EB 0668 Why Cherry Trees Die

Rugose Mosaic Verticillium Wilt

Cherry Rasp Leaf

Bacterial Canker COOPERATIVE EXTENSION -~ Washington State University

CONTENTS

INTRODUCTION ...... 1

NURSERY STOCK AND CULTURAL PRACTICES ...... 1 Nursery Stock ...... 1 Time of Planting ...... 2 Poor Planting ...... 2 Lack of Pruning ...... · 2 Heat Canker ...... \ 2 Mechanical Injury ...... 2 Fertilizer Injury ...... 2 Lack of Sufficient Water ...... 3 Too Much Water ...... 3

VIRUS DISEASES ...... 3 Black Canker ...... 3 Cherry Mottle Leaf ...... 3 Cherry Rasp Leaf ...... 4 Cherry Twisted Leaf ...... 4 Little Cherry ... ; ...... 4 Necrotic Rusty Mottle ...... 4 Rugose Mosaic ...... 5 Rusty Mottle ...... 5 Sour Cherry Yellows ...... 6

MYCOPLASMA DISEASE ...... 6 X-Disease ...... 6

BACTERIAL DISEASES ...... 6 Bacterial Gummosis or Canker ...... 6 Crotch Gummosis ...... 7 Crown Gall ...... 8

FUNGUS DISEASES ...... 8 Verticillium Wilt ...... 8 Crown Rot ...... 9 Cytospora Canker ...... 10 Armillaria Root Rot ...... 10 Wood Rots ...... 11 Insects ...... 11 Winter Injury ...... 12 Rodent Injury ...... 12 Deer Damage ...... 14 Incompatibility ...... 14 Herbicide Damage ...... 14 Arsenic Residues ...... 15 Fire Damage ...... 15 Heating Oil Damage ...... 15 Spray Oil Damage ...... 15

WHY CHERRY TREES DIE

By Earle Blodgett, Ph.D., formerly Plant Pathologist, Washington State University and Washington State Department of Agriculture, Prosser, Washington

Revised by D.A. Johnson, Ph.D., Extension and Associate Plant Pathologist, Washington State University, IAREC, Prosser, Washington and G. G. Grove, Ph.D., and E.H. Beers, Ph.D., Assistant Plant Pathologist and Extension Entomologist, Washington State University, TFRC, Wenatchee, Washington

INTRODUCTION standard black cherry. Natural mutations of sweet cherry which develop into varieties are rare. Rainbow Cherry growing in the Pacific Northwest is not only Stripe is one example. Mutations giving rise to the an interesting and sometimes profitable endeavor but undesirable kinds such as crinkle and deep suture are one attended by many problems. This publication dis­ fairly common in Bing and Black Tartarian. cusses several factors that lead directly or indirectly to the untimely death of cherry trees. It is likely there With rare exceptions sweet cherry varieties are self­ may be other factors that have not yet been recogniz­ sterile; this gives rise to cross pollination problems. ed. Nevertheless, the known problems are numerous and many are serious. The sweet cherry has become a favorite orchard tree of Western United States. Favored locations for The Sweet Cherry large plantings are found in Washington, Oregon, Utah, Idaho, Montana, British Columbia, and Early literature on cherries comes from the writings California. Although cherry growing is beset with of Theophrastus in Greece about 300 B.C. It is believed, many uncertainties, attractive returns in some years however, that cherries were cultivated even before this have maintained grower interest in the crop, and in period. Propagation by grafting has been known since recent years plantings have greatly increased. Part of very early times. Perhaps one reason for the early this has been on a renewal or replacement basis, such recognition of the cherry, even more important than as after the 1955 freeze which killed or severely damag­ the use of its fruit, was the value of the trees as wood. ed thousands of trees. Many large, new orchards have been planted in Washington, not only in the Yakima The presently cultivated sweet cherry is known Valley but near Wenatchee and especially in the Col­ Prunus avium L. Its wild ancestor, alsoP. avium, is umbia Basin. now known commonly as Mazzard. At present this is one of the two favored rootstocks. The other is While the sweet cherry is characteristically a large, Mahaleb (Prunus mahaleb L.), a small, black strong-growing, long-lived tree, it does have problems. (sometimes red) fruited kind which originated in about With domestication and concentration of orchards this the same region and now grows wild over most of tree is subject to a long list of ailments or weaknesses, Europe. Both Mazzard and Mahaleb are native and some of them perplexing as well as serious. Complaints abundant in the Black and Caspian Sea regions and from both new and experienced growers about trees occur in great numbers and variations in northern dying or declining have been more frequent the last Turkey and eastward. few years. It recalls the philosophy of an orchardist friend in Idaho years ago who remarked, "Sweet cherry Until comparatively recent times sweet cherries were trees just can't wait to die." grown mostly in backyards, on estates, and in small semi-commercial plantings. Their culture now extends The following discussions may be helpful in deter­ to temperate climates throughout the world. The sweet mining the causes and possible control of some of these cherry is one of the most delicious of fresh fruits. ailments of sweet cherry trees.

