Compendium of Apple and Pear Diseases and Pests, Second Edition

Part II. Noninfectious Disorders Nutritional Disorders Fruit trees require a certain supply of minerals to grow and develop normally. If the supply of any of the essential minerals is below the minimum, the trees may show a range of symptoms from leaf chlorosis to dieback or from small fruit size to misshapen fruit. In contrast, if the supply of a given element is too high, it becomes toxic, and various organs of the trees show symptoms of toxicity. For each essential element, there is a level between deficiency and sufficiency at which some of the functions of the element are impaired, and disorders are present, but typical deficiency symptoms do not develop. This level is termed insufficiency. The terminology of deficient, insufficient, sufficient (normal), and excessive levels is used throughout this discussion to designate nutritional levels and responses of trees and fruit.

A comprehensive analysis of various organs provides a good picture of tree growth, productivity, and fruit quality attributes.

However, leaf analysis is often used for the nutrient status of the tree. Leaves are collected from the middle of the current year’s shoots at the time of terminal bud formation or minimal nutrient flux (usually in mid-­July to mid-­August), dried, ground to pass through 40-­mesh screens, and ashed, and the mineral elements are analyzed by atomic absorption or other appropriate chemical analyses. If the condition of an orchard is to be assessed, only one or two leaves are collected per tree to make up a minimum of 100 leaves. For medium-­sized individual trees, usually one leaf per shoot is used, and 20–40 shoots are sampled.

For fruit, wedges are obtained from a minimum of 10 fruit and freeze-­dried to facilitate pulverization. Seeds and core tissues of fruit are not usually included in the fruit analysis. Both leaf and fruit tissues should be carefully washed in special detergent and rinsed in distilled water before drying and grinding.

shoot leaves during mid-­July to mid-­August, insufficient at 1.4–1.7%, sufficient at 1.8–2.3% (the upper limit is a sufficient rate in areas of the southern United States), and excessive at 2.4% or more. In newer apple cultivars, the levels of N in leaves and fruit should carefully be monitored to ensure a better fruit color. In ‘Fuji’ apple, in which fruit color can easily be impaired by excess N, the sufficiency level for N is 2.0–2.1% during off years (low-­crop years), while it is about 2.2–2.4% during on years (heavy-­crop years). In pears, N is deficient at 1.4% or less, insufficient at 1.5–2.0%, sufficient at 2.1–2.5%, and excessive at 2.6% or more.

In apples with an abundant supply of the nutrient, fruit N levels increase much faster than leaf N levels. Thus, a modest increase of 25% in leaf N, from 2 to 2.5% of dry matter in midshoot leaves in mid-­July, may be accompanied by an increase of more than 100% in fruit N, from 0.20 to 0.45% of fruit dry matter. Such a high concentration in the fruit is conducive to several types of breakdown, described under Calcium-­Related Responses. A high level of fruit N may lead to a rise in the fruit internal ethylene evolution and respiration. This phenomenon can have a major impact on harvest and storage strategies for apple growers. Fruit with excessive N have poor color, and while the grower delays the harvest, hoping to improve the color, internal ethylene production and respiration increase, which leads to a high rate of loss in storage. Also, excess N application can increase the severity of fire blight infection of trees.

Potassium-­Related Responses In general, for apple and pear, potassium (K) is considered sufficient at levels of 1.5–1.8% of the dry matter of midshoot leaves sampled in mid-­July to mid-­August. Deficiency

Nitrogen-­Related Responses Nitrogen (N) affects vegetative growth, flower bud formation, and fruit characteristics of the tree. The growth of trees is generally in direct proportion to the amount of N applied. Restriction of growth caused by N insufficiency is expressed by a reduction in the top-­to-­root ratio, decreased trunk circumference, decreased width and height of trees, and production of pale green leaves, starting from older leaves. The physiological point at which N deficiency begins is difficult to define. Trees are usually not deficient until the appearance of pale green leaves.

Trees with a N shortage have small fruit size, a small crop, and very short shoots (Fig. 154). When trees are insufficiently supplied with N, application of the nutrient increases the size of spur leaves, which in turn increases early-­season photosynthesis and flower bud formation. High levels of N may have the opposite effect on young trees, increasing vegetative growth and delaying bud formation.

In some apple cultivars, N is deficient (pale green leaves are produced) at levels of 1.4% or less of the dry matter in mid-

Fig. 154. Early leaf senescence and small fruit size caused by insufficient nitrogen in ‘Fuji’. (Cour­tesy E. Fallahi)

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symptoms are visible only when the concentration is below 0.75% of the leaf dry weight. The main visible symptom on both apple and pear is leaf scorch. First, the leaf loses its normal green color, and then it displays water-­soaked areas and finally develops necrosis. Scorching proceeds inward from the leaf edge in apple, and it often results in large necrotic areas in the middle of the leaf in pear. Leaves containing 1% K are free of visible symptoms, but the fruit does not attain normal size or develop normal color (e.g., red apples are often dirty brown).

