UNIVERSITY OF CALIFORNIA PUBLICATIONS COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA THE JUNE DROP OF WASHINGTON NAVEL ORANGES A PROGRESS REPORT BY J. ELIOT COIT and ROBERT W. HODGSON BULLETIN No. 290 January, 1918 UNIVERSITY OF CALIFORNIA PRESS BERKELEY 1917 Benjamin Ide Wheeler, President of the University. EXPERIMENT STATION STAFF HEADS OF DIVISIONS Thomas Forsyth Hunt, Director. Edward J. Wickson, Horticulture (Emeritus). Herbert J. Webber, Director Citrus Experiment Station; Plant Breeding. Hubert E. Van Norman, Vice-Director; Dairy Management. William A. Setchell, Botany. Myer E. Jaffa, Nutrition. * Robert H. Loughridge, Soil Chemistry and Physics (Emeritus). Charles W. Woodworth, Entomology. Ralph E. Smith, Plant Pathology. J. Eliot Coit, Citriculture. John W. Gilmore, Agronomy. Charles F. Shaw, Soil Technology. John W. Gregg, Landscape Gardening and Floriculture. Frederic T. Bioletti, Viticulture and Enology. Warren T. Clarke, Agricultural Extension. John S. Burd, Agricultural Chemistry. Charles B. Lipman, Soil Chemistry and Bacteriology. Clarence M. Haring, Veterinary Science and Bacteriology. Ernest B. Babcock, Genetics. Gordon H. True, Animal Husbandry. James T. Barrett, Plant Pathology. Fritz W. Woll, Animal Nutrition. Walter Mulford, Forestry. W. P. Kelley, Agricultural Chemistry. H. J. Quayle, Entomology. D. T. Mason, Forestry. J. B. Davidson, Agricultural Engineering. Elwood Mead, Rural Institutions. H. S. Reed, Plant Physiology. W. L. Howard, Pomology. |Frank Adams, Irrigation Investigations. C. L. Roadhouse, Dairy Industry. O. J. Kern, Agricultural Education. John E. Dougherty, Poultry Husbandry. S. S. Rogers, Olericulture. David N. Morgan, Assistant to the Director. Mrs. D. L. Bunnell, Librarian. Division of Citriculture J. Eliot Coit Ira J. Condtt Robert W. Hodgson * Died July 1, 1917. t In co-operation with office of Public Roads and Rural Engineering, U. S. Department of Agriculture. THE JUNE DROP OF WASHINGTON NAVEL ORANGES A Progress Beporti BY J. ELIOT COIT and ROBERT W. HODGSON INTRODUCTION The Washington Navel variety when grown in the interior valleys of California and in Arizona has always been subjected to large annual diminution of crop by the excessive shedding of the young fruits. This premature fall has come to be known popularly as the "June drop," although it is by no means confined to the month of June but may occur at any time from petal fall in April until the fruits reach maturity. Ordinarily very little dropping occurs after the fruits reach a diameter of two inches. Conservative estimates based on the most reliable data obtainable place the annual loss at from $1,225,000 to $1,750,000. In many localities in the interior valleys where land and water are relatively cheap, soil conditions ideal, pests few, and climatic conditions favor- able to the production of early maturing fruit of excellent quality, the light crops due to this excessive shedding are both disconcerting and unfortunate. For a number of years requests for an investigation of this trouble have been sent to the Experiment Station and at the Davis meeting of the State Fruit Growers' Convention a resolution was passed calling the attention of the University authorities to the urgent need for such an investigation. In compliance with this request, the Divi- sion of Citriculture undertook a study of the June drop phenomenon in the spring of 1916, and has since accumulated a considerable mass of data. While the investigation is by no means complete and no simple and satisfactory method of prevention has yet been worked out, the cause of the trouble and the nature of the phenomena involved seem to have been uncovered. The fact that a fungus has been found connected with certain phases of the drop phenomenon has led some growers to anticipate 1 A preliminary technical report containing pertinent data appeared in the Agricultural Science Series of the University of California Publications, vol. 3, no. 3, September 29, 1917. "Some Abnormal Water Relations of Citrus Trees in the Arid Southwest and Their Possible Significance, ' ' by Robert W. Hodgson. A more complete joint-authorship technical paper is in course of preparation. 