The Citrus Industry, Volume II: The Production of the Crop

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THE CITRUS INDUSTRY

VOLUME II

PRODUCTION OF THE CROP

THE CITRUS INDUSTRY

VOLUME II

PRODUCTION OF THE CROP

Edited by

LEON DEXTER BATCHELOR

Professor of Horticulture and Director, Citrus Experiment Station,

University of California

and

HERBERT JOHN WEBBER

Professor Emeritus of Subtropical Horticulture, Citrus Experiment

Station, University of California; Formerly Director of the

Citrus Experiment Station, and Onetime Director of

the California Agricultural Experiment Station

WITH THE COLLABORATION OF

ALFRED M. BOYCE HOWARD S. FAWCETT WAYNE E. HARMAN ROBERT W. HODGSON MARTIN P. HUBERTY LEO S. KLOTZ RALPH G. LARUE

EDWIN R. PARKER MARVIN B. ROUNDS WARREN R. SCHOONOVER HARRY S. SMITH TRACY L STORER HENRY J. WILDER FLOYD D. YOUNG

First Edition

UNIVERSITY OF CALIFORNIA PRESS

BERKELEY AND LOS ANGELES

1948

UNIVERSITY OF CALIFORNIA PRESS

BERKELEY AND LOS ANGELES

CALIFORNIA

CAMBRIDGE UNIVERSITY PRESS

LONDON, ENGLAND

COPYRIGHT, 1948, BY

THE REGENTS OF THE UNIVERSITY OF CALIFORNIA

PRINTED IN THE UNITED STATES OF AMERICA

BY THE UNIVERSITY OF CALIFORNIA PRESS

AUTHORS OF CHAPTERS

LEON DEXTER BATCHELOR, Ph.D.

Professor of Horticulture and Director, Citrus Experiment Station, University of California, Riverside, California

ALFRED MULLIKIN BOYCE, Ph.D.

Professor of Entomology, University of California, and Entomologist in the Citrus Experiment Station, Riverside, California

HOWARD SAMUEL FAWCETT, Ph.D.

Professor of Plant Pathology, Emeritus, and Plant Pathologist, Emeritus, in the itrus Experiment Station, Riverside, California

WAYNE E. HARMAN

Meteorologist, Fruit Frost Service, United States Weather Bureau, Los Altos, California

ROBERT WILLARD HODGSON, M.S.

Professor of Subtropical Horticulture, Subtropical Horticulturist in the Experiment Station, and Assistant Dean of the College of Agriculture, University of California, Los Angeles, California

MARTIN RICHARD HUBERTY, Engineer

Professor of Irrigation and Irrigation Engineer in the Experiment Station, University of California, Los Angeles, California

LEO JOSEPH KLOTZ, Ph.D.

Professor of Plant Pathology, University of California, and Plant Pathologist in the Citrus Experiment Station, Riverside, California

RALPH GARRETT LARUE, B.S.

Associate Agriculturist, University of California, Assistant Farm Advisor of San Bernardino County, Agricultural Extension Service, San Bernardino, California

EDWIN ROBERT PARKER, Ph.D.

Horticulturist in the University of California Citrus Experiment Station, Riverside, California

MARVIN BIRD ROUNDS

Formerly Associate in the University of California Citrus Experiment Station, River side, California

WARREN RIPPEY SCHOONOVER, M.S.

Agriculturist, University of California Agricultural Extension Service, Berkeley, California

HARRY SCOTT SMITH, A.M.

Professor of Entomology, University of California, and Entomologist in the Citrus Experiment Station, Riverside, California

TRACY IRWIN STORER, Ph.D.

Professor of Zoology, University of California, and Zoologist in the Experiment Station, Davis, California

HERBERT JOHN WEBBER, PhD, D.Agr., LL.D., 1865-1946*

Formerly Professor of Subtropical Horticulture, Emeritus, University of California, and Director of the Citrus Experiment Station, Riverside, California, and onetime Director of the California Agricultural Experiment Station

HENRY JASON WILDER, A.B.

Specialist in Agricultural Extension, Emeritus, University of California, San Ber nardino, California

FLOYD DILLON YOUNG

Principal Meteorologist, U’nited States Weather Bureau, Pomona, California

* By the death of Herbert John Webber on January 18 1946. horticulturists throughout the world lost a beloved and respected leader whose scientific imagination enthusiasm, and resourcefulness were of inestimable value to the advancement of science His vision and persistent effort made the publication of this book possible Fortunately he lived to participate in the editing of most of the chapters in this volume which may serve as a fitting memorial to such a long and useful life i D B

PREFACE

E

ACH VOLUME of The Citrus Industry is complete in itself, and this fact necessitates the restatement in each volume of the general plan of the

I work. The first volume, History, Botany, and Breeding, was issued in 1943 (University of California Press, Berkeley and Los Angeles, xx + 1028 pages, 233 figures). The present volume, Production of the Crop, covers all the ordinary orchard practices employed in producing the crop: nursery methods, choice and use of rootstocks, planting, cultivation, fertilization, irrigation, pruning, protection against frost, treatment of diseases, pest control, and related subjects. The third volume, Harvesting, Marketing, and Utilization of the Crop, is in course of preparation; the date of issue, however, cannot yet be announced.

As was stated in the Preface to the first volume, the treatise is designed to present a comprehensive view of all phases of the great citrus industry, as a source of information and inspiration for growers, students, and investigators. Thus a special attempt has been made to present the material plainly, yet in a thoroughly scientific way, so that it can, in general, be understood by intelligent growers. The editors, however, have considered it fundamental that the scientific principles on which the various practices are founded should be explained. Some of the explanations in this volume may be too technical to be understood thoroughly except by specialists.

Here, as in Volume I, the different chapters are prepared by authors chosen because they have special knowledge of the subjects treated and have investigated them intensively. There is naturally some overlapping of the different subjects, and—as occurs in all fields of science—some of the phenomena presented may be subject to different interpretations. The editors have at tempted, so far as seemed desirable, to unify the statements; but each author, as an investigator, is entitled to his own opinion, and thus some slightly conflicting statements—of no more than minor importance, however—may be found in different chapters. Each author is responsible for the material presented in his own chapter.