At or below the 1% level, the growth of the tree is impaired; reduced branch diameter and shoot length or shoot dry weight are common indicators. Thus, leaf concentrations above the level at which deficiency symptoms become visible are still considered essential, and levels below 1.5% are insufficient.

Fertigation or effective foliar application of K to apple trees with K deficiency may improve fruit size, yield, and color, while K application to the trees with sufficient K may not improve these quality attributes.

Water deficiency and drought can lead to a sharp reduction in leaf and fruit K concentrations. Trees irrigated with a sprinkler system may have higher K levels in the leaf and fruit tissues than do those irrigated with a drip system.

Calcium-­Related Responses Calcium (Ca) affects fruit senescence and quality by altering intracellular and extracellular processes, and the rate of fruit softening depends on the fruit Ca status. Ca also plays a regulatory role in various processes that influence cell function and signal transduction. At least 60% of the total Ca in the plant is associated with the cell wall fraction. The structure of the cell wall is composed of cellulose microfibrils embedded in a gel-­ like matrix made of several noncellulosic polysaccharides and glycoproteins.

Chlorosis of leaves caused by Ca deficiency is rare in orchards in the United States, but it has been induced in sand or liquid cultures, and develops in orchards in some other countries, e.g., China. If deficiency is induced, chlorotic leaves are found on young, rapidly growing shoot tips. Symptoms of deficiency on leaves are observable only if the leaf Ca content is 0.5–0.6% of the leaf dry weight. Fruit show deficiency symptoms if the leaf Ca content is 1% of the leaf dry weight; these symptoms are not expected when the level is 1.8% or more. In

Fig. 155. Cork spot on a ‘Delicious’ apple. (Cour­tesy G. M. Greene)

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trees older than about 25 years, low Ca levels cause a dramatic loss of vigor that cannot be restored by severe pruning or nitrogen (N) application. The normal Ca content of fruit flesh ranges from 0.01 to 0.03% of the dry weight. Peel or core Ca levels are two to four times higher. A level of 0.025% in the flesh is sufficient to prevent most disorders associated with Ca. Low levels of Ca per se may not cause disorders if other causes, such as excessive vigor or high N content in the trees, do not exist.

Consequently, if the cause of a disorder is present, Ca levels must be sufficient (0.025% or higher) to prevent the disorder, but if the cause is not present, the fruit is free of the disorder even at very low Ca levels.

The tissue concentration of N has an overriding effect on the development of Ca-­related disorders. The ratio of N to Ca in the flesh of apples may vary from 10 to 30. When the ratio is 10, the fruit will be normal, but at a ratio of 30, disorders almost always arise. Fruit N levels change rapidly with increasing N application, whereas the Ca content is unchanged or perhaps somewhat decreased. Changes in N concentration are the major determinant of the ratio of N to Ca. Thus, limiting N rather than increasing Ca is often more practical in correcting Ca-­related disorders.

Cork spot and bitter pit. After normal cell division, healthy, enlarging cortical cells undergo redifferentiation by direct nuclear or amitotic divisions in a Ca-­related disorder, resulting in cork spot in pears. The nucleus divides, and a cell wall is formed within the cell between the two daughter nuclei. The newly formed nuclei may divide again and again. As a result, the original cell is braced with internal cell walls, and the tissue hardens, ruptures, and eventually browns. Ca deficiency or a high ratio of fruit magnesium/calcium, potassium/calcium, or nitrogen/calcium may cause bitter pit in apples, which has symptoms similar to those of cork spot in pears. The spot is always in the flesh. In ‘York Imperial’ apples and in pears, spots are about 1.0–1.5 cm beneath the skin; in ‘Delicious’, ‘Braeburn’, and ‘Fuji’ apples, spots usually touch the skin. The cells of the affected area do not enlarge during the course of fruit growth, and this causes a slight depression on the surface of the fruit (Fig. 155). Because of the high metabolic activity in tissues surrounding these spots, anthocyanin forms prematurely in the apple skin, and the depressed areas turn red, usually during early August. The red coloring does not develop in pears.

The disorder does not develop further in storage.

Several cultural practices and physiological factors, such as light crop, excessive pruning and vigor, and excessive nitrogen, magnesium, and potassium applications, may increase the chance of bitter pit developing in apples. Bitter pit symptoms may be less visible at harvest, but they get worse after storage.

Braeburn and Golden Delicious are among the apple cultivars sensitive to bitter pit.

Deep cracking. The cell walls of apples with low Ca levels are thick and not as elastic as those of healthy fruit. As a result, the fruit may crack after irrigation or rain, when their turgor pressure is high. The cracks are deep if the Ca deficiency is severe in certain apple cultivars (e.g., Stayman); cracks develop only around the calyx and are usually superficial if the deficiency is mild.