204 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION that spraying with fungicides might prove an effective means of control. Unfortunately neither winter spraying nor spraying in the bloom has proved effective. Inasmuch as our investigations have discovered the reasons for the failure of sprays and have indicated in a general way the lines along which efforts for prevention or control must be directed, a report seems warranted at this time. The shedding of the young navel oranges has been ascribed to vari- ous causes among which are irregular or deficient irrigation, cold irrigation water, irregular fertilization, injury due to thrips, fungus parasites, and climatic conditions. A somewhat comprehensive pre- liminary study of the situation with extended observations in the field very soon eliminated all but the last two possibilities. Therefore, attention was early directed toward attempts to isolate fungi from the stigmas of the young flowers and from the navel ends of the small fruits. ALTERNARIA CITRI AND ITS CONNECTION WITH THE JUNE DROP For many years growers of Washington Navel oranges have ex- perienced losses from a black-rot disease of the fruit which usually is first observed in the interior of the navel end and gradually extends inward toward the center. The infection while not extensive occasions annual losses varying from three to seven per cent, This disease was noted by Newton B. Pierce in 1892 and was first described by him in 1902 as "Black Rot of the Navel Orange" caused by Alternaria citri. The fruit is infected when quite small, probably just before or soon after the style is shed, through the cracks and imperfections in the proliferations of the navel. The fungus is a weak parasite and remains quiescent, or nearly so, during the growing period of the young fruit, at which time the fruit is more or less resistant to the encroachments of parasites. "With the decline in vigor incident to approaching maturity the fungus becomes active and exerts a stimu- lating influence on the fruit, causing it to take on a deep reddish yellow color and ripen earlier than the normal fruit. In a small and restricted area the cells of the pulp are broken down and become a nauseating mass of black fungus mycelia. The rind is left uninjured until the disease has made considerable progress within but ultimately a black and decayed spot appears on the surface near the navel end. A certain proportion of the infected fruits early shows a yellow spot about the navel end and drops from the tree when the fruits are one to two inches in diameter, or even larger. The remainder persist to maturity, the disease showing up at picking time, in transit, in storage, or not until in the hands of the consumer. JUNE DROP OF WASHINGTON NAVEL ORANGES 205 During the spring and summer of 1916 hundreds of cultures were made using the flowers and young fruits from many citrus sections of the state, from Oroville to San Diego, and in a high percentage of the cultures Alternaria citri was obtained, though often associated with Macrosporium and some other fungi. Later in the season when the fruits were one inch to two and one-half inches in diameter more cultures were made, using only the fruits which were on the point of dropping. Alternaria was found in almost all of these fruits. Fig. 1. — Typical Alternaria drops. The discolored area under the navel is char- acteristic of the larger fruits. Examination of these larger fruits showed a dark area just under the navel end which contained gum. Cultures from these dark areas usually resulted in Alternaria. From work done in the spring and summer of 1916 it appeared evident that the June drop, to some extent at least, was due to the same fungus parasite causing Navel end rot; and that those fruits which manifested the rot on maturity, instead of measuring the extent of infection with Alternaria, really represented the few fruits only which were able to withstand the infection, the larger proportion having been shed. From these results the theory was advanced that excessive transpiration from the leaves caused water together with enzymatic poisons secreted by the fungus 206 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION to be drawn back through the vascular system of the young fruits through the pedicel, causing the fruit to drop. 2 (See Fig. 3.) In an attempt to determine the distribution of Alternaria, a large number of open-air exposures with culture plates was made in various parts of the state, from Oroville to Whittier. In practically every case Altemaria citri was among the fungi obtained which indicated that the spores are almost ever-present. Examination during winter and spring of mummied oranges and other debris under the trees disclosed the fact that the fungus overwinters on much material. In spite of the prevalence of the spores the amount of infection was very much greater in 1916 than in 1917, but this is perhaps accounted for by the fact that the season of 1916 was very cool, being entirely with- out hot, dry north winds, while in 1917 the bloom was five weeks late and the weather subsequently was remarkable for its long-continued high temperatures. In 1917, however, after making cultures of the very young fruits, it was found that a great many were not infected with Alternaria. The proportion of infected fruits to the total number shed increased as the season progressed. Therefore