Factors Influencing the Setting and Bearing of Fruit Trees

New Zealand Journal of Agriculture, Volume 84, Issue 4, 15 April 1952, Page 260

Factors Influencing the Setting and Bearing of Fruit Trees

MAINTENANCE of consistent and heavy crops of fruit is essential to success in commercial fruit growing. Orchardists whose trees do not always crop consistently attribute this to various causes, including in particular the weather at the time of fruit setting. Poor or irregular fruit setting and bearing may be caused by many factors other than climatic conditions, and in this article T. Conway, Horticultural Instructor, Department of Agriculture, Hastings, describes those which, individually or collectively, may be the cause of disappointing crops.

FRUIT SETTING is a very complex process, and present production methods tend to aggravate the position, as in many cases they are contrary to Nature. For instance, a limited number of varieties best suited to markets and modern cultural operations is usually selected and planted in fairly large blocks without special consideration of pollination; in later years, when the mistake is appreciated, difficulty is found in correcting it without considerable replanting or reworking of trees to provide pollinating varieties. When a block of fruit trees has been planted the owner has an understandable desire to see the trees bear fruit in commercial quantities as soon as possible. However, Nature usually declines to be . hurried, but under intensive culture and by the use of modern knowledge the period during which the trees are unproductive can be shortened considerably. In general, the new orchardist must exercise patience, as the time of bearing is related largely to the kind and variety of tree, the root stock, the soil, and the need to build a strong framework capable of supporting heavy crops. Overburdening a tree with crops too early may seriously curtail its development and restrict its future bearing capacity.

Fruit crops sometimes fluctuate pronouncedly from season to season over whole districts or between orchards within districts. When variations occur within districts the poor crops are often most evident in varieties which tend toward biennial bearing. The over-all crop in such an orchard may not be affected unduly because, while one or two varieties give poor results in any one year, the other varieties produce so well that weaknesses are largely offset. Nevertheless, retention of varieties which produce irregular crops is unwise; either they should be replaced or measures should be taken to regulate their cropping habits. A factor such as the weather may cause a severe setback to relatively stable production from orchards. In such circumstances not only the individual orchardist but also the market supply in general can be affected seriously. Bearing Life of Trees The time of. beginning to bear and the age at which full production is reached normally vary considerably between kinds and varieties of fruit trees. Peaches and plums will generally crop very much earlier after planting than other kinds, such as apples

and pears, and some varieties begin bearing much sooner than others. Precocity in cropping is not always an advantage, as it may retard growth to an undesirable, extent. Normally apples can be expected to begin bearing reasonable crops 6 to 8 years from the time of planting, pears 8 to 12 years, peaches 3 to 5 years, apricots 4 to 5 years, cherries 5 to 7 years, Japanese plums 4 to 5 years, English plums 5 to 7 years, quinces 5 to 6 years, Meyer lemons 3 to 4 years, and other citrus trees 6 to 10 years. . General management and local conditions have a considerable influence on the time of bearing. Fruit trees normally reach maximum production at about the following ages: Apples 15 years, pears 18 years, peaches 8 years, plums 10 years, apricots 10 years, quinces 14 years, Meyer lemons 12 years, and other citrus fruits 18 years. Neglect, disease, and age gradually have a serious effect on the cropping of fruit trees, and there are limits beyond which economic production cannot be maintained. Therefore regular replacement of exhausted trees is necessary even in the bestmanaged orchards. Even under good management maintaining fruit trees in full commercial production becomes increasingly difficult as they progress beyond the following approximate ages: Apples 40 years, pears 50 years, peaches 15 years, plums 30 years, apricots 25 years, cherries 30 years, quinces 30 years, and citrus fruits 35 years. However, much

depends on the root stocks used, the soil conditions, and the incidence of disease.