Raised lenticels. Usually, the first symptom of an insufficiency of Ca in apple fruit is abnormally raised lenticels.

Raised lenticels are especially prominent on yellow cultivars.

The cause of this phenomenon is not known. Lenticels are such good markers of the insufficiency that an experienced analyst can estimate the fruit Ca content by visually examining the lenticels.

Sunburn and crinkle. Although there is no progression from one to the other, both sunburn and crinkle are apple disorders caused by exposure to solar radiation. Some cultivars (e.g., Granny Smith, Golden Delicious, and Fuji) are more sensitive to sunburn than are others, but fruit of all cultivars become bleached if the fruit Ca content is low. If the exposure is in-

tense, the fruit tissue loses water until the cells collapse, and the tissue under the skin browns. Crinkle usually affects a large area of the fruit surface, mostly on the southwest side of the tree, and fruit on pendulous branches are more susceptible.

Tree pit and tissue collapse. Occasionally, close to harvest, sunken areas (pits) appear on apples attached to the tree. The pits resemble symptoms of bitter pit, and, because they appear while the apples are still on the tree, the phenomenon is called tree pit. The disorder is associated with low Ca levels in the fruit. Tissue collapse may occur as early as August if the fruit Ca content is very low, and the tissue collapses in relatively large areas. Tree pit and tissue collapse are well correlated with the Ca concentration in the fruit, and they may be considered the only true symptoms of Ca deficiency.

Premature ripening. Ripening is thoroughly discussed in books of postharvest physiology. It should be mentioned here that apples, and perhaps pears, produce ethylene prematurely and ripen earlier if the Ca concentration in the fruit is low. This is especially important in summer-­ripening cultivars that require multiple pickings. The fruit of the first harvest is lowest in Ca. When summer cultivars are harvested, there are yellow and green fruit on a tree at the same time. The difference in their Ca contents is considerable; the green fruit may contain twice the concentration of Ca as the yellow fruit.

Phosphorus-­Related Responses Because of limited reports of positive responses to phosphorus (P) fertilization of apple orchards, P fertilization has received much less attention than has nitrogen, potassium, and calcium fertilizations. Several conditions have been identified when apples respond to P fertilization. These include times when the apple root length is limited, such as when trees are newly planted and when replant disorders further inhibit root growth, or when low soil P levels limit the P availability to the roots. P application also may be essential for trees grown on volcanic soils with high anion-­fixing capacity. Applications of P may increase the vigor and accelerate flowering of newly planted trees. Foliar application of soluble P compounds can increase the fruit P concentrations and reduce the susceptibility to low-­temperature breakdown in ‘Cox’s Orange Pippin’ and the firmness of ‘McIntosh’ apples, but it is not known whether soil P applications would result in similar improvements in fruit quality.

Drip fertigation has improved the availability of P to apple trees by allowing mass flow delivery of high P concentrations directly to the root surface, thereby proving to be as effective at increasing first-­year tree P uptake as P application in the planting hole. Therefore, P fertigation has become a standard first-­ year recommendation for growers in southern interior British Columbia, Canada.

Mg levels and vice versa. Thus, an accurate interpretation of both potassium and Mg in leaves is essential.

Apple fruit with high magnesium/calcium develop bitter pit symptoms during storage. Thus, vacuum infiltration of Mg is practiced in some apple-­growing regions, including Chile, to force the bitter pit–disposed fruit to display the symptoms before storage.

Iron-­Related Responses About 80% of the iron (Fe) in plants is located in the chloroplasts. Fe-­deficient plants show various degrees of interveinal chlorosis. Young leaves usually become chlorotic first because of the poor development of the chloroplasts (Fig. 157). The deficiency may be caused by low Fe levels in the soil, and it is often induced by bicarbonate ions, which are present in high­pH soils or in irrigation water and render Fe unavailable to the roots. Leaves on trees with bicarbonate-­induced deficiency are almost white, whereas leaves on trees in soil with low Fe levels are chlorotic. Severe deficiency causes necrotic spots to develop in the chlorotic leaves. When multiple deficiencies of manganese, zinc, and Fe are induced in apple, the expression of Fe deficiency predominates.

Green leaves often develop when the level of bicarbonate in the soil is very low, usually when the soil moisture is low.

When the soil moisture and the bicarbonate level increase with rainfall or irrigation, the newly formed leaves become chlorotic from bicarbonate-­induced deficiency. Bicarbonate ion formation requires calcium, carbon dioxide, and water in the soil.

Therefore, Fe deficiency symptoms are often visible in leaves

Fig. 156. ‘Macoun’ apple leaves with interveinal chlorosis and marginal necrosis caused by magnesium deficiency. (Cour­tesy W. F. Wilcox, from the files of K. G. Parker)

Magnesium-­Related Responses Magnesium (Mg) deficiency causes interveinal chlorosis, and the chlorotic areas become necrotic only in extreme cases (Fig. 156). The affected leaves drop early in the season. By mid-­August, Mg-­deficient trees are partially defoliated. The degree to which the symptoms develop varies greatly. Only a few branches may be affected, or entire trees may be uniformly defoliated. Mg is a mobile ion, and in case of deficiency, it is transported from old to young leaves. Consequently, the old leaves drop first.