it was quite evident that while Alternaria was probably the cause of much of the late dropping, it could not be alone responsible for all of the early drop. A survey of the literature on the abscission of fruits and other plant parts brought out the fact that abnormal water relations in plant tissues due to peculiar or unfavorable environmental conditions are fre- quently the cause of shedding in a number of other plants. Therefore a careful study of the climatic and soil conditions in the locality under consideration was undertaken. CLIMATIC CONDITIONS AND THE SHEDDING OF THE YOUNG FRUITS It has long been noted that the yield per tree of Washington Navel oranges is much less in the interior valleys than in the coast regions, and the decrease is greater the more harsh the climatic conditions encountered. There seems to be a marked correlation between the atmospheric temperature and humidity and the yield, the lower the average humidity and the higher the average daily temperature dur- ing the growing period, the smaller the yield. As far as yield is concerned there is little doubt but that the Washington Navel variety is not well adapted to the intense dry heat of the interior valleys. The' tree thrives and produces a high quality of fruit where it has 2 See article in Univ. Calif. Jour, of Agr., Sept., 1916, "The Cause of June Drop of Washington Navel Oranges/' J. Eliot Coit and E. W. Hodgson. JUNE DROP OF WASHINGTON NAVEL ORANGES 207 water, but the yield is greatly reduced. Careful surveys were made to obtain data as to the relative yield in different districts. Placing the yield of the most climatically favored district in the state at 100 per cent, the least favorable locality produced only 25 per cent of a crop where other conditions of production were fairly comparable. This reduction in yield is not strange when it is remembered that the citrus tree is undoubtedly a mesophyte of tropical origin and as such is not well adapted to withstanding intense heat and especially the extremely dry air of the interior valleys. WATER RELATIONS AND THE SHEDDING OF IMMATURE PLANT PARTS All the more recent fundamental plant physiological work has indicated that, for the temperate zones at least, the water relation is the limiting factor in plant growth in the open. Under normal conditions the plant absorbs sufficient water from the soil to supply its needs, make up for that lost by transpiration, and maintain its water content approximately constant. Plants well adapted to arid or desert regions where the air is very hot and dry are often provided with various special structures for resisting too great water loss. However, plants not so well equipped for reducing evaporation, when grown under arid conditions frequently lose more water by transpiration than the roots can absorb in the same time from the soil. The tissues are dried out and a water deficit occurs. If such conditions are prolonged wilting and ultimately death from dessica- tion occurs. But, as is sometimes the case, the plant can absorb sufficient water to supply itself during the night and the greater part of the day when climatic conditions are not at a maximum of severity. There is then a period during the middle of the day when there is a water deficit but later in the afternoon and during the night this loss is made up and the plant' recovers. This is what happens on certain days in the cotton fields of the Imperial Valley where during midday the leaves actually appear wilted but are fresh and turgid the next morning. Under such a periodic daily fluctuation in water content many plants shed certain parts, such as flowers, leaves, or immature fruits. The shed portions are still alive and this process of abscission, as it is called, must not be confused with the shedding of dead plant parts, such as certain autumn leaves, which is known as exfoliation. Plants which shed immature fruits under such conditions possess a delicate mechanism which exists in the pedicel between the fruits and the shoot 208 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION to which they are attached. This mechanism is not possessed by all plants, but where it occurs it may be brought into action and the abscission process started by certain stimuli which vary for different plants. In some plants variations in moisture content or sudden fluctuations in temperature are causative stimuli. In others mechan- ical injury, such as insect punctures or chemical injuries due to fungus attacks, are sufficient to cause the shedding. In some, as in the case in cotton, either will cause abscission. With these facts in mind a comprehensive series of experiments was carried on to ascertain whether pronounced water deficits occur in citrus trees and at what hours of the day. Suffice it to say here that extended