In this volume, as in Volume I, the plan has been to present the matter as applicable anywhere in the citrus-growing world. Since the text is prepared in California by California investigators, it is inevitable that emphasis is placed on California practices; but this circumstance may render the treatment even more valuable to producers and specialists in other countries who are already familiar with the methods employed in their own regions

Two chapters in this volume deal with rootstocks. The first of these, chapter ii, discusses the rootstocks commonly used and their reactions. It is a general summary of our understanding up to the present, based, in large part, on trial and error methods. The relation of rootstocks to the spread or prevention of various virus and fungus diseases has stimulated, in all citrusproducing countries, an intense interest in rootstock problems, and thus it has seemed to the editors that this volume should contain a rather full statement of the results obtained in the rootstock experiments of the California Citrus Experiment Station, which have been in progress for a quarter century. These are probably the most extensive and longest-continued experiments ever conducted with citrus rootstocks, and the results obtained have not been fully published. Chapter iii outlines, in the main, the results and conclusions thus far reached. Some data from them are also used in chapter ii to illustrate specific items; and there is thus some little repetition in the two chapters. There are in the data here presented (chaps, ii and iii) many seeming inconsistencies that cannot at present be entirely accounted for: different reactions under different conditions as yet little understood. Much more extensive data must be acquired before the inconsistencies can be explained.

Chapter xi, Diseases and Their Control, by Howard S. Fawcett and Leo J. Klotz, is designed to summarize and supplement the much fuller treatment of this subject published by Dr. Fawcett in a special voume (Citrus Diseases and Their Control, McGraw-Hill Book Company, New York and London, 1936; 656 pp., 187 figs.). The subject of chapter xiv, on Insects and Mites and Their Control, by A. M. Boyce, is more fully treated in a special text (Quayle, Henry J., Insects of Citrus and Other Subtropical Fruits, Comstock Publishing Co., Ithaca, N.Y., 1938; 583 pp., 577 figs.). We are indebted to Professor Quayle for many helpful suggestions in the preparation of chapter xiv and for the use of many of his photographs. The general form of this chapter follows much the same composition as the special text on this subject. The two books referred to thus constitute, in effect, volumes in this series on the citrus industry and should be in the libraries of all citrus growers and technicians.

The editors wish to express to the authors of chapters in this volume their heartfelt thanks for the kind and efficient cooperation they have given to the task of preparing and editing their chapters; without such assistance the work could not have been satisfactorily completed. It is also fitting that we should acknowledge the generous support given to this project by the authorities of the University of California, under whose general direction the work of the editors and most of the authors of chapters is conducted. We wish especially to thank Dr. Claude B. Hutchison, Vice-President of the University and Dean of the College of Agriculture, and Mr. Samuel T. Farquhar, Manager of the University of California Press, who have at all times given encouragement to the editors in their work.

It is a special pleasure and privilege, also, to express to our colleagues, the various members of the staff of the Citrus Experiment Station, our appreciation for their continued support and assistance. Almost every member of the staff has rendered some assistance in the preparation of this work, either by supplying ideas, material, or photographs, or by reading manuscript or proof

The drawings and photographs used throughout the text are to be credited to the authors of the chapters in which they occur, unless other credit is specifically given. A number of colored illustrations used in chapter vii, Principles and Methods of Fertilization, were originally made by the Florida Agricultural Experiment Station. Through the kind cooperation of the National Fertilizer Association, plates were lent us for reproducing these colored illustrations. The editors are also indebted to Dr. A. F. Camp, Director of the Florida Citrus Experiment Station, for many valuable suggestions. The drawings illustrating chapter i, Nursery Methods, were made by Mrs. Lucene Hardin Webber (Mrs. H. J. Webber, deceased).

The literature citations given at the end of each chapter in this volume have all been checked and edited by Miss Margaret Buvens, Librarian of the California Citrus Experiment Station. The manuscripts of chapters have been read and edited by Mrs. Frances Hayes, Publications Editor, and by Mrs. Helen Freeland, Administrative Assistant, both of the California Citrus Experiment Station. Mrs. Freeland has also assisted greatly in the mechanical details of preparing the manuscripts. Lastly, much of such merit as the publication may be found to deserve is due to the able assistance of Mr. Harold A. Small, Editor of the University of California Press, in guiding the work into print. To all these and to many others not specially mentioned who have supplied data, or otherwise aided in the work, the editors wish to express their thanks.