Domestication of the sweet cherry over the cen­ NURSERY STOCK AND CULTURAL turies has brought great changes in varieties and culture PRACTICES and in its use in a rapidly expanding commerce. Development in Oregon the last hundred years has seen Nursery Stock three seedlings become popular and important One of the most important items in growing cherries varieties: Black Republican, Lambert, and Bing, the is the quality of the nursery stock. A Nursery Improve-

1 ment Program has been in operation in Washington Approximately half the top should be pruned away for over 40 years, and has upgraded the stock material­ at planting time. If a lower-headed tree is desired, even ly. And, since 1962, a Certification Program has made more of the top can be removed. available stock that is true to name and essentially free from all known virus and virus-like diseases. Heat Canker Any trees, especially those that are slow in start­ By using certified stock, an orchardist can avoid ing or are planted late, may develop heat canker. This the risk of planting a new orchard with poor trees. The is a brown, dead area near the soil line caused by heat best is none too good and certified trees are better than injury to the bark from exposure to the sun. any others. Heat canker can be prevented by placing a large Practices used in growing, handling, and storing chunk of sod against the south side of the trunk at nursery stock have improved during the past few years. planting time (Fig. 1). This will provide shade. A Most trees are in good condition when they leave the shingle or a short board is also effective. nursery. They should be transported under good con­ ditions and planted promptly. If this cannot be done, Mechanical Injury they should be kept from freezing or drying out. Many young cherry trees are injured, disfigured, or killed during cultural operations. Sometimes weed Time of Planting growth is so thick and tall that the young trees are not Trees should be planted as early in the spring as easily visible. Most of the damage from mechanical the ground can be worked. Chances of tree survival equipment can be prevented. (and good growth) diminish rapidly if hot weather ar­ rives before the active root system is re-established. Fertilizer Injury Generally, fertilizers should not be applied to Poor Planting cherry trees the first year. Under good cultural condi­ Holes should be large (18 to 24 inches across) so the tions trees may not need applications of nitrogen un­ roots can be spread out evenly and deep enough so the til they start to bear. bud union can be set I to 2 inches below the soil line. The soil should be firmly packed around the roots and, if moisture is low, the trees should be watered.

Lack of Pruning Unless trees are pruned back at planting time, they have little chance to do well. The newly planted root system will not be able to support the original top.

Fig. 2. Young tree dying due to high water table. Fig. 1. A clump of sod placed at the base of a newly planted Note that some peach and cherry trees are still tree will prevent injury by heat canker. alive.

2 Putting too much fertilizer too close to the trunks and roots has damaged or killed many young trees. Leaf yellowing and scorching and shoot die-back are characteristic symptoms.

Lack of Sufficient Water Young cherry trees are vulnerable to lack of water and to competition from weeds, especially during the first two seasons. This is particularly true of replants. Poor or indifferent orchard care accounts for weak growth or death of many cherry trees. Replants in established orchards need special care and more fre­ quent watering.

Too Much Water Cherry trees usually do poorly or die outright in heavy soils with poor drainage. A high water table, standing water, or excessive water will normally kill them (Fig. 2). They cannot tolerate such conditions.