The weather at the time of blossoming and fruit setting plays a very important part in the cropping of fruit trees; it is, perhaps, the most important single factor. If a reasonable period of warm, sunny, calm weather occurs during this critical period, ■ fruit set should be satisfactory unless some other restricting factor is present. The benefit of shelter in this respect must not be overlooked.

Weather has an important influence on two vital aspects of pollination: It controls the flight and activity of bees and other pollinating insects, and it affects fertilisation of the blossom. For bees to work effectively the weather must be favourable; they are more active in fairly high temperatures (60 to 70 degrees F.) and when conditions are sunny with an absence of high winds. Excessive humidity caused by relatively high temperatures after heavy rain, or conversely very cold conditions, prevents the pollen from maturing and drying, thus making it unsuitable for bees and other insects to carry from blossom to blossom. In rough, windy, wet weather much of the pollen can be washed away from the open blossoms and, though sunny conditions may follow, setting of fruit is often poor.

A very poor setting sometimes results when the blossom opens under favourable conditions followed immediately by a prolonged cold period. In such circumstances a severe drop of the young fruits can be expected during the natural drop period.

If the root systems of the trees are healthy and soil conditions are satisfactory, root activity will begin a short time before blossoming. New root growth may be quite considerable if the soil is reasonably warm, moist, and aerated, with the result that abundant moisture is transmitted to the flowers and leaves as they appear. Such conditions are good for setting of fruit.

On the other hand, if the soil remains persistently cold and over-wet, though atmospheric conditions may be relatively satisfactory, leaf and blossom activity will occur from the stored-up energy in the trees without corresponding root activity. Unless the leeway is made up rapidly by activity in the root system, leaves and fruit buds will not be supplied with sufficient moisture and soil elements, particularly nitrogen. Under such conditions dropping of fruitlets may be heavy.

Excessive rain at the time of blossoming and fruit setting may restrict the availability of a high proportion of the soil nitrogen, and a temporary nitrogen starvation could result at a time when ample nitrogen is of first importance to the trees. The climate of a locality is sometimes the factor deciding what trees can be grown or cropped successfully. Only poor results can be expected if attempts are made to grow fruits in an environment unsuited to them. For instance, successful commercial citrus plantings are possible only in the warmer northern areas of New Zealand, whereas raspberries can be highly productive in the cooler districts but a disappointment in the Auckland district. On the other hand, a wider range of conditions will suit some fruits, including apples, pears, and peaches, though some varieties are successful in certain localities and not

in others. Therefore it has become the practice to recommend certain varieties as being the most successful in specified localities or districts. r x n rrosT Damage Late frosts during blossoming of fruit trees in spring are often responsible for extensive crop failures, particularly in peaches, plums, apricots, and nectarines, which . flower earlier in spring than do pip and berry fruits, The extent of the damage will vary with the severity of the frost and may range from immediate destruction and dropping of the fruit down to distortion and severe russeting. Some light frosts may cause only small losses, tending to thin the fruit and so save thinning costs later. When only minor damage to apples and pears is caused by frost the fruit frequently remains on the trees until maturity, but will show signs of calyx distortion and fairly severe russeting. The damage to stone fruit is generally more severe; the fruitlets, if not destroyed outright, may develop to marble size, but eventually drop from the trees because of damage to the kernel. If fruitlets drop from the trees some time after a frost has occurred, the damage to the fruit at the time of the frost has been confined to the sex organs of the flowers or to the young kernels of the smaller fruitlets. If these are cross-sectioned, a brown discoloration of the young kernels of stone fruit may be observed; in apples and pears the discoloration occurs right down to the ovules, from which the seeds are formed. The liability of blossoms to damage by low temperatures generally increases with their further development