investigation has shown beyond any doubt that such do occur regularly day after day in the fruits and foliage of the Washington Navel orange tree during the hot, dry growing season. During the day the fruits decrease in water content as much as 25 to 30 per cent. It has been definitely established that under severe conditions when the atmospheric pull is high the leaves actually draw water back out of the young fruits to maintain themselves. But this supply is not sufficient and they decrease in moisture content also. The combined effect of this tremendous loss from leaves and fruits results in tensions in the water-conducting system of the tree. These tensions as well as the water deficits have been found to be at their maxima when environmental conditions are most severe, that is, be- tween 10 a.m. and 3 p.m. Meteorological records show that the atmospheric humidity of the interior valleys is quite low during the growing months, relative humidities of 15 per cent being not uncommon. Such humidities may and do occur without marked increase in air temperature. In other words, it is possible for extremely dry weather to occur without the characteristic hot norther. Experiments have been performed in the laboratories at Berkeley in which this process of abscission of leaves on cut branches has been induced by artificial means. The process itself has been studied and found to consist in the gelatinization and dissolution of the cell walls resulting in complete separation of the cells. Although further experiments will be necessary in order to clear up certain points not yet entirely settled, all the evidence so far obtained seems to point to the following conclusions : The major part of the June drop occurs early in the season and has to do with blossoms and small fruits. It is caused by a stimulus to abscission arising from abnormal water relations within the plant due to the peculiar climatic conditions. The major part of the drop JUNE DROP OF WASHINGTON NAVEL ORANGES 209 of the larger oranges is caused by the fungus Alternaria citri. These two causes overlap to a considerable extent, for many of the small fruits which succumb to water relations are also infected with Alter- naria. The relative importance of the Alternaria drop would not be very great were it not for the fact that it continues after the water- relations drop has already thinned the fruit much more than is desired. Fig. 2. — Not all these oranges are infected with Alternaria. as a result of certain climatic conditions. Most of them fell Further evidence that the cause as indicated is substantially cor- rect lies in the fact that in certain orchards which are provided with efficient windbreaks and interplanted with alfalfa and heavily irri- gated, the water deficits in leaves and fruits have been found to be much reduced Such orchards have less drop and are notable for their comparatively large yields. The Kellogg orchard at Bakersfield is planted to alfalfa and is shielded by a fairly efficient windbreak. Meteorological measurements made in this orchard and on the desert to windward show that the climatic complex is greatly ameliorated. This of course is the result of a number of factors working together. 210 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION The alfalfa transpires at a tremendous rate and literally bathes the trees in a moist atmosphere. The windbreak retards the movement of this relatively moist air away from the vicinity. The vaporization of water from soil and plants tends to lower the temperature of the air. As the soil is largely shaded, the high soil temperatures are reduced, which temperatures operate to cut down root absorption at the time of day when water loss from the leaves is greatest. It has been shown by other investigators that changes in soil temperature have a marked effect on the rate of water intake by the roots of plants. Above and below certain temperatures (which vary for different kinds of plants) the rate of absorption by the roots is markedly reduced. It thus seems probable that under the prevalent practice of clean cultivation, during the middle of the day when transpiration is greatest the root absorption is actually reduced, resulting in an increase in the water deficits in all parts of the tree. Not only are clean cultivated orchards subjected to higher soil temperatures, but inasmuch as the root system tends constantly toward the surface layers, it is much reduced by the annual spring plowing which shears off many of the fibrous feeders, thus reducing the root area just before blooming and at the very time the trees are under the greatest strain. PREVENTION AND CONTROL When we come to consider the prevention or control of June drop we find that there are two lines along which we may work. In the first place, it