This problem can be controlled by selection of pro­ per sites, care in irrigation, and construction of drainage systems, if needed. Since trees on Mazzard rootstocks are more shallow rooted, they usually do better in heavy, wet soils than those on Mahaleb rootstocks. Fig. 3. Sweet cherry tree infected with black canker. The galls are larger and more localized than the swollen, rough areas VIRUS DISEASES caused by San Jose scale.

A number of virus diseases affect cherry trees. Some kill or weaken trees. Others make them uneco­ nomic and force their removal.

Black Canker Extensive, rough, black cankers occur on scaffold limbs and branches of affected trees (Fig. 3). Cankers may completely girdle branches causing dieback.

Transmission occurs by grafting with shoot tissue from affected trees. Use virus-free nursery stock and scion wood. Remove diseased orchard trees when they are first discovered.

Cherry Mottle Leaf Symptoms consist of an irregular light green to yellow mottle on expanded leaves, which become distorted and puckered (Fig. 4). Fruit set may be small and the fruit may ripen late and lack flavor. Lack of terminal growth and shortening of leaf petioles makes infected trees appear rosetted.

The cherry mottle leaf virus is spread by budding or grafting and by the· microscopic scale mite, Eriophyes inaequalis. Trees on the orchard's edge are Fig. 4. Cherry mottle leaf also causes distortion usually infected first. Cultivars Bing and Napoleon and puckering.

3 (Royal Ann) are severely affected. Black Tartarian, Van, Chinook, Lambert, Rainier, and Sam are symp­ tomless carriers.

Remove infected trees to prevent new infections. Destroy stands of wild cherry near commercial or­ chards. Use virus-free nursery stock and scion wood.

Cherry Rasp Leaf Symptoms include prominent lumps or outgrowths on the underside of leaves between lateral veins and along the midrib (Fig. 5). Leaf narrowing, folding and distortion may occur; the upper leaf surface turns rough and pebbly in texture. Symptoms first appear on lower leaves in newly infected trees. Trees develop an open, bare appearance (Fig. 6) as spurs and lower branches die. Fig. 5. Outgrowths form on underside of leaf, between veins. The dagger nematode, Xiphenema americanum, and grafting spread cherry rasp leaf virus, which causes this disease. Spread within a tree or orchard is usually slow. The virus does not affect pears, but causes flat apple on apple trees.

Remove infected trees and other trees close by. Fumigate soil with a nematicide before planting cherry or apple trees. Use virus-free nursery stock and scion wood.

Cherry Twisted Leaf A characteristic symptom is an abrupt kink in the leaf midrib or petiole. Leaves twist, curling down and to the side; leaves remain small, or develop asymmetri­ cally. Sections of tissue die on the underside of the midrib and lateral veins. Shortened areas between spur joints cause spurs to bunch together.

Cherry twisted leaf virus causes this disease, which is spread through budding and grafting in orchards. The cultivar Bing is very severely affected. Napoleon, Rainier, and Van may also show symptoms, which can be severe. Most other cultivars do not show symptoms when infected. Fig. 6. As spurs and lower branches die, trees To prevent spread, remove infected trees as soon appear open and bare. as symptoms appear. Destroy choke-cherry stands near commercial orchards. Little cherry virus causes this disease, which is spread by the apple mealy bug, or by budding and Little Cherry grafting. Sweet cherry, sour cherry, and ornamental Symptoms are seen as a delay in fruit ripening. Fruit flowering cherries are hosts of the virus. on the same branch vary· widely in maturity. At harvest, fruit are somewhat pointed or angular, small and Use virus-free planting stock. Destroy infected trees medium red to pinkish red, and bitter in taste. Some and remove suckers from nearby ornamental flower­ varieties develop reddish purple leaves in early fall. ing cherry trees.