L. D. B.

H. J. w.

Riverside, California

September 1, 1948

CONTENTS 1

CONTENTS 1

CHAPTER I NURSERY METHODS BY HERBERT JOHN WEBBER

CHAPTER II ROOTSTOCKS: THEIR CHARACTER AND REACTIONS BY HERBERT JOHN WEBBER

CHAPTER III CHOICE OF ROOTSTOCKS BY LEON D. BATCHELOR and MARVIN B. ROUNDS

CHAPTER IV SELECTION OF ORCHARD SITE BY EDWIN R. PARKER

CHAPTER V PLANNING AND PLANTING THE ORCHARD BY RALPH G. LARUE and MARVIN B. ROUNDS

CHAPTER VI CULTIVATION, OR TILLAGE BY WARREN R. SCHOONOVER and LEON D. BATCHELOR

CHAPTER VII PRINCIPLES AND METHODS OF FERTILIZATION BY LEON D. BATCHELOR

CHAPTER VIII COVER CROPS AND GREEN MANURES BY LEON D. BATCHELOR

CHAPTER IX PRINCIPLES AND METHODS OF PRUNING BY ROBERT W. HODGSON

CHAPTER X PRINCIPLES AND METHODS OF IRRIGATION BY MARTIN R. HUBERTY

CHAPTER XI DISEASES AND THEIR CONTROL BY HOWARD S. FAWCETT AND LEO J. KLOTZ

CHAPTER XII BIOLOGICAL CONTROL OF INSECT PESTS BY HARRY S. SMITH

CHAPTER XIII BIOLOGICAL CONTROL OF CITRUS INSECTS BY PARASITIC FUNGI AND BACTERIA BY HOWARD S. FAWCETT

CHAPTER XIV INSECTS AND MITES AND THEIR CONTROL BY ALFRED M. BOYCE

CHAPTER XV QUARANTINE AND QUARANTINE SERVICE BY HARRY S. SMITH

CHAPTER XVI INJURY BY RODENTS AND ITS CONTROL BY TRACY I. STORER

CHAPTER XVII PROTECTING THE CITRUS ORCHARD AGAINST FROST BY FLOYD D. YOUNG AND WAYNE E. HARMAN

CHAPTER XVIII EFFECTS OF FREEZES: TREATMENT AND RECOVERY OF INJURED TREES BY HENRY J. WILDER

INDEX

CHAPTER I

NURSERY METHODS

BY

HERBERT JOHN WEBBER

T

HE METHODS1 used in the propagation of citrus trees do not differ materially from those employed with other fruit trees. The principles are the same for all plants; but experience has shown that for citrus some methods are preferable to others. The nursery methods now practiced in this industry are about the same the world over.

Fifty years ago, the first and most difficult decision to make was whether to use seedlings or budded trees (Webber, 1897, p. 471). Fortunately, this matter is no longer in question. It is now well understood in all citrus-growing regions that the building of a successful commercial citrus industry must rest on the propagation and planting of only selected, disease-free varieties known to be suited to the region and to produce fruit of known good quality. Since the best commercial varieties do not reproduce true by seed, they must be propagated by budding or grafting, by cuttings, or the like.

In general, the propagation of citrus varieties is accomplished by budding into seedlings of species known to give good results when used as rootstocks. It is highly important to use rootstocks known to be adapted to the variety to be grown and to the region and soil on which the trees are to be planted (see chaps, ii and iii, below).

Budding is by far the most generally used method of propagation. Grafting apparently gives just as good results, but it is more difficult and slower. Sometimes propagation is effected by means of layers, marcotts, and cuttings; but these methods are at present little used in the principal citrus regions, though they apparently have a definite place in the industry in some sections.

The propagation and growing of trees in a nursery is highly specialized, intensive work, and only those who give careful attention to all phases of the subject are likely to obtain satisfactory results.

SEEDS FOR ROOTSTOCKS

KIND OF SEED TO BE USED

The seeds most commonly used for citrus rootstocks in the United States are those of the sweet orange, sour orange, grapefruit, shaddock, Rough lemon, and trifoliate orange. In other citrus-growing countries different species or varieties are sometimes used, as, for instance, the sweet lime in Palestine, the yuzu orange in Japan, and the Calamondin and mandarin in the Philippine Islands. The stock to be used for any particular variety in a given place must be carefully chosen in accordance with what is known to be good practice in that place (see chaps, ii and iii, below). A stock successful with a certain variety in one place may not be satisfactory in another.

In the determination of the stock to be used, emphasis has heretofore been placed on the species; it was thought to be sufficient merely to decide between sour orange, sweet orange, or grapefruit, and the like. It is now generally recognized that the seedlings produced by different varieties of any of these species will themselves differ and that it is highly important that the seed for growing stocks should be taken from selected trees of the species to be used. The evidence thus far available indicates that this is very important where the sour orange, sweet orange, or grapefruit is used; it is less important where varieties such as Rough lemon, Palestine sweet lime, or Sampson tangelo are to be employed, since they are known to reproduce remarkably true to type through seed propagation.

Until special rootstock varieties are available for propagation, especially to produce seed for nursery purposes, it will be necessary to take the seed from selected healthy trees of the species chosen. In California, good trees of rootstock types known to be free from the virus of scaly bark are being chosen and registered by state agencies as approved sources for rootstock seeds.

The reactions of various stocks on different varieties of citrus and the methods of stock selection are fully discussed in the next chapter.

SOURCE OF SEED

The seed for use in each country or each citrus region is generally obtained from local trees. Citrus seed very soon loses its vitality if dried; it is thus usually impractical to obtain seed in bulk from any great distance. Most of the sour orange seed that for many years has been extensively used in California for nursery purposes is produced in Florida and Cuba2 —probably a unique situation; at any rate, the writer does not know that the citrus industry elsewhere depends in any marked degree on importations of seed for its nurseries. Most of the Rough lemon seed for nursery use in California is imported from Florida, but only small quantities are used.

In almost all citrus-growing countries there are trees enough of the kinds desired for stocks to assure a local supply of seed from known good trees. Nurserymen should propagate and grow a sufficient number of trees of the best stock varieties to produce, for themselves, a supply of seed of known varieties and of known freedom from disease. They will thus be able to demonstrate to growers the type of stocks they are using, a matter likely to become increasingly important.

Whatever species or variety may be used for stock, the seeds should always be taken from good, fully matured fruits grown on vigorous, healthy trees.

Very few diseases are known to be transmitted through the seed or on the seed, except to a very limited degree; but in any case, seeds from healthy trees are safer to use and are generally more vigorous than those from unhealthy or inferior trees. The seeds from frost-injured fruits are evidently as good to use as those from unfrosted fruits. This was found to be true with frozen fruit in Florida after the very severe freeze of 1894-95.

EXTRACTION OF SEED, AND AMOUNT NECESSARY

The most common method of extracting the seed is to separate the fruits into halves by making a shallow cut through the skin and twisting them apart. The contents are then squeezed into a large sieve having a mesh coarse enough to let the juice and pulp be washed through. Another common method is to place the fruits in barrels, cover them with water, and permit them to stand until the soaking and rotting softens the peel so that the whole fruits can be thrown into a large sieve and the seeds washed clean of pulp. Some nurserymen have devised threshing machines that tear the fruit into shreds, wash the seeds out, and separate them from the pulp; but such machines are not regularly manufactured.

It is important to provide a larger number of seeds for planting than the number of seedlings desired. In the majority of citrus species and varieties the seeds are polyembryonic, and frequently from two to three seedlings develop from one seed. The number of seedlings obtained may, indeed, exceed the number of seeds planted; but this rarely occurs. Usually, a stand of one- half to three-fourths as many seedlings as seeds planted may be considered a fair return. The small, slow-growing seedlings should be discarded, a process which further reduces the number. It is thus safe to say that at least twice as many seeds should be planted as the number of seedlings desired.