is entirely possible for many growers, who are so sit- uated as to make it practicable, to modify environmental conditions, both above and below ground, * sufficiently to reduce the extent and amount of the water deficits referred to. In selecting a site for a navel orchard it is well to keep in mind the direction of prevailing winds, the relation of the site to extensive irrigated areas and to the degree of exposure to these influences. Fruit growers should accus- tom themselves to think of climate not only in terms of counties, great valleys and states, but also in terms of strictly local conditions. A half acre of irrigated alfalfa isolated on the desert will exert a most profound influence on the climate a few yards to leeward and a few feet from the ground. Everyone is familiar with the coarse, rough appearance of oranges grown in fully exposed positions in the tops of the trees. The reason why oranges borne on the inside of the tree are of smoother texture and thinner skin is because the environ- ment inside the tree represents quite a different climate than that of the outside. .TUNE DROP OF WASHINGTON NAVEL ORANGES 211 In order to compare the evaporating power of the air inside and outside of an orange tree at Edison, two atmometer cups were oper- ated, one in the midst of the tree and the other at the same height from the ground but midway between two trees. The readings cov- ered seventeen days from May 22 to June 9, 1916, during which time the maximum daily temperatures ran from 80° to 95°. The average daily water loss from the inside cup was 36.8 c.c. and that for the outside cup 47.9 c.c, or 30 per cent greater. While it is obviously impossible for growers to completely offset the effects of hot north winds such as occurred June 14-17, 1917, when in addition temperatures between 110° and 120° F. were registered in many parts of the state south of the Tehachapi, it is possible to Fig. 3. — Structure of the navel orange. The central pith of primary fruit acts as the stem of secondary fruit. accomplish a good deal toward ameliorating such conditions. The careful selection of the site ; the growing of summer cover crops such as cow peas or alfalfa; adequate irrigation; the reduction of leaf area by moderate winter pruning ; closer planting of the trees where possible; the growing of adequate windbreaks; the mulching of the ground to reduce the soil temperature ; all are ways to accomplish the results desired. It should be understood however that the authors are not making a general recommendation for the growing of summer crops in citrus orchards, but merely pointing out that where they are grown, some reduction in the amount of June drop may be expected. Whether or not it may be practicable for any such crops to be grown in a given orchard must be decided for each individual case separately. The second line of work which holds some promise is the discovery and propagation of chance sports or mutations which may be satis- 212 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION factory in other ways, yet to a degree resistant to the conditions which produce the June drop. Every grower should watch for such mutations. The proportion of the shedding caused by the fungus alone is relatively small. Moreover, the mode of infection is a serious hindrance to successful intervention by means of fungicide sprays. The authors have therefore been led to conclude that spraying with fungicides alone can hardly accomplish enough good to pay for the materials and labor. Cooperative spraying experiments carried out during the last year have shown no measurable reduction of the June drop. SUMMARY June drop of Washington Navel oranges causes an annual loss of 1,225,000 to $1,750,000 in California and is the limiting factor in profitable navel-orange production in many parts of the interior valleys of California and Arizona. The drop itself can be separated into two parts, that occurring from petal fall until the fruit is about an inch in diameter, and that occurring after that time. The first part is much the more serious and is due to abnormal water relations which serve as a stimulus to abscission. The second part is less important and is due to the fungus Altemaria citri of wide dis- tribution which is also the cause of black rot of Navel oranges. The fruits showing the black rot represent those infected fruits which managed to survive to maturity, other infected fruits having fallen, chiefly during the months of July and August. The water-relations drop bears a definite relation to climatic conditions and all efforts looking toward prevention or control must be either in the nature of modifying environmental conditions, or in selection for dry-heat-resistant strains. Of these two the former seems to promise the more immediate results.