The fruit requires considerable amounts of Mg. If trees are marginally supplied with the nutrient, those with fruit display symptoms of the deficiency first or develop more severe leaf symptoms. Mg-­deficient leaves usually do not produce sorbitol but accumulate starch, and carbohydrates are not transported into the fruit. Consequently, the fruit of Mg-­deficient trees are small.

Mg and potassium show strong antagonism with each other.

Leaves and fruit with higher potassium levels often have lower

Fig. 157. Iron deficiency symptoms on leaves of ‘Delicious’ apples. (Cour­tesy E. Fallahi)

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formed after rains or near irrigation ditches (where water is more plentiful) if the other two factors are present.

Boron-­Related Responses Apple and pear trees respond to boron (B) with a range of symptoms. The normal B concentration in midshoot apple leaves during mid-­July to mid-­August is 35–40 parts per million (ppm) on a dry weight basis. If B is excessive (60–70 ppm), the fruit may ripen and fall prematurely. Excessive B increases the hydrolysis of starch to simple sugars and increases internal browning in apples (Fig. 158). At 25 ppm, the B content is insufficient, and budbreak is usually 1 week later than it is on trees well supplied with the nutrient. The fruit from trees with this insufficiency are flat, especially on cultivars such as York Imperial apple, and severe B deficiency may result in fruit cracking in pears. In B-­deficient trees, calcium translocation is impaired, and thus disorders related to calcium deficiency may appear.

Internal cork (drought spot) develops when the leaf B concentration is 14–21 ppm, appearing as brown spots within the core area. Affected pits of the core cease to enlarge, and the fruit becomes misshapen. The disorder is similar to cork spot in many respects, but internal cork develops within the core rather than in the flesh. When the leaf B concentration is 12 ppm or less, the vegetative parts of the tree are also affected. At this concentration, shoots die back during late summer. Leaves on terminal shoots may become chlorotic with a reddening of the veins. Small lesions develop on the bark near the tips of the shoots, and the inner bark under the lesions turns brown. The internodes of the tree are short, and the leaves form rosettes.

When foliar B is deficient (about 17 ppm) in pears, flowers wither and appear as if infected with Pseudomonas syringae

Fig. 158. Internal breakdown of apple caused by excess boron content. (Cour­tesy F. Peryea)

van Hall (=P. syringae subsp. syringae van Hall); the symptom is similarly called blossom blast. Autumn foliar spraying is usually more effective in preventing the disorder than is spraying in the spring. However, soil application or fertigation with B is needed to solve the problem. The true nature of the problem should be decided by determining the B level, attempting to isolate the bacterium, or both.

Zinc-­Related Responses The most characteristic symptom of zinc (Zn) deficiency of apple and pear is the rosetting of leaves (Fig. 159). The leaves produced are small, narrow, mottled, and bunched together at the end of the shoot tips; hence the name “little leaf disease” for this deficiency. Rosetting is most easily recognizable during the first flush of growth. Affected terminals may die the following season, and many of the laterals produced are weak. The narrowing of the terminal leaves is the best indicator of incipient deficiency. Leaves with a Zn content of 25 parts per million (ppm) or less are deficient; they are pale green or yellow if exposed to the sun but may develop normal coloring if shaded.

Zn-­deficient terminal leaves also exhibit interveinal chlorosis.

The deficiency often develops in trees on sandy soils containing limited quantities of Zn or on soils with a high pH or high in phosphorus and calcium. As a common mistake, some fruit growers apply excessive phosphorus to the soil with low phosphorus and, thus, induce Zn deficiency in the tree.

Manganese-­Related Responses The normal level of manganese (Mn) in midshoot apple and pear leaves in mid-­July to mid-­August is 70–85 parts per million (ppm) on a dry weight basis. The typical reaction of apple and pear to Mn deficiency is interveinal chlorosis, which develops in older leaves during midsummer and becomes more pronounced as the season progresses. In severe deficiency, heavy defoliation may occur, especially following strong winds. Leaf chlorosis caused by the deficiency, unless severe, is not associated with loss of tree vigor.

Mn toxicity may be found in acidic soils, especially in ‘Delicious’ and ‘Jonathan’ apples. It may cause chlorosis, early leaf abscission, reduced flower bud development, and internal bark necrosis. The Mn level in affected tissue may be high when internal bark necrosis develops, but foci of accumulation, rather than an overall high Mn content, are characteristic of the disorder. Affected tissue can eventually contain Mn at levels as high as 500 ppm. Excessive and repeated applications of urea as the source of nitrogen to a small radius around the trunk of the trees often leads to excess Mn or even Mn toxicity in ‘Delicious’ and ‘Fuji’ apples and this practice should be avoided.