4 Necrotic Rusty Mottle Chronic Symptoms. Leaves appear with twisted tips. Symptoms include angular spots or large areas of Small outgrowths of dark green tissue may develop dead tissue on leaves 3 to 6 weeks after bloom. Spotted along the midrib on lower leaf surface (Fig. 8). Ter­ leaves develop shotholes. Severely decayed leaves drop. minal growth and elongation between nodes or joints Before harvest, remaining leaves turn mottled yellow, is often reduced. Severe strains cause uneven matura­ with green bands or islands. Buds on terminal shoots tion of fruit. Green fruit may be cylindrical. fail to open or swell; eventually they die. Leaves are earlier than normal. Bark symptoms consist of shallow Grafting, seed, or pollen from diseased trees spread areas of decay with gum blisters or deep gum pockets. strains of Prunus ringspot virus, which cause this disease. Mild forms show leaf symptoms, with little or no effect Necrotic rusty mottle virus causes this disease, which on fruit. Fruit ripening may be delayed 7 to 10 days spreads through infected budwood. A vector is with no leaf symptoms present. Severe forms exhibit unknown. Cool, late springs promote earlier and more blossom blast or withering, severe leaf deformity, pro­ severe symptoms. Leaf symptoms usually are not nounced leaf curl, reduced shoot growth, fruit ripen­ noticeable during summer and fall where temperatures ing delayed 10 or more days, and reduced tree vigor. remain high. Remove infected trees before bloom. Use virus-free Remove affected trees. Use virus-free nursery stock planting stock and scion wood. Use the serological test, and scion wood. Several pollinating cultivars are symp­ ELISA, to detect symptomless, infected trees. tomless hosts. Test for virus before taking budwood for propagation.

Rugose Mosaic Two different types of symptoms, acute and chronic, are produced on cherry.

Acute symptoms. Brown, dead tissue spots develop suddenly on leaves of one limb or one side of the tree in late April to mid-May. Dead tissues drop out, giv­ ing leaves a tattered or lacelike appearance (Fig. 7). Fruit symptoms may not be apparent the first year. Fruit may ripen unevenly. Fig. 8. Outgrowths of tissue grow along midrib.

Fig. 9. Older leaves turn a mottled yellow. Rusty Mottle Fig. 7. Leaves appear lacelike or tattered in Symptoms are seen as older leaves turn a mottled May. yellow midway between blossom and harvest (Fig. 9).

5 Leaves show variable yellow rings and patterns. Leaves on affected trees turn abruptly to yellow and red fall coloration. Symptoms appear first and most intense­ ly on leaves in the lower center of the tree. Two to three weeks before harvest, rusty-yellow colored leaves drop, taking 30 to 70 percent of a tree's leaves. Infected trees decline in vigor, main limbs die back. Fruit ripening may be delayed on severely affected trees.

Rusty mottle virus causes this disease, which is spread ·by infected budwood, or by a leafhopper.

Remove diseased orchard trees when they are first discovered. Plant virus-free nursery trees. Fig. 10. Pink, small fruit develop on same spur as dark red fruit in X-disease. Sour Cherry Yellows On sour cherry trees, leaves develop striking pale X-disease mycoplasma causes this disease, which green to yellow areas 3 to 4 weeks after petal fall. is spread by leafhopper, budding, and grafting. X­ Later, entire leaf turns yellow except for green along disease attacks peach, nectarine, sweet and sour cher­ larger veins. Leaves soon drop. Growth is willowy, and ries, and chokecherry. Symptomless hosts are flower­ the number of fruit is reduced. ing cherry and plum.

