soil is of paramount importance to the success of the greenhouse. There are three important points that we must consider in the study of a healthy soil. They are : ( i ) texture, (2) fertilizers, (3) soil flora.

Texture

Texture deals with the character of the particles which make up a soil, and with their arrangement in relation to each other. Clay soils are generally made up of very minute particles. Silt is made up of large grains. Coarse sand or gravel is composed of the largest grains. A soil is said to be porous when air or water can circulate through it freely. The porosity depends on the various proportions of clay, silt and sand which that soil contains. Plant growth, and incidentally plant health, is closely interwoven with the soil structure. Compact, sticky clays will be far more unfavorable to greenhouse crops than a clay loam.

The greenhouse man has the advantage over the ordinary farmer because he can modify the texture of his soil so as to make it ideal for his crops. By combining the proper amounts of clay, silt, sand and humus, he may give to the plants a most congenial place to thrive in. To obtain such a result the gardener must exercise his best judgment. By varying the texture of the soil we may often influence the plants unfavorably. Flowering plants may be made to produce excessive foliage and few blossoms, while others may be differently affected.

Fertilizers

Crops require certain food elements to make growth possible at all, and they further require specific substances to enable them to accomplish definite purposes. The carnation, for instance, requires peculiar food elements to attain maximum growth. It further requires special nutritive elements to enable it to produce flowers and to avoid going altogether to foliage. The four leading plant foods needed by greenhouse crops are nitrogen, phosphorus, potassium and lime. All the other plant food elements are present in nearly all soils.

The effect of nitrogen is to stimulate leaf and stem growth, and to add green color. An overdose of it, however, may result in soft plant tissue, and thus retard fruiting. Acid phosphate stimulates root growth, and an overdose of it encourages an excess of root formation over foliage. Phosphorus also stimulates earliness in fruiting. The effect of potassium is to help the plant in assimilating other plant food, and indirectly in the manufacture of starch. It also encourages the production of finer plant tissue, thus increasing the plant's resistance to disease.

The aim of the greenhouse man is to produce early truck crops or cut flowers and this is directly concerned with feeding. Because the four plant food elements above mentioned are of extreme importance, their application cannot be indiscriminate. The greenhouse man must know how much of them to use in combination or separately. He must know also which element will especially benefit the particular crop with which he deals. In his investigations with the fertilizer requirements of lettuce, Stuart * reached the following conclusions : Potash when used in any considerable amount either alone or with nitrate of soda is unfavorable for growth (fig. 1, D.). Acid phosphate alone, in combination with nitrate of soda, or in combination with muriate of potash, stimulates growth (fig. i. A, B, C). For lettuce the use of chemical fertilizers proved slightly superior to stable manure, while nitrate of soda was found to be superior to dried blood. Wheeler and Adams,f in their work with radishes, found that an application of partially composted horse manure at the rate of 75 tons per acre gave better results than any other combination of fertilizers used. Working with carnations, Darner + and his associates

* Stuart W., Indiana Agr. Expt. Sta. Bui. 84, Vol. 10: 11 5-142, 1900.

t Wheeler, H. J., and Adams, G. E., Rhode Island Agr. Expt. Sta. Bui. 128: 183-194, 1908.

t Darner, H. B., et al., Illinois Agr. Expt. Sta. Bui. 176: 365- 386, 1914.

found that the use of nitrogen and acid phosphate caused an increase in the quantity and quality of the blossoms, but that the excessive use of potassium sulphate and dried blood would act injuriously on the plants. In his work on roses, Muncie * concluded that nitrogen in the form of farm manure, liquid manure or blood is very beneficial. The same seems also to be true for acid phosphate when used at the rate of 4 to 8 tons per acre. Lime should be added only when necessary to sweeten the soil. In this case, finely ground limestone may be used as a top dressing at the rate of 10 pounds per 100 square feet of bench space.

From the above discussion, it is evident that the proper handling of fertilizers underlies the success or failure of greenhouse crops. The cattleman, the poultryman, and others who deal with live stock now fully appreciate the importance of a properly balanced ration. Plants are similarly living organisms and consequently they too derive most benefit from a balanced ration (fig. 1, A.).

Aside from a consideration of the relation of the fertilizer to plant growth, its relationship to the soil must not be overlooked. Certain fertilizers, such as nitrate of soda, yield a residue of sodium, the accumulation of which sweetens the soil, and in the long run makes it alkaline. In clay soils serious physical effects may be the consequence. On the other hand, muriate and sulphate of potash, and sulphate of ammonia leave an acid residuum, the

•Muncie, F. W., Illinois Agr. Expt. Sta. Bui. 196: 5x1-564, 1917.

accumulation of which may render the soil sour. It therefore becomes imperative to so use or to so mix these fertilizers that their residues will combine and thus neutralize each other. One reason perhaps why greenhouse men favor the use of manure is that they have experienced the bad effects of the residue of improperly mixed fertilizers.