Selected References

Fig. 159. Rosetting of apple shoot leaves caused by zinc deficiency (right) and healthy leaves (left). (Cour­tesy S. V. Thomson)

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Bramlage, W. J., Drake, M., and Lord, W. J. 1980. The influence of mineral nutrition on the quality and storage performance of some fruit grown in North America. Pages 29-­39 in: Mineral Nutrition of Fruit Trees. D. Atkinson, J. E. Jackson, R. O. Sharples, and W. M. Walter, eds. Butterworths, London.

Conway, W. S., and Sams, C. E. 1985. Influence of fruit maturity on the effect of postharvest calcium treatment on decay of Golden Delicious apples. Plant Dis. 69:42-­44.

Drake, M., Bramlage, W. J., and Baker, J. H. 1979. Effects of foliar calcium on McIntosh apple storage disorders. Commun. Soil Sci. Plant Anal. 10:303-­309.

Fallahi, E., Righetti, T. L., and Raese, T. J. 1988. Ranking tissue mineral analysis to identify mineral limitation on quality in fruit.

J. Am. Soc. Hortic. Sci. 113:382-­389.

Fallahi, E., Conway, W. S., Hickey, K. D., and Sams, C. E. 1997. The role of calcium and nitrogen in postharvest quality and disease resistance of apples. HortScience 32:26-­30.

Fallahi, E., Colt, W. M., and Fallahi, B. 2001. Optimum ranges of leaf nitrogen for yield, fruit quality, and photosynthesis in ‘BC-­2 Fuji’ apple. J. Am. Pomol. Soc. 55(2):68-­75.

Ferguson, I. B., and Watkins, C. B. 1989. Bitter pit in apple fruit. Hortic. Rev. 11:289-­355.

Marlow, G. C., and Loescher, W. H. 1984. Watercore. Hortic. Rev. 6:189-­251.

Neilsen, G. H., Neilsen, D., and Peryea, F. 1999. Response of soil and irrigated fruit trees to fertigation or broadcast application of nitrogen, phosphorus, and potassium. HortTechnology 9:393-­401.

Neilsen, G. H., Neilsen, D., Herbert, L. C., and Hogue, E. J. 2004.

Response of fertigation of N and K under conditions susceptible to the development of K deficiency. J. Am. Soc. Hortic. Sci. 129:26-­31.

Neilsen, G. H., Neilsen, D., Hogue, E. J., and Herbert, L. C. 2004. Zinc and boron nutrition management in fertigated high density apple orchards. Can. J. Plant Sci. 84:823-­827.

Neilsen, G. H., Neilsen, D., Dong, S. F., and Toivonen, P. 2005. Application of CaCl2 sprays earlier in the season reduce bitter pit incidence in ‘Braeburn’ apple. HortScience 40:1850-­1853.

Shear, C. B., and Faust, M. 1980. Nutritional ranges in deciduous tree fruits and nuts. Hortic. Rev. 2:142-­163.

(Prepared by M. Faust and R. F. Korcak; Revised by E. Fallahi)

Disorders Caused by Environmental Factors Low-­Temperature Injury Low temperatures in the spring, autumn, and winter or rapid temperature fluctuations during those seasons can severely injure apple and pear trees and buds. The latter can often be avoided by growing apples near large bodies of water that moderate temperatures and by growing apples on slopes that allow cold air to drain away from the orchards.

Spring injury. As temperatures increase in late winter and early spring, tissues deharden and become more susceptible to injury from rapid drops in temperature. Both floral and vegetative meristems within the buds can be damaged if temperatures fall sufficiently low during the winter or when temperatures drop rapidly in the spring or autumn or following winter thaws.

Hardiness continues to be lost as the buds open, and the tissues of the blossoms and young fruit, especially the ovules and developing seeds, are very susceptible. At maximum hardiness, some apple cultivars can withstand temperatures of –35°C or below, but open blossoms and young fruit can be damaged at temperatures of –2°C. In apple, the center or “king” flower in each cluster is the most advanced and therefore the most susceptible. Early freezes may damage this flower, causing it to have a shorter stem than the lateral buds or it may be killed completely, leaving the lateral buds intact. Freezes that take place when the flowers are more advanced may kill only the pistil, ovules, or developing seeds. In some cases, distorted, seedless fruit may be produced if they are large enough to survive without the seed (Figs. 160–162). Severe freezes may kill the entire developing fruit (Fig. 163). If the injury is less severe, frost rings can be formed at the calyx or the equator of the fruit, where ice crystals have lifted the epidermis, or a sector of the fruit may be severely russeted (Fig. 164). The damage is sometimes confined to the apical region of the fruit; it grows little thereafter, but the basal portion continues to develop, giving rise to fruit resembling those damaged by the rosy apple aphid (Dysaphis plantaginea (Passerini)).