Sweet cherry trees may show yellow rings, mottl­ Use disease-free rootstock and scion wood. ing or shotholes on leaves, or may show no symptoms Remove diseased trees. Destroy chokecherry stands at all. No harmful effects have been recorded for sweet near commercial orchards. cherry. BACTERIAL DISEASES Symptom development requires warm days and cool nights. During some years, infected trees may be Bacterial Gummosis or Canker symptomless. Trees grown in warm climates may never This disease, caused by Pseudomonas syringae, is show symptoms. widely distributed and occurs on many kinds of plants. It is also called lilac blight. It attacks vigorous young Prune dwarf virus causes this disease. It is spread cherry trees. Cankers develop on the trunk or scaffold by budding, grafting, infected seeds, and pollen. branches (Fig. 11) and great quantities of gum may form around the enlarging cankers (Fig. 12). The Establish sour cherry orchards with virus-free trees diseased bark becomes dark, looks water-soaked, and isolated from older orchards. Rogue out and replant the area sinks, leaving a definite margin between infected sour cherry trees in orchards 1 to 5 years old. diseased and healthy tissue. Rogue out and do not replant trees in orchards 6 to 10 years old. Older orchards do not benefit from rogu­ If cankers girdle the branches or trunk, the leaves ing out trees. above the canker roll and turn yellow. Growth stops and the branch or tree eventually dies. In some areas MYCOPLASMA DISEASE of the West, a "dead bud condition" is another phase of the same disease. This phase of the disease can be X-Disease confused with nutrient deficiencies. Symptoms may consist of light pink and dark red fruit occurring on the same spurs (Fig. 10). Pink fruit Normally the cankers do not extend below the soil tend to be small. Initially only a few spurs on one line (Fig. 13) and the tree may send up suckers-a branch are affected. The infection eventually spreads characteristic symptom. Trees 2 to 12 years old are throughout the tree. Terminals are slightly rosetted. most susceptible. Nursery trees may develop trunk Leaves of terminal may be slightly undersized and cankers that make them unfit for planting. bronze or rusty green in color. Stipules may be enlarg­ ed. Trees on Mahaleb rootstock may collapse and die Rain or water splash, and pruning tools spread the within one season. bacterium. Bacteria overwinter in active cankers, in

6 l l

Fig. 11. Surface of bacterial canker appears dark purple. Fig. 13. Cankers seldom extend below the soil line. A side shoot or sucker frequently arises below the canker and can be developed into a new tree if it originates above the bud union.

weeds and trash. This helps keep the trunk and crown dry.

Since nursery infections occur in the fall or during mild winters, spraying young cherry trees with a cop­ per fungicide before digging will help provide control.

Crotch Gummosis Crotch gummosis is a condition that is often con­ fused with bacterial gummosis. It is common and has caused many cherry growers concern. In the winter

Fig.12. Gum exudes from canker margins. II infected buds, and on the surface of infected and l healthy trees and weeds. Prune off cankered limbs well below visable canker. Avoid pruning in early spring and fall when bacteria are most active. Sterilize pruning tools before pruning healthy trees.

Trees may be protected with copper fungicides ap­ Fig. 14. Gumming in narrow angle crotches can be exten­ plied to the trunks and lower branches. The area sive during wet weather and is often wrongly diagnosed as around the base of young trees should be kept free of bacterial gummosis. No cankers are present however.

7 season (November to March) during rainy periods, large amounts of gum may form in narrow crotches, on the trunks and branches, and near large pruning wounds or branches that have been topped (Fig. 14). The gum often forms large masses and drips to the ground.

This is not bacterial gummosis and when warmer spring weather arrives the gum dries up. Some growers become alarmed by tree crotch gumming but there is no real problem as long as cankers or dead areas do not develop. Wide angle crotches seldom form gum, so early tree shaping is desirable.

Crown Gall · Crown gall, caused by the bacterium Agrobacterium tumefaciens, is characterized by swelling or galls on roots. If the galls are numerous or severe, they can cause tree decline and death, especially if they are on the crown or main roots of young trees (Fig. 15). Many young trees in some orchards have aerial galls which also cause severe decline (Fig. 16). Crown gall is Fig. 16. A young tree with a large aerial gall widespread and during some seasons has a high in­ which is girdling the entire trunk. cidence in nursery stock. Mazzard roots are much

Fig. 17. Lower leaves of new growth tum color with Verticillium wilt.

more susceptible than Mahaleb. Only healthy clean stock should be accepted for planting.

Avoid wounding the tree trunk or major roots at or below the soil surface. Galltrol is a bacterial in­ oculant that will aid in the control of this disease on nursery stock.