As a guide to the number of fruits required to supply the planting seed desired, one fairly accurate method is to make an estimate of the average number of seeds per fruit by counting the seeds in from twenty-five to fifty fruits of the stock variety. The number of seeds produced by the same variety may vary from one year to another, and accurate methods of calculation are therefore desirable.

Seeds of different species of citrus also vary greatly in number per fruit and in size (see Vol. I, fig. 191, p. 799), and the estimate of seed number for one species has no relation to the numbers to be expected in other species or m different varieties of such species.

Citrus seed for nursery use, especially of the sour orange, is frequently harvested in quantity and sold in bulk by the pound or bushel. Estimates of the number of seeds for such units of measure are so variable as to be of little value. Seed supplied in large quantities in bulk is almost invariably taken from unselected trees, and the use of such seed of unknown heritage for rootstocks should be discouraged.

Citrus seeds of the same variety vary greatly in size, even in the same fruit, and there are many rudiments with undeveloped embryos. Some growers have taken the trouble to separate and discard the small seeds; but the rudimentary seeds in which the embryos are not developed are light and most of them can be floated out when the seeds are in water and being washed clean of pulp. This is the only separation that need be made. A separation based on size or weight is of no value with citrus seeds, for a small seed may have one large embryo whereas a much larger and heavier seed may have several embryos none of which is as large as the one embryo in the smaller seed.

STORAGE OF CITRUS SEED

If citrus seed is allowed to dry, it soon loses its vitality. If the seed is to be kept for some time, say several months, it should be surface-dried quickly after being thoroughly washed, and should then be mixed with equal parts of ground charcoal, packed in a tight wooden box or tin container, and kept in a damp, cool place, preferably between 38° and 55° F.

Sweet orange seed and grapefruit seed are somewhat more quickly injured by drying than the seed of the sour orange or the Rough lemon; but although seeds of different species may show some difference in susceptibility to injury by drying, this difference is slight, and no citrus seed can safely be kept for long. Sometimes the seed is harvested in the early winter, stratified in clean sand (instead of ground charcoal), and kept through the winter in a cool, damp cellar. This is not a practice to be generally recommended.

THE SEEDBED

LOCATION AND ARRANGEMENT

If only a few seedlings are to be grown, the seeds may be planted in boxes of convenient size, containing soil 8 to 10 inches in depth. The boxes should be put in a warm, partly shaded place, and the soil should be kept moist but not wet. If large numbers of seedlings are to be grown, special seedbeds should be prepared in comparatively warm places on good, well-drained soil. Almost any type of spil may be used, but a rather light sandy loam is to be preferred. It is desirable to use virgin soil, if that is obtainable, or soil which has not for several years been used for the growing of citrus seedlings or vegetables. If a seedbed is grown on the same land for several years in succession, the soil is likely to become so much infected with damping-off fungi as to cause serious loss. It is further desirable to select a site at some distance from other citrus plantings, in order to reduce the danger of infestation with pests.

The soil, if fairly rich, may not require fertilization, but on light soils that are deficient in plant food some fertilization should be given before planting. For this purpose commercial fertilizers, rich in nitrogen, such as are recommended for young trees and vegetables, should be used. In California an application of some soluble nitrogen carrier, such as sulfate of ammonia or nitrate of lime, is usually all that is required. About the same quantities should be used as would be applied in the fertilization of an area of equal size in a mature citrus grove.3 The use of organic manures on seedbeds is not usually considered safe, owing to the greater probabilities of introducing damping-off fungi. Any fertilization given before the seeds are planted should be spread and thoroughly mixed with the soil by tillage a month or more before the seeds are planted. Young seedlings are frequently burned and injured if the seeds are planted too soon after the application of the fertilizer. Land for planting should be thoroughly and deeply tilled and put in good physical condition. If too loose, it should be firmed with a light roller. It is then ready for planting.

In all citrus regions, seedlings are generally grown under some sort of shed (fig. 1), in order to provide a certain amount of protection from the full rays

of the sun, to check the evaporation of moisture from the plants and the soil, and also to serve as a protection against injury from freezing. They make a better growth when thus protected. In California such sheds are usually covered with building lath nailed to boards in standard-sized sections, with open spaces of equal width between laths. In this way a half shade is provided. The laths are usually run north and south in order to provide a more uniform distribution of the sunlight exposure and to prevent burning of the foliage.

In Florida similar sheds are employed, but boards (1 in. by 3 in. by 16 ft.) are generally used instead of lath for the cover, the boards being placed 3 inches apart to give the half shade. Sheds covered with a thin cloth, such as cheesecloth, are also sometimes used.

It is by no means necessary, in humid countries, to place the seedbeds under shade. Some nurserymen plant part or all of their stock in the open without shade. If this is done, it is necessary to give more careful attention to the water ing during the early period of germination and growth, er the proportion of seedlings obtained is likely to be much lessened. Unshaded seedlings in California rarely give as good a growth as those in sheds. On the other hand, one of the most experienced nurserymen in Florida states: "The better plan is to provide ample irrigation and grow the seedlings in the open, i.e., without shade, and sow the seed late enough to escape frost injury. In Florida, experience has shown that in this way excellent seedbeds may be grown on the same soil for many years, and, if careful attention is given to irrigation when the seedlings are coming through the ground, there is no danger of having the crooked stems [i.e., when just coming through the soil] injured by sun and wind." (Hume, 1926, p. 166.)

There are many ways of arranging seedbeds. In all, however, care must be taken to provide for tillage, and if the beds are in a lath house the arrangement must be adapted to the type of irrigation to be used and to the spaces between the posts.

When furrow irrigation is employed, it is a common practice to plant the seeds in long beds or strips about 10 to 12 inches wide, with spaces of similar width between the beds to provide for the irrigation furrows and for tillage. Approximately every sixth bed or row is omitted to allow for a pathway to facilitate working around plants. This, with slight variations, is the common practice in California in lath-house culture. If facilities for irrigation by sprinkling are available, seedbeds 3 to 4 feet wide may be planted, with paths 12 to 18 inches wide left between them to make cultivation easier (fig. 1). Almost any arrangement of beds may be used, if satisfactory provision is made for irrigation and tillage.