after the first spring movement. At tight cluster a temperature of 22 degrees F. may sometimes be withstood without undue loss, at late pink 25 to 26 degrees may damage the blossoms severely, and at full bloom a temperature of 27 degrees may almost ruin the crop. A series of temperatures near these points can have a cumulative effect with disastrous results. One damaging frost at full bloom may not destroy the whole crop, as the laterdeveloping buds may not be damaged an may set satisfactorily if another frost does not occur. Winter injury of dormant buds often occurs overseas, but, fortunately, ternperatures are rarely low enough in New Zealand to cause such damage, A considerable degree of frost protection can be given to fruit trees by oil burners placed at intervals throughout the orchard and lit when low ternperatures are likely to cause harm. The use of such burners has been extended greatly in recent years, » x ex l ' Root stocks The productivity and vigour of a fruit tree are much influenced by the root stock on which it is propagated. The stock also influences the time at which a tree begins to bear profitable crops after being planted; trees on vigorous stocks begin bearing later than those on weaker stocks and therefore are later in coming into full production, but a strong root stock eventually will produce a tree of bigger framework and capable of carrying heavier crops than will a weaker stock. Best results can be achieved only by the choice of a root stock to suit the variety, soil type, and location, as some stocks perform better under specific conditions.

The use of intermediate stocks on some fruit trees, particularly citrus, may affect the quality as well as the production of the fruit. Propagation of trees with intermediate stocks involves a method of double budding or grafting in which a variety providing certain influences is placed on the original stock, followed by final working with the desired variety. Soil Type Satisfactory fruit setting and bearing depend essentially on the suitability of the soil type for the kind of fruit. Good-quality, well-drained, and friable soils greatly assist proper functioning of the root systems, and this has a very important bearing on the success of fruit production. Good physical structure and humus content of soils are essential. Heavy clay soils of poor physical structure may cause rapid deterioration or poor functioning of the root systems, which is detrimental to the health of the trees and seriously curtails their cropping capacity. In some districts or localities the soil may be best suited to one or more kinds of fruits, and growers need to know the fruits their soils are capable of producing satisfactorily. In general, citrus fruits do best on the lighter soils and peaches, nectarines, and apricots prefer deep, friable, well-drained soils. Pears and quinces grow better on the heavier soils and can stand more soil moisture without detrimental effects than can citrus or stone fruits. Apples and plums . show a wide tolerance for soils, but grow better on the heavier types. Disappointment is sometimes the result of insistence on trying to grow fruits which are unsuited to local conditions. Drainage Many New Zealand orchard soils are reasonably well drained as a result of either the natural fall of the land or the friable and free-draining texture of the topsoil and subsoil and have little need of tile or other artificial drainage. This is one of the factors influencing selection of land for fruit growing. The retentive nature of much of the Auckland and Nelson clay subsoils presents its own drainage problems. However, in many orchards drainage cannot cope efficiently with the heavy rains of autumn, winter, and early spring. Under such conditions root activity „ during autumn and early spring is restricted severely through lack of oxygen at the rooting area. The main damage is to the small feeding roots which begin to develop in spring. These fine roots are very important to the trees, and any breakdown of or interference with their proper functioning causes a severe check to the trees at a critical time. The buds may begin to burst open in spring and then wilt and die or become stagnant because root action is insufficient to maintain growth as the demand increases. Drainage, either natural or by tiles, must be adequate to prevent over-saturation of the soil at all times, but particularly in spring. Good drainage also allows the soil to warm satisfactorily, whereas over-saturated soil tends to remain - very cold and retards spring development of the trees.

If the soil remains over-saturated for long periods, the roots in the lower strata of the soil become weakened and die, leaving the trees dependent on the roots closer to the surface for survival. If dry conditions follow, the root systems cannot maintain the top growth and the fruit fails to develop properly or may drop from the trees. Furthermore, trees debilitated by lack of drainage over long periods cannot set or carry satisfactory crops, as dieback occurs and lack of growth and foliage become severely restricting factors.

Some kinds of fruit trees are more tolerant of inefficient drainage than are others. Quinces and pears are in this category, but peaches, nectarines, and apricots must have particularly welldrained conditions.

