This phenomenon is widespread throughout many perennial trees and shrubs, but is not universal. Perennial fruit crops initiate flower buds for next season’s crop in the current season, and for most deciduous fruit species, the alternation of large and small crops is caused by competition between the current season’s crop and the coming season’s flower buds.
Excessive crop in the "on year" depletes the nutrients needed to form new fruit buds; however there also is evidence that seed-produced hormones exported from the developing ovules have a direct inhibitory effect on flower development. Apple is a heavily-studied species with regard to alternate bearing (AB), and research indicates that floral inhibition is caused by seed produced hormones, especially gibberellins (Jackson, 2003), while the data for pear are much less clear in this matter (Dennis, 2003).
This "on-off" sequence does not always follow a regular biennial pattern. For example, Bosc pear, a cultivar that is highly susceptible to AB, may exhibit two consecutive off-years following a heavy crop. For this reason, the term "biennial bearing" is a less appropriate label for this phenomenon.
While internally regulated, AB is often triggered by an external factor (such as unfavorable weather or poor crop management). Once triggered, the fluctuation is likely to continue for years after the triggering event. This "ripple effect" differentiates AB from other cropping irregularities.
Alternate bearing is a significant economic problem for a number of fruit and nut industries worldwide. In a heavy "on-crop" year, the problem is too many small fruit that have a low cash value because of their small size and over-abundance. In an "off-crop" year, fruit are generally too large and of poor quality, with increased potential for physiological disorders. Net return to the grower is low in an off-crop year because there are too few fruit. Thus, AB results in price instability and erratic annual returns to growers. The limited supply of fruit in the off-crop year also can lead to loss of market share that is not always regained the following year.
The severity of AB can be calculated in two ways (Monselise and Goldschmidt, 1982):
Formula 1: Alternate bearing index (I) = (year 1 yield) - (year 2 yield)/(year 1 yield + year 2 yield), where I = 0 is no alternate bearing and I = 1.0 is complete alternate bearing; and
Formula 2: Alternate bearing = (current year's yield) - (5-year running average yield)/5-year running average yield; when the current yield is 20% greater than the 5-year average yield, it is an on-crop and when the current yield is 20% less than the 5-year average, it is an off-crop.
Among deciduous fruits, AB is documented in apple, pear, plum, prune, apricot, cranberry and blueberry (Monselise and Goldschmidt, 1982). Cultivars (varieties) within a given species vary in the degree of crop regularity, along the full spectrum of regularity. Comparing several commercial apple cultivars for example, Gala, Jonagold, Granny Smith and Idared have a low AB index, Rome shows mild susceptibility to AB, Delicious and McIntosh show moderate susceptibility to AB, Golden Delicious shows strong susceptibility, and Fuji is highly susceptible to AB. Additionally, specific strains of a given cultivar may differ in susceptibility to AB. A typical example of this variation would be standard growth habit (more regular) versus spur type (more alternate) strains of Delicious.
Other crops, especially peach (including nectarine), have the internal capacity for a full crop every year. Peach flower buds are not as cold hardy as apple in winter and bloom earlier, thus are more susceptible to crop losses caused by unfavorable weather. However these increased risks for crop irregularity are strictly due to unfavorable weather, and peach has the potential to return to a regular pattern of full crops unless winter cold injury is so severe that it damages the perennial parts of the tree. Peach yields are influenced only by external factors in the previous winter and current growing season.
Role of Cultural Practices and Pollination
Since AB is a naturally occurring and internally regulated process, good management practices are necessary to minimize its occurrence. Generally speaking, flower formation in perennial deciduous fruit crops is promoted under growing conditions that contribute to good plant health and promote moderate vigor. It is cliché, but moderation is truly a key to production of annual crops. For example, both extremes--excessive fertilization (especially nitrogen) or mineral nutrient deficiencies--can lead to reduced flowering and low productivity (Dennis, 2003). Adoption of a production philosophy that promotes the principal of moderation can help to alleviate loss of profitability caused by AB and other cropping irregularities.
Practices that promote plant health and moderate vigor include annual pruning, irrigation to prevent water stress, regular testing for leaf and soil mineral nutrient content, use of fertilizer to maintain optimal nutrient levels, timely mowing and irrigation to minimize water stress and maintenance of tree health through the judicious use of herbicides and crop protectant sprays. While none of these practices are sufficient to promote annual cropping, it would be difficult to prevent AB if these preconditions were not adequate.