FUNGUS DISEASES

Fig. 15. An extremely severe case of crown gall V erticillium Wilt on Mazzard rootstock. Mazzard is much more Verticillium wilt is caused by the fungus Ver­ susceptible to crown gall than Mahaleb. ticillium dahliae. Its first symptom is the wilting of

8 leaves and killing of spurs along the main branches of young, vigorous trees. Further evidence may be wilting and die-back of current shoots on one or more branches (Figs. 17 and 18).

The disease clogs the water-conducting tissues in the branches, resulting in leaf wilt and twig die-back. When affected branches are cut to expose the woody vascular system, dark streaks or bands are often found (Fig. 19).

Usually only one branch on one side of a tree is affected. The rest of the tree is perfectly normal.

Any part of the tree, or the entire top, may be af­ fected. The leaves may fall from the diseased branch­ es or they may stay attached longer than normal, a symptom also found in San Jose scale. Normally there is no gumming, suckering, or cankering.

Affected branches and trees may die over the winter or slowly recover over several years.

Verticillium wilt commonly occurs in the Yakima Fig. 18. Leaf drop follows. Only part of a Valley and the Columbia Basin. Currently it is one of tree may be affected. the most common and devastating diseases in cherry orchards. Once infection occurs, no satisfactory con­ trol is known.

Cherry trees should not be planted on land that has been in potato, tomato, pepper, raspberry, strawberry, peach, apricot, or other crops that are susceptible to Verticillium. Control weeds in orchards e.g., phlox, geranium, shepherds purse, lambsquarters, nightshade. Soil fumigation will reduce the fungus population in the soil, but recontamination occurs. Manure from

Fig. 19. Branches cut lengthwise show dark streaks and bands, principally in the vascular tissue, a symptom often Fig. 20. With bark removed, crown rot bands found in Verticillium wilt. appear on trunk. (S. Mircetich photo).

9 feedlots where potatoes have been fed should not be used as fertilizer, because this may spread the fungus. Trees on Mahaleb rootstocks are less susceptible than on Mazzard.

Trees should not be forced into excessive growth. Avoid excessive irrigation, high nitrogen fertilization, severe pruning, or other practices that promote suc­ culent growth. Severely weakened branches and trees should be removed as soon as there is good evidence they will not recover.

Crown Rot Crown rot can be caused by several soilborne species of Phytophthora, all of which attack other plants in addition to cherry. Phytophthora cactorum is the most frequently isolated species from diseased cherry roots in Washington. However, researchers in California have reported P. cambivora, P. drechsleri, Fig. 21. Cytospora has infected this old, mature sweet and P. magasperma parasitic on cherry roots and cherry tree, killing the wood on the trunk and branches. crowns. The fungus infects young, vigorous trees and destroys the bark near the crown (Fig. 20), including trees in bark injuries, colonizes the injured tissue, and the main root system. The foliage of diseased trees fre­ eventually begins to grow into adjacent healthy tissue. quently is a lighter green color than healthy trees. The The cankers are dark and sunken; they can enlarge to leaves eventually assume a yellow or bronze color earlier a point where they girdle the branches and main trunks than those on healthy trees. Infected trees eventually die. resulting in branch die-back or tree death. Minute, pinhead-sized spore-producing structures called pyc­ The key to controlling crown rot is prevention. nidia (Fig. 21) can form in the diseased tissue and are There is little than can be done to cure a tree once it frequently found 4 to 6 weeks after the infected branch is infected. However, several pre- and postplanting dies. Under moist conditions, the pycnidia can exude practices can help in preventing the disease. Plant trees millions of spores in small reddish to orange tendrils. on Mazzard rootstock; research has shown that Splashing rain or irrigated water can disperse the Mahaleb is more susceptible to infection. Phytoph­ spores to other wound sites. thora fungi thrive under moist soil conditions. Avoid overwatering or planting trees in poorly drained soils. As the various fungi that cause the disease are readily transported in irrigation water, no soils should be con­ sidered free of the fungi once they are irrigated. Metalyxyl (Ridomil) is a systemic fungicide that shows promise for the prevention of collar rot. It is registered for use on nonbearing trees (trees that will not bear a crop within 12 months of application) and is applied as a soil drench around the base of the tree. Mix 1 quart of Ridomil per 100 gallons of water and apply: 1 quart per tree where trunks are less than 1 inch in diameter, 2 quarts per tree where trunks are 1 to 3 in­ ches, 3 quarts per tree when trunk diameters are 3-5 inches, or 4 quarts per tree when trunk diameters are greater than 5 inches.