Cultivation

Care needs to be exercised during spring, cultivation, as. damage by implements to roots, both large and small, may cause a severe reduction in the supply of moisture and nutrients to the trees at the vital fruit-set period. Some of the very fine roots are close to the surface, even down the middle of a row or bay, and deep cultivation, particularly by ploughs and disc harrows, can cause damage which will, at least temporarily, cut off supplies to the trees. This is usually more evident in soils which are rather shallow or have been mulched, irrigated, or highly fertilised, as in those conditions a large proportion of the fine roots are close to the surface. An orchard with a dry, friable soil in loose cultivated condition, or one which is carrying tall and extensive weed or plant growth, is more liable to frost damage by light or marginal frosts in spring than is one with a fairly firm and moist soil which is free from weeds. A firm, moist, weed-free soil may give protection against a frost of 2 or 3 degrees, and this aspect is worth consideration when spring cultivation is being undertaken. The ideal would be to cultivate the soil - to a reasonably, fine tilth as early as conditions allow and then firm it with a Cambridge roller to - draw the soil moisture close to the surface. Cultivation can then be continued after danger from frost has passed. Such a system may not be necessary or possible, however, on heavy clay soils.

Pruning Unless pruning achieves proper balance of fruiting 'and non-fruiting wood, biennial bearing or partial crop failures may result. New fruiting wood must be encouraged in the tree so that the fruit is borne mainly on reasonably young growth. This entails cutting away much of the older fruiting wood which has become exhausted and cannot be relied on to crop regularly. Fruit buds on the younger growth are healthier and stronger, and ample replacement of old wood and mainten-? ance of young fruiting wood in the trees can contribute much to regularity of cropping. Proper development of fruit buds is associated in a . high degree with the provision of ample light and air throughout the trees. Training and pruning of trees to facilitate penetration of light is important to the proper development of the buds. Overcrowded trees bear mostly on the outer surface and this restricts their bearing capacity. To foster regular cropping trees must be kept in moderately strong growth once the bearing habit has become established. At the same time the balance between fruit-producing wood and new growth must be regulated to maintain stable cropping and good health in the trees. Fairly severe pruning of old and debilitated trees often stimulates them into much-improved growth and fruit bearing. Heavy thinning of fruit spurs over a period of years results in better lateral growth and new fruiting wood of better type for consistent cropping. Very often insufficient detailed pruning is attempted, with the result that . too much old wood and too many spurs are left on the trees.

Excessive pruning causes undue stimulation of the trees into vegetative growth at the expense of fruiting. Lack of pruning retards the development of new growth and usually, results in an excess of weak buds which will not set fruit. Fairly heavy spur pruning when a heavy crop is expected is claimed to offset the tendency to biennial bearing by reducing the fruit buds and spurs and preventing the trees from exhausting themselves by overcropping. For best results the relation between fertiliser application and pruning must be close. Heavy pruning and heavy application of nitrogenous fertiliser can result in excessive vigour at the expense of fruiting, whereas light pruning without fertilising may be followed by heavy cropping requiring extensive thinning to produce fruit of good size. An intermediate course should be followed. Excessive pruning of younger trees delays fruit bearing. This tends to upset the balance between root and top growth, and some years may elapse before adjustment has taken place. Vigorous trees are best pruned lightly to encourage and maintain fruitfulness. Biennial Bearing Kinds and varieties of fruit trees differ considerably in form, type of fruiting wood, and cropping habits. Some varieties, particularly of apples, naturally tend to bear well in one season and to carry a poor crop in the following season. This biennial bearing is the cause of many otherwisegood varieties losing popularity.