All deciduous fruit crops are pollinated by insects, thus providing for adequate pollination by bringing bees into the planting at bloom is a standard management practice. As with the cultural practices listed above, providing bees for pollination does not directly influence AB, rather it reduces the risk of cropping irregularities, which could trigger AB.
Site Selection and Pre-Plant Practices
Site and cultivar selection can contribute to reducing the risk of AB and other cropping irregularities. Spring flowering deciduous orchards are preferentially planted on sites with low risk of frost or freeze damage. Upland sites with slope and elevation greater than the surrounding land allow cold air drainage and are preferred, as are sites situated near large bodies of water that moderate springtime temperatures. Woodlands, windbreaks or other obstructions below orchards may prevent good air drainage and increase the risk of frost damage.
Orchards perform best on fertile, well drained soils with adequate aeration to promote root health. Among deciduous tree fruits, pear is the most tolerant of excess soil moisture and peach is the least tolerant. Because of the fairly exacting site requirements, most new orchards are situated on existing orchard land. Orchard replant sites should be tested for adequate soil physical and chemical properties, and for the presence of replant disease organisms. Pre-plant remedial action should be taken when testing shows it is warranted.
Cultivar selection is primarily a marketing decision; however consideration should also be given to crop bearing potential, as certain cultivars are known to produce more reliable crops. A listing of cultivars that are considered to be regular croppers and well-adapted to the region can be found in the Penn State Extension Tree Fruit Production Guide .
Most cherry cultivars, many plum cultivars, and almost all apples and pears are self-incompatible (Westwood, 1978). In these instances, reliable annual cropping is greatly promoted by the placement of adequate pollinizers in the planting. By contrast, most peach and nectarine cultivars are self fertile. The nursery can provide specific information on pollinizer requirements when fruit plants are purchased.
Crop Load Management
The single most important practice for minimizing alternate bearing is judicious crop load management. Perennial fruit crops tend to set more fruit than a) can be matured to adequate size and quality to meet market expectations; and b) than can allow for adequate flower production for the subsequent season. Thus adjusting crop load is important not only for annual bearing, but for economic sustainability in the current season as well as the next. Several production practices can be used to reduce crop load, but the chief practices for doing so are fruit thinning and pruning.
In order for fruit thinning to increase return bloom, it must be done in the four to six weeks following petal fall. This window of time is thought to correspond to the period when floral initiation occurs, and no amount of crop removal after this time can be relied upon to promote flowering in any but the most annual cultivars. There is evidence that this window of opportunity is shorter for strongly alternating cultivars and longer for more annual bearers. Early thinning timing also is the most beneficial for enhancing fruit size and quality in the current season.
Apple and pear can be thinned chemically using several materials labeled for this purpose (Agnello, et al., 2004). These materials are applied during or shortly after bloom by means of conventional air-blast spray equipment. The following paragraphs summarize the characteristics of the most commonly used chemical thinners, and are not intended to be used as guidelines for the application of these registered compounds. Refer to the Penn State Extension Tree Fruit Production Guide and your local crop advisor for details on the regulatory status and current recommendations for use of chemical thinners.
Certain carbamate insecticides have thinning activity, and carbaryl (e.g., Sevin®) and oxamyl (Vydate®) are labeled for this use. Carbamates at concentration of about 600 parts per million (PPM), have an established track record of being mild but dependable chemical thinners with a relatively wide window of time for efficacy. Being mild thinners, carbamates are often used in combination with stronger thinners, or tank mixed with oil to boost thinning response.
Napthaleneacetic acid (NAA) is a synthetic auxin (a class of plant hormone) with strong thinning activity at concentration between 2.5-20 PPM. NAA (e.g., Fruitone®) has been labeled as a thinner for both apple and pear for several decades, and detailed recommendations have been developed for specific cultivars and different regions. Apple thinning activity is strongest when fruit diameter is between 10 and 15 mm, and progressively milder when applied earlier. Thinning activity becomes less certain as fruit diameter exceeds 17 mm, and fruits become insensitive to NAA when diameter exceeds 20 mm. Pears also are responsive to NAA, but typically require earlier timing (4 to 7 days after petal fall) and higher rates (7.5-15 PPM) than most apple cultivars. NAA can have negative side effects of growth retardation and leaf chlorosis with certain cultivars of apple and pear, and an amide salt formulation (Amid Thin®) is labeled for use with sensitive cultivars. In addition to fruit thinning, NAA has a direct promoting effect on flower formation (see the following section on Return Bloom sprays).