If seedlings are to be grown on a very large scale in the open, they may be planted in rows or narrow beds with a 21- to 31-foot space between them to provide for cultivation.

PLANTING AND GERMINATION OF SEED

The seed is usually planted in the early spring after the danger from frosts is past. In both California and Florida most of the planting is done between early March and late April, though planting may be done during any of the spring or summer months, whenever the seed is available. It is preferable to get the seed planted fairly early in order to have the benefit of the full growing season. The only exception, among the species now used as stocks, is the trifoliate orange, the seed of which is planted in October or as soon as the fruit is fully ripe.

Citrus seeds will start to germinate when the soil temperature somewhat exceeds 55° F., which is approximately the zero temperature of growth (see Vol. I, p. 58). The temperature of the soils of southern California commonly reaches this point about the first of March. If planting is too early, a warm period, which causes germination to start, may be followed by a cold period, and the seed is then likely to rot in the ground. Good germination is usually obtained when the mean daily temperatures range between 58° and 65° F., and where citrus is grown in California these temperatures are usually reached in the middle or latter part of March or in April. Under conditions permitting the temperature to be controlled and kept uniform, optimum germination is obtained at a temperature of approximately 80° to 90° F. (see Vol. I, pp. 56-58; also Fawcett, 1929; Camp, Mowry, and Loucks, 1933).

The seed, if not freshly extracted from the fruits, is often placed in moderately warm water and allowed to soak for twenty-four hours or more before planting Some nurserymen incubate the seed in warm, moist chambers for several days before planting, mixing the seed with an equal amount of moist sand and holding it in a moist chamber at a temperature of about 90° F. The object of these measures is twofold: first, to hasten the process of germination, and second, to prevent the development of gooseneck or bench root in the seedlings.

Frequently, a rather large proportion of the seedlings will show a more or less S-shaped curve (gooseneck or bench root) in the young root-stem or hypocotyl (fig. 2) as they emerge from the seed, and this is thought to be injurious to the future growth of the seedling. Ralston (1915) published data indicating that this bending of the hypocotyl is caused, to an appreciable degree, by the resistance which the dry seed coats present to the emergence of the young roots. The proportion of bench-root seedlings formed was considerably reduced by soaking the dry seeds in water for 24 to 36 hours before planting, or by planting fresh seed direct from the fruit. Nasharty,4 however, has found that the crowded nucellar embryos developed in citrus seeds frequently grow in such positions that the hypocotyl (root end) may be turned at various angles away from the micropylar end of the seed, from which the roots emerge in germination, and that the abnormal position of the embryo may cause various degrees of curvature or even complete turns to be made by the roots as they emerge. The gooseneck formed in the roots of such embryonic seedlings apparently could not be much influenced by the soaking and softening of the seed coats. .

Experiments conducted by several nurserymen, and observed by the writer, seem to show that the gooseneck condition exhibited by seedlings is commonly not so abnormal as to result in permanent injury. Apparently, only very severe cases of gooseneck are to be considered as injurious (fig. 2), and seedlings showing such abnormalities should be discarded when they are dug from the seedbed.

Citrus seed is ordinarily planted by hand, the seeds being placed about 1 inch apart each way in the beds, and from 34 inch to 1 inch deep. A method commonly followed is to spread the seed as thickly as desired on the surface of the soil in the beds and then to press them into the soil by placing a board over them and walking on it. This presses the seed down firmly in the soil in the normal position—they should lie on their sides as they naturally fall. The bed is then covered to the desired depth, about 34 inch, with a layer of clean river sand. The ground should always be moist at the time of planting. The seed may also be sown in shallow furrows and afterward covered with ordinary soil, if sand is not available. The sand covering helps to prevent both the spread of damping-off fungi and the formation of a surface crust which might hinder the emergence of the tender young shoots. Under the most favorable conditions the young shoots may appear above the ground in two weeks, but usually a month or six weeks is required.

CARE AND TILLAGE

The seedbed must be handled carefully if it is to be maintained in a healthy condition. It must be kept free from weeds and just moist enough to encourage growth. An overmoist condition which would encourage the development of damping-off should be avoided. The fungi producing the disease known as damping-off cause more damage in seedbeds than any other disease-producing agents. They are likely to be present in soils containing decaying organic matter, and to infect the young shoots at or near the surface of the soil. They spread very rapidly in the soil, under favorable conditions, and it is not uncommon to find that the plants on a sizable area have withered in a single night.

Conditions favorable to damping-off are wet soil, dense shading, humid atmosphere, and overcrowding. The control methods that can be used successfully are mainly preventive, such as irrigating at less frequent intervals and using a clean sand covering for the seeds. The sand covering will allow the surface of the soil, where infection most frequently occurs, to dry off quickly after irrigation. If an infection appears, a thorough, shallow cultivation should be given in order to dry off the surface soil, and the shade should be reduced if it is too dense. The seedlings may also be pulled up for a space of a foot or more around the diseased spot and thinned out in a circle several feet in diameter. Spraying with bordeaux mixture or treatment with Semesan is also advocated by some.

Weindling and Fawcett (1936) in experiments with Rhizoctonia solani, one of the most common damping-off fungi attacking citrus seedlings in Cali fornia, found that applications of aluminum sulfate gave a marked degree of control. The chemical was applied to the surface of the soil before seeding, at the rate of 35 grams per square foot, and raked into the top inch of the soil. By this means the loss was reduced from more than 31 per cent, as shown by untreated plots, to 1.7 per cent for the treated plots. The effect of the treatment was explained as primarily due to the action of the aluminum sulfate in increasing the acidity of the soil and thus favoring the growth of a secondary parasitic fungus, Trichoderma, commonly present in soils, which attacks and destroys the Rhtzoctonia fungus.

A similar control was obtained by covering the seed to a depth of about 34 inch with peat. Neither of these treatments has as yet been commonly used in commercial nurseries.

A type of gummosis due to the brown rot fungus sometimes causes damage in the seedbed after the seedlings have reached a height of 8 inches or more. It appears as an exudation of gum just above the ground. If the disease is found to be prevalent, the plants should be sprayed with weak bordeaux mixture (2-2-50), care being taken to cover the stems near the ground.