Soil Flora

By a soil flora is meant the bacteria or fungi, whether beneficial or harmful, which thrive in that soil. Science has proved definitely that a soil can no longer be regarded as a conglomeration of dead, inert particles of rock. The soil teems with life which to a large extent determines its fertility. The more numerous the beneficial bacteria and fungi it contains, the more fertile it will be. On the other hand if the beneficial micro-organisms are absent, or perform their work imperfectly, or if the soil is overridden by harmful parasitic bacteria or fungi, we speak of it as a sterile or sick soil. In the greenhouse, the soil flora is often entirely different from what it is outdoors. This is due to the fact that the soil is artificially made up of a mixture of various ingredients with the object of making it ideal for plant growth. It is imperative that the greenhouse manager possess some knowledge of bacteria and fungi, and that he understand the functions and the requirements of the soil micro-organisms, if he wishes to secure proper control of his soil and to make it ideal for plant growth.

A. Bacteria. Bacteria are minute microscopical plants that consist of a single cell. They are composed of a cell wall of protoplasm and average about 1/25000 of an inch in length. These simple organisms multiply by fission, that is, the original mother cell divides in two equal parts, which may separate or remain united, giving the appearance of a thread. It has been estimated that a single bacterium divides about every twenty minutes. Granting that this rate of division is uninterrupted for twenty-four hours, the descendants of a single one within a day would be in round numbers 1,800,999 trillions. These when placed end to end would make a string two trillion miles long, or a thread long enough to go around the earth at the equator 70,000,000 times. However, multiplication at such a rate cannot occur because food conditions are restricted. The three main types of bacteria are : 1 . the cocci, 2. the bacilli or rods, 3. the spirilla or spirals (fig. 2, c). The greater number of the soil bacteria are beneficial, the most common being the saprophytes, or those which help to decay the dead organic matter from either animal or plant. The parasites on the other hand are those which produce disease.

B. Fungi. Fungi are low forms of microscopic plants, of a slightly higher type than bacteria. Fungi arc made up of colorless feeding threads technically known as hyphse or mycelium. The spores which correspond to the seed of the higher plants are borne

Fig. 2.

a. Effect of a balanced fertilizer on corn and clover, b, various organisms isolated from a soil particle, c. types of bacteria, Coccus, Bacillus and Spirilla (after P. E. Brown); d. pycnidium (after C. L. Shear), e. conidiophores of Penicillium.

either in sacs, known as pycnidia (fig. 2, d) or on free stalks, known as conidiophores, meaning stalk bearing spores (fig. 2, e.). Fungi, like bacteria, depend on animals or plants for their food. Like bacteria, they are differentiated into saprophytes and parasites.

Relationship of Micro-organisms to the Fertility of a Soil

Bacteriologists are continually engaged in discovering the possible function of numerous groups of the soil organisms. A recent exhaustive study * of Actinomyces, or thread bacteria, in the soil, for instance, seems to show that they serve to decompose grass roots, being more numerous in sod than in cultivated land. Other groups of bacteria undoubtedly perform other important functions.

The mere presence of friendly micro-organisms in the soil, however, would be insufficient to assure the welfare of our cultivated lands. These minute organisms must find the conditions necessary to induce a maximum activity in th^ performance of their work, which is to act as chief cook in the dietary of the plant. Most of the plant's food, as it is found in the soil, is in a crude and unavailable form. The bits of mineral matter, the manure, or fertilizer added to the greenhouse soil, all contain plant foods, but in a form which plants cannot readily

♦Conn, Joel H., New York (Geneva), Agr. Expt. Sta. BuL 52: 3-11, 1916.

use. They must be softened and predigested, and this work is done by the friendiy micro-organisms. The supply of plant food is therefore directly dependent on the work of these minute scavengers. An intimate relation exists between the higher and the lower form of plant life, the one depending on the other for sustenance.

A. Number of Micro-organisms in Soil. Investigations by Waksman * and others clearly show that micro-organisms are present in soils everywhere (see Table I and fig. 2, b).

It should be remembered that differences in the physical and chemical nature of the greenhouse soil, the sort of fertilizers used and the amount of temperature and moisture will all be important factors in determining the number of micro-organisms present.