Benzyladenine (6BA) is a synthetic cytokinin also known for strong chemical thinning efficacy at 75-175 PPM. 6BA was registered as an apple thinner in the mid-1990s, and several formulations are now labeled as chemical thinners for apple (MaxCel®, Excelis Plus®, and RiteSize®). MaxCel® was registered for pear in 2007. For thinning apple, 6BA is usually tank-mixed with a carbamate as the combination is more effective than either product applied separately. 6BA has none of the negative side effects of NAA, and promotes cell division in developing fruit, an effect which can increase fruit size over that obtained from thinning alone. 6BA does not stimulate flowering, thus return bloom is only enhanced by the resulting reduction in crop load.
Ethephon (e.g., Ethrel®) is an ethylene releasing plant growth regulator used for apple thinning at 150-600 PPM. Ethylene is a naturally-occurring plant growth regulator that directly stimulates fruit abscission as well as flower formation. Thinning results with ethephon can be unpredictable, and the potential for over-thinning is perhaps the strongest with this material. Ethephon is a strong thinner out to about 23 mm fruit diameter, and frequently is used as a "rescue" treatment when earlier attempts to adjust crop load have failed to remove enough fruit. Ethephon also may be applied after fruits become unresponsive to chemical thinners for its direct promotion of flower formation (see the following section entitled "Return Bloom Sprays").
Obtaining the optimal crop load for fruit size, quality and return bloom with chemical thinning is challenging. Any factor that influences tree health or vigor also will have an impact on fruit set, thus it is challenging to determine the amount of chemical thinning that is required to supplement the endogenous crop load adjustments that are taking place naturally. Weather, especially incident sunlight and air temperature have a profound effect on both natural and chemical fruit thinning. Choice of materials, rates used, timing and spray application method all influence the outcome. Many fruit growers consider chemical thinning to be the single most important and most challenging practice they perform each season.
Still, there is evidence that AB has become a less significant problem for apples and pears than it once was (Monselise and Goldschmidt, 1982). This may be attributed in part to selection of annual bearing cultivars, but far more so to the commercial development of effective chemical thinners. Chemical thinning provides apple and pear growers with an economical means of removing excess crop early enough in the season to permit adequate floral induction for the coming year. It is the first line of defense against AB in apple and pear, and without an adequate thinning program, no other practice can be relied upon to provide adequate numbers of flowers for annual cropping.
Crop load management also can be accomplished by pruning and by hand thinning. While these methods, especially pruning, are used for apple and pear, the role of these alternative methods is somewhat diminished on these two crops because chemical thinning is both timely and more labor efficient. These practices remain vitally important for adjusting crop load of most other deciduous fruit crops. Most commonly, both practices are utilized as a crop load management system to obtain an optimal crop load reduction with maximum labor efficiency.
Pruning removes bearing surface (fruit buds) and stimulates vegetative growth from remaining buds. This promotion of vegetative vigor prevents many of the remaining buds from becoming floral. Thus pruning reduces cropping by two mechanisms. Pruning is a non-selective mass-thinning technique, and therefore is reasonably labor-efficient compared to hand thinning.
Environmental factors, especially sunlight, play an important role in flower induction. Apple buds and especially the supporting spur leaves must receive at least 30% of incident solar radiation in order to initiate flowers (Dennis, 2003). Proper pruning practices for maintaining the fruiting potential of tree fruit include the use of thinning cuts, in which whole branches are removed to open the canopy to light penetration (Ferree and Schupp, 2003). By contrast, excessive pruning or the use of heading cuts can lead to invigoration of vegetative growth and insufficient flowering. Size-controlling rootstocks also play a role in maintaining good light distribution in apple tree canopies (Dennis, 2003).