Unless some relief is given in a year of heavy cropping by blossom or fruit thinning, overcropping may occur and development of fruit buds for the following season will be affected seriously. At this point the biennial habit of fruit trees is initially fostered. Care must be exercised in the management of the trees to prevent overcropping in any year. The tendency toward biennial bearing is most pronounced in some of the spur-bearing varieties, such as Dunns, Ballarat, and Statesman apples and Winter Nelis pears, whereas the more lateral-bearing varieties are more consistent in their production. Therefore the aim should be to prune biennialbearing trees to encourage the formation of laterals on which to bear their fruit. A correct balance between foliage, fruit spurs, and new fruiting wood is a big factor in maintaining regular crops of these varieties. The early development of vigorous and ample foliage in spring is important to assure good crops the following year, as the leaves perform the special function of starch or carbohydrate manufacture. Generally in a year of heavy crop the tree produces smaller leaves than when carrying a light crop, and such foliage is less efficient in producing the fruit buds for the subsequent crop. The proportion of foliage to fruit is important in the production of the current season’s crop, a fairly high relationship being desirable for good results. Any variety may be thrown, at least temporarily, into alternate bearing by some physical upset to the trees, such as total loss of crop because of frost damage or severe fireblight during blossoming, by waterlogging caused by excessive rain, or perhaps by failure to prune trees. Pollination The relation between fruit setting and the formation of properly developed and viable seed in the fruit is important. Pollination must be effective to assure good crops. Though exceptions are . numerous, usually only blossoms which have been fertilised set fruit that will remain on the tree to maturity, as growth of the fruit depends on satisfactory seed development. Lack of satisfactory pollination is often the cause of poor setting of some fruits, particularly pears, sweet cherries, plums, and apples.

Unless pollination is completed, the small fruits, which may appear to have set, will not develop and will fall from the tree shortly after blossoming. Pollination provides for the ovules of the flower being fertilised by the nuclei from the pollen grains. For this purpose the pollen grains must be deposited on the stigma, where they will germinate. Pollen of fruit trees is transferred by bees and other insects. The stigma is coated with a sticky material which holds the grains and allows them to germinate and put out a germ tube which penetrates the style. The nuclei necessary for fertilisation pass down this tube to the ovules. Some varieties of fruits are self sterile and require cross-pollination with other varieties of the same kind. Other varieties are self fertile and do not require cross-pollination; they in-

elude the great majority of peaches and nectarines, and sour cherries, large areas of single varieties of which can be planted. Some varieties . cannot be self pollinated because the flowers are imperfectly formed, prohibiting normal fertilisation. This difficulty is experienced with strawberries because the flowers of some varieties are pistillate. The pollen produced by some kinds of fruiting plants is sterile, which prevents their being of any value in pollination, but these varieties usually are very receptive to the pollen of other compatible varieties, and where they are associated with them good crops can be set. If self-sterile non-compatible varieties are interplanted, crops will be poor, particularly of pears, cherries, plums, and apples.

Almost all Japanese plums are crosscompatible, provided the flowering periods coincide, though in some exceptions the choice of compatible varieties as pollinators is limited. Some varieties are relatively self fruitful. Even varieties of fruits which are considered self fertile often set a better crop of fruit if provision is made for cross-pollina-tion. Such provision is a good form of insurance. In the so-called self-sterile varieties the pollen is not effective on flowers of the same variety and they will not produce seed and carry their fruit properly unless they are cross-pol-linated with a suitable variety. The pollen of self-sterile varieties may function satisfactorily on the flowers of

other varieties.

No cultivated variety of sweet cherry sets fruit with its own pollen, so they must be cross-pollinated with a compatible variety. Much investigation has been carried out to separate sweet cherry varieties into suitable crosspollinating groups. Varieties within these groups will not pollinate each other, and plantings must be arranged with varieties from each group closely associated to assure satisfactory setting. When data are complete and available much will be possible to avoid crop failures with cherries. a ++ + • H • j ++>> Tr.hn An attempt is Being matte at tne apnn Innes Horticultural Institution, England, to breed self-fertile varieties of fruit, but this is a very slow process and many years will elapse before these varieties become available commercialiy. Planting a large block of one variety is unwise unless its self fertility is is unwise unless ns seii leiuiiiy is certain. It is preferable to intersperse the varieties at regular intervals No me varieties at leguiar inteivais. imu sKiuld^erhlps 3 be changed every third row. Some authorities specify that a pollinating variety should be placed at every third tree in every third row as a minimum reouirement However if orchard is u}duW m&ed,.difficulties the Ul arie*io^ S and harvesting dates of tne varieties. The . effectiveness of cross-pollination can be ascertained by hanging flowermg laterals from other varieties which flower at the same time m jars of water fairly high up in the trees. Results can then be checked with untreated trees and data secured about both the need for pollination and the compatibility of the varieties. tried. If results are satisfactory, permanent pollinators can be budded or grafted on to the trees.