Fig. 160. Distortion of mature ‘Idared’ apples caused by frost injury following bloom. (Cour­tesy W. F. Wilcox)

Although vegetative tissues are more resistant to freeze injury, entire buds or shoots can be killed by severe freezes. Leaf tissues can also be disrupted by milder freezes, which cause crinkling of the leaf blade (Fig. 165).

Autumn injury. Autumn freezes can damage fruit on the tree, making it unmarketable except for juice. Freezing can take place at temperatures approaching –2°C, the extent of injury increasing with exposure time. The injury appears as a browning and softening of the flesh. Such fruit should be allowed to thaw before being harvested, because bruising accentuates the injury. The symptoms in thawed fruit vary from slight discoloration and a water-­soaked appearance to complete browning

Fig. 161. Distortion of ‘Bartlett’ pears caused by frost injury about 1 month after bloom. (Cour­tesy A. L. Jones)

Fig. 162. Late-­s pring frost injury on ‘Bartlett’ pears. (Cour­tesy W. F. Wilcox, from the files of K. G. Parker)

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of the flesh and a spongy texture. Injury to vegetative tissues can also occur in the autumn if they have not hardened off in response to low temperatures and short photoperiods. As hardening progresses, the temperature required for injury decreases from –5°C (nonhardy) to about –35°C (maximum hardiness).

Apple tissues are generally hardier than pear tissues, although some pear cultivars are hardier than some apple cultivars. The extent of injury depends on the minimum temperature, the rate of temperature decline, and the period of exposure. Because

Fig. 163. Frost injury in the interior of young apples frozen about 4 weeks after bloom. (Cour­tesy F. G. Dennis, Jr.)

Fig. 164. Frost rings on apples and pears. (Cour­ tesy D. A. Rosenberger)

trees harden from the top down, the injury is generally greatest in the crotches and at the base of the trunk. Phloem and cambium tissues in these regions may be killed by severe autumn freezes; bridge grafting may be required for repair. Heavy cropping trees tend to be more susceptible to injury than light cropping trees, and late-­harvested trees are more susceptible than early-­harvested trees of the same cultivar. Similarly, late-­ ripening cultivars are generally more susceptible than early-­ ripening cultivars. Trees under environmental (e.g., drought, flooding, hail, or wind) or biotic (e.g., early defoliation or foliage damage cause by insects or disease) stresses are more susceptible than healthy trees. Trees should be pruned only lightly before they attain maximum hardiness because heavy pruning can delay hardening.

Winter injury. At maximum hardiness, the phloem and cambium are hardier than the xylem, and low winter temperatures can result in blackheart, the death and discoloration of the ray parenchyma cells within the xylem. The xylem cells themselves are already dead, but reserve carbohydrates stored in the ray cells are immobilized, and freezing injury may permit the entry of pathogens. Damage to buds can take place at temperatures of –25°C or below, the severity varying with the cultivar and previous conditions. At very low temperatures, the trunk may split as water is withdrawn from the cells and the tissues shrink.

As temperatures increase in late winter and early spring, tissues deharden and critical temperatures rise. If a prolonged warm period is followed by a sudden drop in temperature, the bark, flower primordia, and vegetative meristems may be damaged. The temperature of bark tissues exposed to direct sunlight can rise to 25–30°C, even when the air temperature remains at or below 15°C; this leads to rapid loss of hardiness and subsequent injury when freezing occurs. Damage to tissues on exposed trunks and large scaffold limbs (termed southwest injury, in the Northern Hemisphere, or sunscald) consists of blistered bark and cracks of various depths. The bark of the trunk and scaffold limbs may split, exposing the cambium, which dries out and dies. Internal browning of the phloem and cambium is often apparent, but growers should use caution in evaluating injury. If a sufficient number of cells survive, new cambium cells can be produced, and the tree will recover. Such trees should be pruned lightly, if at all, and reexamined after spring growth begins. A number of cultural practices can be used to prevent or reduce winter injury. Trees should not be overfertilized or pruned so heavily that they continue growth late into the autumn. Soil management and irrigation practices should also be considered as means of controlling growth. Control of insects and diseases is essential for optimum hardening, to avoid the diversion of photosynthates from the process. The removal of excess fruits (thinning) also permits better hardening. Trunks exposed to the direct rays of the sun can be shaded or painted with white latex paint to reflect light and thus decrease bark temperature.