Hand thinning serves one of the same purposes as chemical thinning--that of removing a portion of the crop, thereby alleviating some of the competition between an excessive crop and its inhibitory effect on flower formation. Hand thinning is labor-intensive, and it must be done early in the growing season in order to have a beneficial effect on AB. Hand thinning can be done by selectively plucking individual fruits from a branch, a technique commonly used with larger fruited species, or by striking the limbs with a rubber- or cloth-covered stick. The latter method is more labor efficient, being a non-selective technique; however it is difficult to obtain consistent results with limb tapping. Large-fruited species may be "pre-thinned" using a combination of pruning and limb tapping, to reduce the cost of selective hand thinning to make the final crop load adjustment.
Return Bloom Sprays
Chemical thinning alone may not be sufficient to promote annual bearing for several commercially important apple cultivars that possess a strong genetic tendency to AB. Examples include York Imperial, Mutsu, Fuji, Macoun, Honeycrisp and spur-type strains of Delicious. In these special cases, plant growth regulators that directly stimulate flower formation should be considered. Two thinner chemistries are labeled for this use (Figure 1). Several weekly or bi-weekly sprays of 3-5 PPM NAA, or 100-200 PPM ethephon may be used, starting when fruit diameter has exceeded 24 mm, (about five weeks after bloom), and up until eight weeks after bloom. These low rates of chemical are recommended to avoid undesirable side effects of these treatments, such as reduced fruit size and premature fruit ripening.
Cropping and Tree Age
Yields of deciduous fruit crops tend to decline gradually with plant age. Although this decline is not solely attributed to AB, the additional tree stress and limb breakage from excessive over-cropping in repeated "on" years can shorten the productive life of an orchard. This decline can be attributed either to a decline in plant vigor and tree health as exhibited in both fruit number and size, or to a decline in plant number per acre caused by mortality. These two mechanisms are not mutually exclusive, but the extent to which each mechanism contributes to yield decline differs by crop. For example, pome fruit (apple and pear) yield decline is attributed mainly to declining number and size of fruit, and is very gradual. Yield decline of these crops may only become evident after 40 years in well-maintained orchards. On the other hand, peach trees tend to have a shorter life (Westwood, 1978), and tree mortality often closely follows any perceived decline in fruiting. Peach orchard yields decline much more rapidly, due primarily to tree mortality.
Alternate Bearing (AB) is an alternating pattern of large and small crops occurring in many fruit species that is internally regulated by the plant. AB has a number of undesirable consequences that cause economic losses to fruit growers. Cultural and pollination practices, and variety and site selection are important preconditions to achieving annual production, but are not adequate in themselves to prevent AB. Adjusting crop load to a moderate level early in the growing season is the single most important key to preventing AB. Crop can be adjusted by a variety of methods, depending on the fruit type, and often several methods are used in combination. The development and use of effective chemical thinners has been successful in reducing AB in apple and pear production over the past 50 years. Apple varieties that are strongly AB can also benefit from sprays of growth regulators that promote flowering.
Agnello, A.M., A.J. Landers, W.W. Turechek, D.A. Rosenberger, T.L. Robinson, J.R. Schupp, J.E. Carroll, L. Cheng, P.D. Curtis, D.I. Breth, and S. A. Hoying. 2004. Pest management guidelines for commercial tree-fruit production 2004. Cornell Coop. Ext. Publ.
Dennis, F. Jr. 2003. Flowering, pollination and fruit set and development. In: D.C. Ferree and I.J. Warrington, eds. Apples: Botany, Production and Uses. CABI Publishing, Cambridge, MA. Pp. 153-166.
Ferree, D.C. and J.R. Schupp. 2003. Pruning and training physiology. In: D.C. Ferree and I. J. Warrington (eds). Apples: Botany, Production and Uses. CABI Publishing, Cambridge, MA. pp. 319-344.
Jackson, J.E. 2003. Chapter 9: Flowers and fruits. In: Biology of Apples and Pears. Cambridge University Press, New York, N.Y. pp. 268-308.
Monselise, S.P., and E.E. Goldschmidt. 1982. Alternate bearing in fruit trees. In: J. Janik, ed. Horticultural Rev. 4:128-173.
Westwood, M.N. 1978. Temperate Zone Pomology. W.H. Freeman and Co., San Francisco, CA.