A good - pollinator must provide pollen that will germinate well, it must blossom at the same time as the variety to be pollinated, and the varieties must be compatible. Though English and Japanese plums are not usually compatible, some English plums will pollinate certain Japanese varieties. Some varieties of grapes require cross-pollination. b xa/ai-L- nf Rppc vvorK OT The work of the honey bee is the most important factor in the transference o f pollen from blossom to blossom f or the fertilisation of deciduous fruits, Wind does not assist in the pollination mos deciduous fruits. Other insec s are also believed to assist in pollination, but only to a minor extent compared with honey bees. qnravs which are poisonous or sprays ' vni s n are poisonous be repellent to bees should not be orJuirti rinrinp blncLcnmin? hot applied during blossoming, but should be delayed until a stage. f blossom fa ll when there is n ±Sitv to bees caused K “"fe s’spraying, 0 a" d^a sa£ 0 ,... rrl x the Reelreeninp- industry guard to We beekeeping industry IctTrohibUs ’of Sayl poisonous to bees until almost all the blossoms have fallen from the trees. & subsequent article bv D Roberts Apiary instructor, Department of Agriculture, Auckland, will deal with the I,™™ the nniiina+inn nf hard fruits P ' c_;i KJ.|+ r ; A n+c N e ■ Nutritional requirements of trees setting fruit are rather complicated. The first growth movement in the tree in spring depends on the nutrients stored

in the tree parts before dormancy was reached in the previous season, but it is important that ample nutrients, particularly nitrogen, be available in the soil to maintain development of this growth satisfactorily during the vital blossoming and fruit-set period. Soil conditions require to be as near perfect as possible at this period, with a good relation between soil moisture, ternperature, and nutrients. A deficiency of nitrogen at this time is likely to cause undue dropping of fruitlets. With the development of early-spnng growth nitrogen is utilised in fairly large amounts, resulting in a heavy draw on available supplies. If soil conditions and root activity are not satisfactory, the supply to the trees is restricted, though ample nitrogen may be available. x , n -x x Trees affected by a nitrogen shortage may bloom quite normally, but will not fruit nrnnpr -i v voune fruits x irui J properly, ine young irrnts turn e n ow or. light green soon after natal fall and are ouicklv shed from f he trees . In sto ned fruit y this usually results - in an excessive drop at the fertSlsers“a “Sfd th?y nitrogenous rermisers are uteu, uiey must be applied a short time before the fIowCTS beg “ ‘° pen ' The activity of certain microorganisms in the soil has an important bearing on the supply of nitrogen. If s °il conditions are unsatisfactory, the micro-organisms cannot convert nitrogenous matter in the soil to a suitable condition for assimilation by the roots, In these circumstances the addition of nitrogen in the form of nitrates will overcome any shortage which may be caused. The stimulating effect of nitrogen on the roots gives them greater foraging power under dry conditions, as well as sponsoring their early development.