Nature and Function of a Healthy Soil Flora

The function of a normal soil is to provide available plant food. About 95 per cent of the weight of a growing plant is made up of carbon, hydrogen, oxygen and nitrogen. The remaining 5 per cent constitutes the mineral or the non-combustible part or ash of the plant. Carbon, hydrogen, and oxygen are absorbed in the form of carbonic acid and water; nitrogen is usually derived from nitrates produced by micro-organisms out of organic matter in the soil. Neither the organic nor the mineral elements are in

The Healthy Soil ii

Table i

Actinomyces

Fungi

Source of Soil Used

Bacteria or Thread

Bacteria

7,202,000

711,000

313,000

8,257,000

611,000

375,000

10,133,000

900,000

925,000

New Jersey forest

2,088,000

1 20,000

218,000

2,600,000

150,000

Milltown bogs

185,000

33,000

Buckalew bogs

450,000

12,000

43,000

220,000

281,000

113,000

10,000,000

2,000,000

119,000

California fertilized

3,840,000

680,000

108,000

California unfertilized

6,444,000

356,000

36,000

7,900,000

1,400,000

400,000

Oregon white land

3,400,000

300,000

300,000

Porto Rico soil

2,140,000

960,000

300,000

2,070,000

933,000

30,000

North Dakota flax

1,730,000

263,000

23,000

4,335,000

665,000

76,000

6,035,000

566,000

330,000

2,125,000

574,000

30,000

2,440,000

1,560,000

230,000

Maine Aroostook loam

4,650,000

250,000

85,000

Maine Aroostook infested. . . ,

15,000,000

2,200,000

300,000

1,600,000

1,100,000

112,000 a form which plants can use. They must at first be acted upon by certain definite micro-organisms in the soil.

A. The Transformation of Carbon. Cellulose, which is but a form of carbon, constitutes a large per cent of the woody tissue of plants. Soils contain large amounts of cellulose and this undoubtedly helps to maintain their proper physical condition. It is found in large quantities in straw, manure, or in green vegetable matter. But because of its complex form, plants cannot make use of it, until it undergoes a certain decomposition. This is accomplished by a group of soil bacteria known as Amylobacter, which, feeding on the dead vegetable cellulose, break it up, and reduce it to carbon dioxide, hydrogen and fatty acids. The carbon dioxide either returns to the air to replenish the atmospheric supply, or it unites with water to form carbonic acid and soil carbonates. The carbon dioxide is taken by the plants either directly from the air through the leaves, or from the soil in some carbonate form. Thus we see that it is not the cellulose nor the product of its decomposition that furnishes plant food, but certain inorganic elements which are set free in its decomposition. ,

B. Elaboration of Available Nitrogen. From the viewpoint of plant nutrition, nitrogen is unquestionably the most important of all elements. The nitrogen of the air, although totaling about 79 per cent of it, is not in an available form. In the transformation of proteids into available nitrogen in the soil, three definite processes take place, all thanks to the work of certain soil micro-organisms.

1. Ammonification, In this process, the soil bacteria attack the complex proteids and convert them into ammonia. The odor of ammonia from decomposed urea, manure, or any other organic matter is always an indication that ammonification takes place. According to Sackett * and others the ability to bring about this change is attributed to the following soil bacteria: Bacillus mycoides^ Bacillus proteus vulgaris^ Bacillus mesentericus vulgatus^ Bacillus suhtilis^ Bacillus janthinus^ Bacillus coli commu' nis^ Bacillus megatherium^ Bacillus fluorescens liquefaciens^ Bacillus fluorescens putidus and Sarcina lutea.

Recent investigations by Waksman f and others indicate that certain classes of fungi are even stronger ammonifiers than are bacteria. Trichoderma Koningi and the Mucorales fungi were found to be strong ammonifiers. Fungi, too, are very strong cellulose decomposers. Further extensive investigations on soil fungi will no doubt more strongly establish their relationship to ammonification.

2. Nitrification. In order to be readily available for plants, ammonia and ammonia compounds must be changed still further into simpler compounds or, as the process is known, must undergo nitrification. The ammonia is first oxidized into nitrous acid and nitrates. This is accomplished by soil bacteria, Ni-

♦Sackett, W. G., Colorado Agr. Expt. Sta. Bui. 196: 3-39, 1916. t Waksman, A., Soil Science 2: 103-155, 1916.

trosomonas and Nitrosocoscus. The nitrates are then oxidized into nitric acid and nitrates, through the work of the bacterium, Nitrobacter. The nitrates are the only forms of nitrogen which plants can use.

C. Action of Soil Flora on Mineral Substances. Inert mineral substances, like the organic matter in the soil, must first be acted upon by certain soil bacteria to be converted to a form which plants can readily assimilate.