Sunburn

Fig. 165. Crinkling of apple leaves from spring frost injury and tearing of leaves by hail. (Cour­tesy S. V. Thomson)

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Sunburn and sunscald are injuries to the fruit or vegetative tissues of apple trees. Damage to fruit by exposure to solar radiation is usually described as sunburn, whereas sunscald is injury to the bark and underlying tissues caused by freezing (see Low-­Temperature Injury). Sunburn, as the term implies, develops when fruit is exposed to direct solar radiation. Sunburn is a perennial problem in arid areas. It is seldom a problem on red-­skinned cultivars in humid areas of North America unless shaded fruit are suddenly exposed to direct sunlight as a result of summer pruning or because of sudden shifts in branch positions that sometimes happen with heavy crop loads. Green and yellow cultivars, such as Granny Smith and Golden Delicious, are particularly susceptible, but even red cultivars may suffer sunscald on hot, sunny days. Affected areas become white, tan, or brown (Fig. 166). If the injury is mild, the flesh itself may

not be damaged, but longer exposure can result in severe injury to both skin and flesh, with sunken areas and dead tissue extending 1 cm or more into the fruit. As limbs bend under the weight of the growing fruit, the fruit in the interior become exposed and susceptible to injury. To avoid sunburn, growers can encourage vigorous growth to provide more foliage for shade, and they can support branches to minimize movement and the resultant exposure as the crop matures. In arid areas, sunburn can be averted by cooling fruit with overhead sprinklers. Applications of kaolin clay particle films or wax-­based coatings have been shown to reduce sunburn. Once harvested, fruit should be moved quickly to shaded areas or packinghouses to avoid injury on newly exposed surfaces.

Fruit Cracking The severity of fruit cracking varies with the cultivar. ‘Stayman’, ‘York Imperial’, ‘Cox’s Orange Pippin’, and ‘James Grieve’ apples are especially susceptible. Cracks may originate on the cheeks (‘Stayman’) or at the calyx (‘York Imperial’) and may extend from the stem or calyx in meridional lines toward the cheeks (Fig. 167). Cracks often appear in areas affected by abnormalities, such as russeting apple scab lesions, or by sunburn. The cracks may be up to a centimeter deep.

Cracking is not associated with rapid changes in soil moisture, as when the soil is saturated after a period of drought.

There is a strong association, however, between cracking and a depressed rate of transpiration for 6 or more hours. Portions of fruit tissues beneath areas susceptible to cracking have higher osmotic potential than do areas that are not affected. More water moves to areas with high osmotic potential during periods of high relative humidity, promoting the swelling of such tissues. If the hypodermal tissues are unable to expand tangentially, as when the surface has been damaged by russet or scab, cracking appears. Overmature fruit of some cultivars are also prone to cracking in the stem cup area, especially when rains collect in the stem cup just prior to harvest.

Covering fruit with brown paper bags several weeks before harvest greatly reduces the incidence of cracking. Applications of gibberellic acid have also been shown to suppress cracking.

drought continues, leaves become chlorotic, then light gray, and eventually light brown; however, this takes place only under the most severe conditions. If the drought persists for a full season, flower bud formation and flower development may be reduced the next year. In humid production regions, drought stress may predispose trees to cankers caused by Botryosphaeria dothidea (Moug.) Ces. & De Not.

Flooding Symptoms of anoxia are similar to those of drought. Apple and pear are more resistant to “wet feet” than are stone fruit and can survive up to 6 weeks of flooding under experimental conditions. Waterlogging of the soil in the spring and summer is more detrimental to mature apples than is waterlogging in the autumn. As with drought, shoot and leaf growth are affected first, followed by wilting, leaf chlorosis and browning, and finally defoliation. Roots may exhibit necrosis and a characteristic blue-­purple mottling. The following year, the trees at first appear to thrive, but then the leaves begin to wilt and fruit drop and dieback takes place (as with severe drought) as a result of the inability of the roots to provide water.

Hail Hail damage to foliage, fruit, shoots, and scaffold limbs can happen any time during the growing season. The degree of injury depends on the size and density of the hail and the stage of vegetative and reproductive development at the time of injury.

Small hailstones damage the more tender parts of the tree, such as the leaves and fruit (Fig. 168); even pea-­sized hail can virtually destroy a crop when fruit are developing on the tree. If the

Drought Inadequate soil moisture can induce different physiological and morphological symptoms, depending on the severity of the drought. Short dry periods during the growing season can decrease shoot, leaf, trunk, and fruit growth without affecting the appearance of the leaves. The stomata remain open, and the leaves continue to fix carbon because apple has the ability to adjust osmotically to increasingly negative water potentials.

On the other hand, a long period of sustained drought may induce cupping of the leaves, wilting and defoliation, decreased fruit set, increased fruit drop, and even shoot dieback. As the

Fig. 166. Sunburn of ‘Delicious’ apples. (Cour­tesy A. L. Jones)

Fig. 167. Cracking of ‘Stayman’ apples. (Cour­tesy K. S. Yoder)

Fig. 168. Hail injury on apples. (Cour­tesy K. S. Yoder)

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injury takes place early in the season, suberization heals the wounds, which then appear as large scars or deformed areas on the fruit. If it takes place later in the season when growth is almost complete, the wounds do not heal and the injury resembles bird damage. Large hailstones can affect all parts of the tree, shredding leaves and damaging small shoots and scaffold limbs. Succulent tissues damaged by hail are very susceptible to infection by fire blight. Bark wounds can be observed on the top side of the branch; in some cases, they cover 50% of the circumference of the limb. Although these wounds can heal and appear like small galls or cankers, they weaken the limb and provide openings for the entry of pathogens.