Reichert and Perlberger (1936) and Reichert (1938) have made extensive studies of the diseases causing injury in citrus seedbeds in Palestine. Twentyeight different diseases were identified, and losses of from 30 to 90 per cent of the plants were recorded. The most important diseases were found to be fungus root rots caused by species of Rhizoctonia, Sclerotinia, and Phytophthora, and a physiological disorder, albinism. The best method of control for these fungus diseases was found to be: (1) the disinfection of the seed by soaking for 30 minutes in a solution of U’spulun or Ceresan, strength 1: 1,000; (2) germination in cold seedbeds; (3) daily airing f removal of covering shade for short period]; and (4) weekly spraying with bordeaux mixture, strength 0 5 to 1 0 per cent [2-2-50 to 4 4- 50 mixture].

Albinism, or the production of white, chlorophyl-lacking seedlings, which caused an estimated loss of 30 per cent in seedbeds of sweet lemon (the Palestine sweet lime) in Palestine (Reichert and Perlberger, 1936), was eliminated by disinfecting the seeds with a solution of Ceresan, strength 1: 1,000. With sour orange seeds, a disinfection with 1:2,000 solution gave complete control.

After the seedlings reach a height of 4 or 5 inches, they are not so susceptible to attacks of damping-off and may then be irrigated rather more liberally to force the growth. Where the soils lack plant food, several light applications of fertilizer may be made during the season. Frequent shallow cultivations should be given and all weeds should be removed.

The seedlings are usually allowed to remain in the seedbed for one or two years, or until they reach a diameter of about %6 inch to 14 inch. In many places this will require two years, but where the soil is rich and the temperature is continuously high they will attain the required size in one year. It is poor policy to transfer seedlings to the nursery before they reach the proper size, as their care is less expensive in the seedbed. Little saving in time is made by their transfer earlier, as it is not advisable to bud them until they reach a diameter of about % inch. Some very rapidly growing rootstocks, such as the Rough lemon, may attain the proper size earlier than seedlings of sour and sweet orange, which are slow growers. It should be said that some nurserymen bud their trees when they are much smaller and rarely or never leave the trees more than one year in the seedbed and one year in the nursery before budding.

DIGGING AND SELECTION OF SEEDLINGS

The seedlings are dug from the seedbed just before they are transplanted to the nursery. The beds should be thoroughly irrigated several days before the digging, so that the soil will be in good condition.

The seedlings are dug from the bed by lifting them with a spade or spading fork. If they are comparatively small, the best plan is to use a spading fork to loosen the earth, after which they can be lifted out by hand. If, however, they are as large as they should be after two years in the seedbed, it may be necessary to use a sharp spade to cut the taproots about 8 to 10 inches below the surface and loosen the plants before lifting them out by hand.

As the plants are dug they should be placed in tubs, with their roots in water, for removal to the packing shed for grading and preparation. The packing shed should be well protected from drying winds and should exclude sunlight. If a moderate wind is blowing, a canvas should be thrown around the packing shed.

In the packing shed the seedlings should be examined and the good ones selected for planting in the nursery. All small, diseased, or imperfect plants should be discarded. Too much emphasis cannot be placed on the importance of rigid selection at this time. Most of the dwarfish, off-type, variant seedlings, worthless as stocks, will be eliminated if the small seedlings are thrown out. (For full discussion see chap, ii, p. 139.) All seedlings showing any disease should also be discarded, as well as any showing severe gooseneck (bench root) or other imperfections. If this selection is properly carried out, probably a fifth to a fourth of the seedlings will be discarded at this time.

After the seedlings are graded, the tops and roots are cut to the required length. The roots are usually cut back to a length of about 8 inches, and about one-third of the top is removed. The seedlings are then packed in pails or tubs, with the roots in water, for removal to the nursery; or they are bunched and packed in boxes with damp sphagnum moss or sawdust for shipment. Everything must be done quickly, for citrus plants are easily injured by drying of the roots.

THE NURSERY: ITS ARRANGEMENT AND CARE

WHY PLANT IN A NURSERY?

Seedlings are usually transplanted to a special nursery in which they are grown close together until they reach the proper size for budding. They are then budded, and the buds are forced and grown into the standard nursery trees.

Growers who produce their own trees sometimes transfer the seedlings from the seedbed direct to the orchard and there do the budding at the appropriate time. The trees may then continue their growth without the interruption of a second transplanting from the nursery to the orchard. But this means that one must walk over, cultivate, and irrigate an acre of orchard land instead of the 100 feet of a nursery row. Furthermore, the loss of a seedling in transplanting or budding is much more serious in the orchard than in a nursery row. Experience has shown that little time is saved by direct transplanting to the orchard, and the greatly increased expense and hazard resulting from this method render the use of a nursery desirable.

SELECTION OF NURSERY SITE

The nursery should be in a warm place where the danger from frost injury is reduced to a minimum. It should preferably be on a deep, rich, fine sandy loam or loam soil, free from stones and containing clay enough to make the soil hold together and to permit balling, if this method of transplanting is to be used. If the trees are to be balled, it is important that the nursery soil be of the same texture as the soil of the grove into which the trees are to be planted; or, if not the same texture, then lighter. If the nursery trees are to be transplanted bare-root to the orchard, it probably does not much matter what type of soil is used in their production. However, Nusbickel has stated: It is my belief that stock raised in a light soil will give many more fibrous roots than that raised in a heavy soil, and that a tree which is to be transplanted ‘bare root’ should have the fibrous roots which a sandy or loam soil produce.5

If the soil is not rich enough, it may be improved by a cover crop turned under six weeks or more before the seedlings are planted, or by a dressing of well-rotted manure or commercial fertilizer turned under several weeks before the seedlings are planted. The ground should be plowed deep and should be thoroughly tilled before planting.

PLANTING THE SEEDLINGS

In California the seedlings are transplanted to the nursery in early spring, preferably in March or April. In Florida the ordinary time of transplanting is in November or December, but the seedlings are often planted in summer, when rains occur (Hume, 1926, p. 167).