Nitrogen is preferably applied in spring, whereas applications of phosphates and potash are better made in late autumn or winter, but a correct balance of these nutrients is essential at all times. Any excess of growth arising from too liberal use of fertilisers may restrict the development of fruit buds, and lack of plant nutrients can result in weak blossom buds and poor general tree health. The fruit buds of most deciduous fruits are formed during the preceding season, and if soil nutrients and general growing conditions are not satisfactory during this, period, the result will be a light crop in the following year. Thinning of Surplus Fruits As blossoming of fruit trees is completed, all blossoms which have not been fertilised fall off the trees. The small fruits which remain increase slightly in size until late November or early December, when in apples and pears a natural drop of surplus or weaker fruits occurs; for stoned fruit this period is about the end of October to early November. This is a normal procedure and is referred to as the “natural drop”. Except, in “offbearing” years following some climatic upset, usually more fruit is left on the trees than is necessary to provide a satisfactory crop. Only a very small proportion of the mass of original blossoms needs to set properly to provide a good crop to be carried to maturity. The period of growth up to about early December is vital in the life of fruit trees, as new shoot growth, fruit setting, root growth, and bud development are all occurring during a relatively short time. That points to the need for thinning fruit as early as possible, but normally thinning should take place only after the natural drop, for all calculations may be upset if further natural dropping occurs. If thinning is neglected, not only will production from the trees be affected in the following season, but the fruit size in the current season will be small.

Though blossom thinning is known to be more beneficial in ensuring a satisfactory crop in the following season than is fruitlet thinning, not only is it a tedious and difficult operation, but it may result in a very poor crop for the current season if a damaging frost occurs afterward. In these circumstances, rather than helping the trees, blossom thinning may throw them into a sequence of biennial bearing. Experimental workers are seeking a spraying chemical and procedure of application for thinning- blossoms during a heavy flowering season. The objects are to lessen the cost of thinning and to reduce the crop as early as possible in spring in the interests of the following season’s crop. The use of chemicals presents many difficulties, as all the . blossom is likely to be destroyed unless the right kind and concentration of the material are applied at the correct stage of blooming. The aim is to kill surplus blossom, but to leave sufficient on the tree for a good crop. Whether this method will become common depends on trials which are being carried out. Proportion of Foliage The proportion of foliage to fruit on a tree is important to the development of both the crop and future fruiting buds. Apples contain about 85 per cent, by weight of water, the other 15 per cent, being made up largely of sugars, fibrous or cellulose materials, pectin and jelly-forming materials, organic acids, and a very small amount of mineral matter. Apart from the water, almost all these materials are synthesised in the leaves of the tree. To carry out this important task the foliage must function efficiently and, as the quality of the crop depends on the amount of efficient foliage, there is a close association between good foliage, ample growth, fruit size, and regular cropping. Therefore the thinning of the crop must be related to the amount of foliage which can be allotted to each fruit.

Diseases and Pests Injury to flowers, buds, spurs, young growth, and roots of trees may be caused by diseases and pests, which thus may affect fruit setting. For example, fireblight affects blossoms, spurs,, and growth; powdery mildew affects new growth; brown rot affects blossoms and growth; black'spot affects young fruits and foliage; and leaf curl affects foliage, young fruits, and growth. Many insects also cause direct or indirect harm. Scales, mites, aphides, leaf hoppers, and thrips cause debility of tree foliage and bark. Thrips can also directly affect the vital organs of the flowers such as the pistils and stamens and have been responsible for drastic reduction of crops in some overseas countries. Fortunately thrips are not unduly common in pip- and stonefruit orchards in New Zealand, but some damage is caused to citrus trees and grape vines. Spray Injury . Incorrect selection of spray materials or time of spraying may cause damage to fruit buds, blossoms, setting fruit, or foliage: Special care is needed with combination sprays because reactions between the chemicals may cause damage to developing fruit and growth. This type of damage is often increased by very high temperatures at the time of spraying or immediately afterward. Care needs to be exercised in the use of winter oils or tar washes when bud movement is near, as damage to the buds may occur. For instance, sulphur and oils must not be used in close association at or after the bud-move-ment stage. . Growers can do much to control the many influences affectingcropping, though certain types of climatic hazards are beyond their control and must be accepted as elements of risk.