Lightning Lightning occasionally kills trees that are struck. The bark may be separated from the wood from the top of a damaged tree to the ground, the whole central leader may collapse, and the foliage turns brown and drops. Whole rows may be killed when lightning hits trellis wires. More commonly, the only response is a dieback from the tip of the highest one or two shoots in each tree. The dieback resembles fire blight, except that there is a distinct margin between healthy and necrotic tissues in lightning-­damaged trees and no bacterial ooze is produced. The pith of shoots may be dry and separated (Fig. 169). The distribution pattern of injured trees is usually irregular. In some cases, trees adjacent to trees with dieback show no injury, whereas those farther away may be affected.

Diagnosis of lightning damage is usually based on the lack of evidence of other factors, the prevalence of dieback in the

Fig. 169. Dried, separated pith of an apple shoot struck by lightning. (Cour­tesy D. A. Rosenberger)

highest shoots of affected trees, and knowledge of lightning activity in the area.

Russet Russet is observed when cork forms on the outer surface of fruit, often in a netlike pattern (Fig. 170). It has been associated with certain environmental conditions (e.g., high humidity, rain or dew on the fruit, and frost), abnormal growth of the epidermal cells, damage from harsh chemicals, improper nutrition, various fungi and bacteria that grow on the cuticle, and interactions among these factors. The fruit are most susceptible between bloom and 30 days after petal fall, which coincides with the visible development of the cuticle on the fruit. If the fruit cuticle is physically damaged during or shortly after bloom, or if the underlying epidermal cells divide too rapidly and cause the cuticle to rupture, an active cork cambium is initiated in the lower epidermal region and cork develops. After their initial formation, the cork cells push outward, the cuticle is sloughed off, and cork eventually becomes the dominant protective layer in that region of the fruit. Susceptibility varies among cultivars and individual sports. Complete russeting of the fruit of certain cultivars of pear (e.g., Beurré Bosc) and apple (e.g., Golden Russet) is a normal varietal characteristic that is appreciated in some markets. Russeting is undesirable in cultivars that normally russet only partially or not at all. Golden Delicious and Cox’s Orange Pippin are examples of apple cultivars prone to this undesirable russeting. Individual sports of ‘Golden Delicious’ show marked differences in susceptibility, with spur types generally being more susceptible than regular types. In some cultivars (e.g., Jonathan and Northern Spy), russeting typically develops in the stem cavity. Frost during bloom may induce russeting (see Low-­Temperature Injury). Spray-­induced russet often develops only on the exposed side of apples, whereas russet from other causes may develop uniformly over the entire surface. Russet often results from interactions among several factors and is therefore difficult to control.

Russet can be controlled by selecting genetically superior cultivars and sports when establishing new orchards, avoiding spraying chemicals that induce the disorder (wettable powders are safer than are emulsifiable concentrates or oils), using a cultural program favoring good nutrient balance (nitrogen not in excessive and adequate phosphorus), pruning properly to encourage fast drying, and not applying chemicals during periods when the weather favors russet (e.g., slow drying conditions, high humidity, and temperatures greater than 32°C). Copper sprays to control fire blight applied after green tip can cause severe russet. Application of the hormone gibberellic acid has been shown to reduce russet.

Selected References Howell, G. S., Jr., and Dennis, F. G., Jr. 1981. Cultural management of perennial plants to maximize resistance to cold stress. Pages 195-­ 204 in: Analysis and Improvement of Cold Hardiness. C. R. Olien and M. N. Smith, eds. CRC Press, Boca Raton, FL. Matteson Heidenreich, M. C., Corral-­Garcia, M. R., Momol, E. A., and Burr, T. J. 1997. Russet of apple fruit caused by Aureobasidium pullulans and Rhodotorula glutinis. Plant Dis. 81:337-­342.

Olien, W. C. 1987. Effect of seasonal waterlogging on vegetative growth and fruiting of apple trees. J. Am. Soc. Hortic. Sci. 112:209-­214.

Pierson, C. F., Ceponis, M. J., and McColloch, L. P. 1971. Market diseases of apples, pears, and quinces. U.S. Dep. Agric. Agric. Handb.

376.

Skene, D. S. 1982. The development of russet, rough russet and cracks on the fruit of the apple Cox’s Orange Pippin during the course of the season. J. Hortic. Sci. 57:165-­174.

Walter, T. E. 1967. Russeting and cracking in apples: A review of world literature. Pages 83-­95 in: Annu. Rep. East Malling Res. Stn. Kent 1966.

Fig. 170. Russet on a ‘Golden Delicious’ apple.

(Cour­tesy T. B. Sutton)

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(Prepared by J. A. Flore and F. G. Dennis, Jr.)