The seedlings are set in straight rows, carefully laid out from 312 to 4 feet apart, with the plants spaced from 12 to 15 inches apart in the rows. A planting line with the proper spacing marked on it may be used to insure good results. A three-wheeled marker with blocks attached for proper spacing drawn down the line will speed up the planting.

The holes in which the seedlings are to be planted are usually made by thrusting a nursery dibble (fig. 3) or spade into the ground and pressing the soil apart so as to leave an opening of the desired depth. The root of the seedling is then placed in the opening and held erect at approximately the same height as in the seedbed while the soil is pressed firmly around it with the foot. In placing the seedling in the hole, it is highly important that the young and flexible main root should not be pushed down with the end bent upward; it should be kept straight so that it will grow in a natural position. Some care is also necessary to make sure that the soil is pressed firmly around the base of the root as well as around its top. In arid countries irrigation should follow the planting in each row as soon as possible.

In the employment of one method of planting in California, irrigation furrows are so made as to mark the planting rows. Water is run into the furrows for a short time before planting, in order to moisten the dry and crumbly surface soil so that it will not rattle into the dibble holes. The seedlings are then planted, and the soil is firmed around them, after which the water is again run down the furrows to firm and settle the soil thoroughly around the plants. Since the water should be run in direct contact with the seedlings, it is best to plant in shallow furrows and place the seedlings slightly higher than would be done in level soil.

In good nursery soil that is not too sandy the planting may be done quickly by opening up a 10-inch-deep furrow with a plow having a good sharp colter. The seedlings are then held in place against the clean vertical cut face, and the earth is filled in around them. The water should follow close behind the filling-in.

In regions where irrigation is not used, planting should be done when the soil is moist, and if rain does not follow soon, the seedlings should be watered to firm the soil and give them a proper start. In most citrusgrowing countries it is difficult to grow good nurseries unless facilities for irrigation are available.

CULTIVATION, IRRIGATION, AND FERTILIZATION

The nursery must be kept thoroughly cultivated and free from weeds at all times. In arid, irrigated sections, water should be applied about every 10 to 15 days. In heavy soils less frequent irrigation may be required. In some very rich soils no fertilization is required, but fairly heavy fertilization usually improves the growth. In California the general practice is to use some form of nitrogen, such as nitrate of lime, nitrate of soda, sulfate of ammonia, or dried blood, applied at a rate to give from 200 to 300 pounds of actual nitrogen per acre. The general policy is to put this on in two equal applications, one in early spring and the other in midsummer. In Florida, complete commercial fertilizers rich in nitrogen, such as are recommended for vegetables, are commonly used, and are given in two or three applications.

Fertilizers are spread broadcast on the soil between the rows and are cultivated in, unless they are in the form of soluble materials such as nitrate of lime or sulfate of ammonia. These materials may be spread in the freshly opened irrigation furrows to be carried down by the water, which is turned on and allowed to trickle slowly through the furrow. In California, such soluble materials are sometimes applied in the irrigation water through the use of special applicators connected to the irrigation system. Every effort should be made to keep the seedlings growing vigorously.

If the seedlings are of good size, about 14 inch in diameter, when transplanted to the nursery, they should reach the best size for budding by the next fall, or within about 6 months. After insertion, the buds should produce good-sized trees suitable for transplanting by the second spring following, which will be about 18 months after the buds are inserted.

The best-sized trees to bud are those that have reached a diameter of from % inch to 12 inch, from 4 to 8 inches above the ground.

BUDDING THE NURSERY SEEDLINGS

WHEN TO BUD

Budding can be done whenever the nursery seedlings reach the proper size and are in such a growing condition that the bark will slip to allow the insertion of the buds. A decade or two ago, budding was most commonly done in the early spring as soon as the bark would slip; but this is generally the rush season for all grove and nursery operations. The seedlings that are ready to bud in the early spring could have been budded the fall before, as there is little growth during the winter. Fall budding, or dormant budding, as it is called, has thus become almost the universal practice in commercial nurseries. Two factors favor fall budding. First, the major operations may be carried out in the early fall, from September to November, when there is a slack labor period. Second, seedlings budded in the fall can be cut back in the early spring just before growth starts. All the growth of the season is thus forced into the bud, and the result is a larger season’s growth than can be got from spring buds.

The principle in fall budding is to insert the buds and get them healed on just before the arrival of the winter’s cold, so that they will remain dormant during the period of arrested development of the rootstock trees. Since the growth stops and the bark sets at different times, because of varying climatic conditions and variation in stock varieties used, careful attention must be given, in any locality, to the time of budding. At Riverside, California, fall budding is usually done between the 10th and 20th of October, but the period varies slightly in different seasons. Trifoliate orange seedlings frequently must be budded earlier. In the Oroville section of northern California, fall budding must usually be done ten days earlier than at Riverside. In Florida it can usually be done successfully as late as November.

PREPARATION OF SEEDLINGS FOR BUDDING

In order that seedling development may be as large as possible, little pruning is done in the nursery. Pruning tends to retard the growth and, if severe, greatly reduces the size of the seedlings and prolongs the period necessary to hold them in the nursery before budding.

It is usually considered desirable to examine the seedlings soon after their growth is well started, for the purpose of finding and removing any vigorously growing shoots that spring from the main trunk near the base, where the bud must later be inserted. This pruning to force the growth mainly into a single trunk up to the height of the bud insertion (6 to 10 inches) is certainly the only pruning of nursery-stock seedlings that is justified, and it should extend only to the removal of branches that threaten to divide the growth of the main trunk. Even this pruning is frequently omitted.

Shortly before the budding, then, the seedlings are pruned to a single bare trunk up to a height of 6 to 10 inches, depending upon the height of budding adopted. Severe pruning on a young tree tends to set the bark so that it will not slip again until new growth starts. It is therefore advisable to do the preparatory pruning two weeks or more before the budding starts; otherwise the pruning should be done at practically the same time as the budding.

" Sour orange, Rough lemon, citrange, and trifoliate orange seedlings tend to grow upright, with a single trunk, and usually require but little pruning. Sweet orange, shaddock, grapefruit, tangelo, and mandarin orange seedlings tend to form low and bushy heads and usually require considerable pruning.