Cytospora Canker Cytospora canker is a destructive disease of tree fruits in Washington. The disease is associated with Fig. 22. Bark and "shoestrings" from cherry killed by Ar­ millaria root rot. Trees die as if they were girdled. winter-injured or mechanically injured twigs, trunks, or scaffold branches. The fungus typically enters the

10 There is no cure for trees once they are infected. However, problems can be minimized through prac­ tices that can help to prevent infection. Minimize winter and mechanical injuries; winter injury can be prevented by painting tree trunks with white paint. Other practices that can minimize winter injury or maintain good tree vigor should be followed. Prune out infected branches, flags, and cankers; remove the infected wood from the orchard and burn it in a desirable location. Make pruning cuts that leave a rais­ ed collar of tissue at the branch junction rather than cuts that leave a stub. Pruning in spring when the wounds heal more rapidly has effectively reduced disease incidence in some areas. New orchards should not be established in proximity to orchards where the disease is a severe problem.

Armillaria Root Rot This fungus disease is not common in most Wash­ Fig. 23. A young, bearing tree severely infested with San Jose scale. One characteristic feature is retention of much of the ington cherry areas, but may cause heavy damage foliage during the dormant season. Rough cankers develop where it occurs. The disease organism produces a mat on heavily encrusted branches and gumming usually occurs of white, felt-like growth between the bark and wood in these areas. of the roots and trunk. It causes the trees to die as if they were girdled. Large black threads of fungus tissue, resembling shoestrings, may be present on affected tissue and in the soil (Fig. 22). The disease is most serious in previously wooded areas, especially where oak trees are native.

Control is difficult and fumigation is very expensive.

Wood Rots Cherry wood is also susceptible to infection by wood-rotting fungi that frequently enter the trees through other cankers or winter or mechanically in­ jured tissue. Broken main stems and trunks, especial­ ly following a wind storm, are the most obvious results of wood rot damage. This occurs because the wood rot fungi invade and weaken the heartwood and sap­ wood of trees.

Infected heart or sapwood is often spongy and discolored. Basidiocarps, the fruiting bodies of the Fig. 24. Close view of Fig. 23. The San Jose scale infested fungi, are often observed in the autumn on diseased branches are in the center. Compare the rough, somewhat limbs and trunks. A number of fungi has been reported swollen area with normal wood on the large branch to the r to cause wood rots of cherry; however, the most com­ right. San Jose scale has been confused with various cankers. mon wood rotting fungi in Washington are Coriolus versicolor and Polyporus hirstutus. Several genera of and dead trees is important because it decreases the wood rotting fungi can attack a cherry tree concur­ amount of inoculum in the orchard. Minimizing winter rently. and pruning injury (as described for Cytospora canker) is helpful in preventing infection by the wood-rotting fungi. Management of wood rots includes good orchard sanitation, minimization of wounds and pruning cuts, Insects and maintenance of adequate tree vigor. Removal of San Jose scale. Quadraspidiotus pernictosus, infests fruiting bodies, infected limbs and stumps, wood piles, cherry, particularly Bing, with such severity that twigs,

11 branches, and whole trees may be killed. Live scale appears as a rough, grayish covering on the bark, which becomes moist when rubbed. Dead leaves that hang on trees over winter are usually a sign of scale­ infested trees (Fig. 23). Affected twigs and branches may develop cankers with abundant gummosis (Fig. 24). Because of this, San Jose scale has sometimes been erroneously diagnosed as bacterial canker of as Cor­ yneum blight. Annual dormant sprays are the best way to prevent scale from building up.