1. Changes of Phosphates. Phosphates as they commonly occur in nature are but little soluble in water. This is why they cannot be used in their first form, although they are required by most plants. Soils deficient in this element may be improved by such fertilizers as superphosphate of lime, ground bone, phosphate rock or Thomas slag. In the process of decomposition of organic matter a large quantity of carbon dioxide is liberated, which unites with the water in the soil to form carbonic acid. This acid attacks the insoluble phosphates, transforms them into superphosphates — the only form soluble in water, — and renders them available to plant life.

2. Changes in Potassium^ Sulphur^ and Iron. The carbon dioxide and other organic acids produced during the fermentation of organic matter, attack the potash feldspar which occurs in the soil. The product is potassium carbonate which is soluble in water and hence readily taken up by plants. The nitric acid which is formed during nitrification may also combine with the raw potash in the soil, forming potassium nitrate which is a form available for plants.

As a result of the activity of soil bacteria, hydrogen sulphate is evolved from the decomposition of proteids. The sulphur may be further changed into sulphur dioxide, and when combining wdth water and oxygen, into free sulphuric acid. The latter readily combines with calcium or magnesium, forming calcium or magnesium sulphate, from which the plant obtains sulphur for the construction of its proteids.

Chapter 2

When a soil is sick, either because its beneficial bacteria do not perform their functions properly, or because of abnormalities in its chemical or physical properties, careful treatment and proper cultural methods may restore it to health. But when a soil becomes sick and unproductive because parasitic forms gain a foothold in it, much greater skill and knowledge are required to cope with the problem. Its solution is of the greatest economic importance to the gardener and to the greenhouse man.

Parasitic fungi, upon finding their way into a soil, do not necessarily interfere with the work of the beneficial bacteria, such as the ammonifiers and nitrifiers, for instance. Nor do they always influence the chemical or physical nature of the soil. Many of them directly attack the crop itself, causing serious diseases in the plants.

This disease is 'very familiar to every grower of plants. It is peculiar to seedlings or tender plants, and is very prevalent in the greenhouse, the hot bed,

l6 the cold frame as well as in the field. It is induced by the presence of definite parasitic fungi, which thrive best in overwatered soils, and when the greenhouse is kept at a comparatively high temperature with poor ventilation. Damping off is also favored by thick sowing and too much shade in the seed bed.

Symptoms of Damping Of, Every experienced grower knows the disease when he sees it. Seedlings freshly damped off are soft and water soaked at the base of the stem. If they are pulled they often break off easily. A more careful examination shows that the root system is entirely decayed, although the upper part of the stem and leaves may still be green, and also possibly fresh. The degree of prostration in the seedlings is determined by the amount of moisture in the soil. If it is slight, the seedlings will become flabby and wilted before they topple over. With a high moisture content, they are more firm, but become prostrate as soon as infection sets in. The trouble usually begins in spots in the bed, thence spreading in every direction. Damping off is usually caused by several fungi, the chief of which is Fythium de Baryanum Hesse. The organism was first named and described by Hesse in 1874. Ward * found it to be a very prevalent parasite in the garden soils of Europe. In America the fungus was first recognized as of great economic importance by Atkinson. t The seedlings of most

*Ward, M., Quart. Jour. Micros. Soc. New Ser. 22: 487, t Atkinson, G. F., New York (Cornell) Agr. Expt. Sta. Bui. 94: 233-272, 1895.

greenhouse plants may become subject to damping off by Pythium. When examined under a compound microscope, Fythium de Baryanum is seen to be made up of coarse non-separate, highly granular, irregularly branched hyaline vegetative threads or mycelium. The younger growing threads are more finely granular. The oldest are coarsely granular or more often empty. These threads penetrate the cells of the host, where they obtain its food.

Pythium de Baryanum does not often fruit on the dead seedlings. The fmiting is better observed when the fungus is grown in pure culture. Under normal conditions it produces two forms of spores, conidia and oogonia. The summer spores, or conidia, are swellings formed at the tip of the hyphse (fig. 3, a.). These swellings readily break off from the mother threads and germinate by sending out a slender tube. This tube penetrates the seedling tissue where it grows and develops and after due incubation reproduces the disease. The oospore, or sexual spore, is the stage which is most commonly found. The female organ (oogonium) first develops as a terminal enlargement which is cut off by a septum from the mother thread. Next or adjacent to it a slender tube is cut off from the mycelium by a septum. This tube performs the function of the male sexual organ and is known as antheridium. The latter then comes into close contact and empties all its content into the female oogonium (fig. 3, b and c). Fertilization thus takes place, and a mature egg, or oospore, or winter resting spore is formed.

The latest investigations have not yet disclosed whether or not Pythium de Baryanum is carried over from year to year by its oospores. It is apparently able to propagate itself indefinitely by its vegetative mycelium.