BUDWOOD FROM SELECTED MOTHER TREES

Buds for propagation should always be taken from carefully selected trees of the variety desired: it is not sufficient to know merely that the buds are being cut from trees in a good grove of the variety. This necessity for exercising careful selection in the choice of mother trees rests primarily on two considerations: first, the variation in varieties and the consequent importance of taking buds from trees of the best strain of the variety; and second, the transmission of virus diseases and the desirability of being certain that the scions are from healthy trees.

As for the first of these considerations, it may be pointed out that bud sports frequently occur in all citrus varieties, and that these off-type sports are transmitted by bud propagation. This has been demonstrated beyond question. If, therefore, no tree and bud selection is exercised, poor and worthless types are certain to be propagated frequently.

All trees from which buds are to be cut should be very carefully examined while they are carrying a full crop of mature fruit, and buds should be taken only from those trees that are found to be vigorous and productive and to bear only good fruits typical of the variety. If the tree is found to bear abnormal fruits, it should be discarded as a source of budwood.

Much has been written on the desirability of taking budwood from performance-record trees (see Vol. I, chap, x; also Shamel, 1920). The policy cannot be too highly commended, as it is only by following the production and character of the fruit through several years that a complete understanding of the heritage and capacity of a tree can be obtained. That it is necessary to take buds from high-yielding trees has frequently been questioned, on the grounds that yield is in large measure determined by environmental factors. This may be granted, but the fact remains that some strains or varieties naturally produce higher yields than others. A tree may be of low or moderate yield by reason of poor rootstock or poor soil; yet the character may possibly be inher ent in the strain, and if so, it would be transmitted by budding. Thus, the policy of taking buds only from high-yielding trees is a safe one, and the only one that can be advocated as reliable.

As for the second consideration, the importance of choosing mother trees which show no indication of disease, little has been known until recently. It has now been demonstrated by Fawcett (1938, 1939, 1943) that citrus trees may be affected by a number of virus diseases which are transmitted by tissue transplantations, as by budding and grafting. Among these are such serious maladies as scaly bark (psorosis), concave gum disease, and others. Some trees that are still vigorous and productive may have contracted such diseases and, if used as a source of budwood, would spread the disease to many or all of the bud progeny produced. In a circular letter of December, 1941,6 Fawcett stated:

"Prevention of psorosis A, psorosis B, concave gum, blind pocket, infectious variegation, and crinkly leaf consists in planting trees propagated from buds taken from virus-free bud parents. As far as known, none of the diseases spread from diseased to healthy trees under orchard conditions except by possible root grafts with an adjacent tree. To insure freedom from potential psorosis and other related diseases, buds for nursery trees should come from trees registered by the Bureau of Nursery Service of the State Department of Agriculture. More than 700 such trees have been registered and others are in process of inspection and registration. Growers are advised to ask the nurserymen for a written statement that trees purchased come from registered trees and to obtain the registration number and location of the parent tree. The rootstocks should also come from seeds from virus-free trees, although seeds rarely transmit the virus." (See also Fawcett, 1939.)

It is evident that the widespread occurrence of scaly bark in almost all citrus-growing regions of the world has resulted from its spread through variety propagation. The taking of budwood and scions from known healthy trees, carefully chosen, is therefore highly important (see chap, iii, below).

In general, it is best to produce nursery trees in the localities where they are to be planted. Trees produced locally can be planted more promptly than those brought from a distance, and the danger of injury during shipment is avoided. Furthermore, local trees are likely to carry only the pests and diseases that are already in the locality, and thus do not ordinarily require disinfection.

TYPE OF BUDWOOD TO BE CHOSEN

Budwood should always be chosen from fairly well-matured wood of the current year’s growth. Citrus trees usually have from two to three distinct periods of growth each year, and budwood from the last or next to the last growth is preferable. Round twigs, or those as nearly round as possible, should be selected. The young growth of citrus wood is at first angular but becomes more nearly round as the twig matures. The basal portions of young branches which are nearly or quite round (fig. 4, A) supply the best buds, with the

exception of the first one or two basal buds, which are usually somewhat imperfect and should be discarded. Where it is difficult to obtain well-rounded wood, angular wood may be used, but it is not so satisfactory since it is less easy to employ and is likely to be too immature (fig. 4, B).

Thorny budwood should never be used if other budwood can be obtained. Most of the standard citrus varieties are now nearly thornless, and it is commonly believed that this condition has resulted from the continuous selection of thornless budwood. Some varieties, such as the Lisbon lemon and the Parson Brown orange, still remain thorny, and newly introduced seedling varieties are usually thorny at first. Since thorny trees are undesirable, every effort should be made to choose budwood as nearly thornless as possible (see Vol. I, p. 825).

On the much-debated question whether to use only the so-called fruit-wood twigs in propagation, or whether sucker wood can also be used satisfactorily, the evidence is fragmentary, and at present definite conclusions cannot be stated. There can be no doubt that both types of wood carry the same heritage, and the writer knows of no convincing evidence to indicate that sucker wood is more likely to throw sports than any other type of wood. There is apparently no marked difference between the two types of wood, other than a physiological one related to rate of growth as determined by position.

Reference is frequently made to three types of wood: fruit wood, intermediate wood, and sucker wood; the names indicate clearly the relationship Sucker wood is merely the rapid-growing wood of the suckers that spring out from the main trunk or main branches of the tree and grow very rapidly up through the top. Such suckers are large and coarse, with very large leaves, and are of continuous rather than of periodic growth. The small, rounded twigs of the typical so-called fruit-wood growth, with short internodes, small leaves, and very compact tissue, are quite different from the typical sucker wood; but there are all degrees of intermediate types of twigs or wood on the same tree. The sucker branches, probably because of their location, spend all their sustenance in vegetative growth to fulfill their function in rejuvenating the top. The fruit-wood branches, on the other hand, grow slowly in marked cycles or periods, store their nutrition, and are thus prepared when the proper season comes to develop flowers and fruits.

Halma’s (1934) experiments with respect to size and age of budwood seem to indicate that these factors have little or no influence on the size of trees produced. He tested seven different sizes of budwood, ranging from 3 to 11 millimeters in diameter, in relation to the