Shot hole borer. Shot hole borer (Scolytus rugu­ losus) is associated with weak or dying trees, but is rarely the causal agent. The small black beetles will attack branches weakened by Verticillium wilt, winter injury, or other causes (Fig. 25). If large piles of in­ fested prunings are left near the orchard, shot hole borer may attack healthy trees. Sanitation is the best prevention in this situation. Because the beetles emerge over a long period of time, control with pesticides is difficult.

Root grubs. Large beetle grubs (larvae) of Polyphylla, Pleocoma, and other genera may eat and girdle cherry roots, causing death of young trees (Fig. 26). While not common, this type of damage is serious Fig. 25. Shot hole borers have en­ when it occurs. Above-ground symptoms are similar tered the trunk of this young tree ailing from the effects of a high to those caused by girdling (for example, by gophers). water table. The entry is prin­ The whole root system may be consumed, leaving on­ cipally near old bud scars. The ly stubs. tunneling has caused gummosis. Winter Injury Sweet cherry can be injured by freezes at temperatures even above 0°F, although well-hardened trees may withstand temperatures of -20°F or below.

Typical winter injury symptoms are splitting and killing of bark on trunks and branches (Fig. 27) and discoloration of woody tissue in twigs and spurs. Dark brown areas or streaks are apparent in the damaged wood when the tissues are cut for examination. A high percentage of flower buds may be killed. Extremely low temperatures with little or no snow cover can result in extensive root damage. Wood rotting fungi may enter through winter-injured tissues and cause addi­ tional damage.

Whole orchards have been severely damaged by sudden, severe freezes or unusually early cold spells. There were reports of tree damage from low temperatures in 1924. A very early, severe cold spell Fig. 26. Bark on the crown and roots of this young cherry tree has been eaten by the Polyphylla beetle larvae shown in November 1935, extremely low temperatures in near the crown. The above-ground symptoms are those of 1948-49, another severe freeze in November 1955, and girdling. sharp freezes in December 1964, 1968, and 1972 have

12 Fig. 27. Severe winter injury on trunk of mature Fig. 28. Young tree which died suddenly due to tree. Good fruit may be produced on this type girdling by gophers. The injury to the crown of tree, but yield is low. near the soil line was not visible until the soil was removed.

all taken their toll in dead and damaged cherry trees and in loss of fruit buds.

Winter injury is difficult to prevent. Damage can be reduced by preventing excessive late growth of young trees and by protecting their trunks with white paint or shading boards to prevent sun scald. Orchard heat­ ing in the winter may be feasible in some circumstances.

Mahaleb rootstocks seem to be more winter hardy than Mazzard. Use of hardy sour cherry interstocks such as Montmorency, Kansas Sweet, and North Star may reduce trunk injury.

Rodent Injury Cherry trees of all ages may be partly or totally gir­ dled near the round line by mice (the short tailed meadow mouse) and gophers or the main root system and the lower trunk may be damaged (Figs. 28 and 29).

Fig. 29. This tree died suddenly in early summer. Gophers may eat the bark or the whole root system Removal showed that bark on the roots and the trunk of young trees, leaving just short stubs of the main below ground had been eaten away by gophers. roots. Trees affected this way may fall over or be easily

13 pulled from the ground. In older trees, rodeqt injury is often hard to discover without considerable digg­ ing. Pocket gophers have probably killed more cherry trees in the West than any other single cause. They have killed trees as large as 12 to 24 inches in diameter. Severe gumming of trunks and scaffold branches often accompanies root injury and partial girdling. There are no cankers in this type of gum formation.

Mice usually eat only the bark and often girdle the crown completely, causing the tree to die sometime after bloom. If a few roots are left, the trees go into a severe, slow decline. Tree symptoms such as leaf yellowing, reduced growth, and eventual collapse are similar, whether the damage is caused by mice, by gophers, or by crown rot. In cases of crown rot, the bark is present, but dead, instead of being eaten away. Careful inspection is sometimes necessary to determine if rodents have caused the damage.

Although either Mazzard or Mahaleb rootstock may be attacked, gophers seem to prefer Mahaleb roots. Mice and gophers may be successfully controlled by using machines which place poisoned bait in artificial burrows and trails. Certain orchard sprays or treated baits have been used for mouse control. Trapping gophers is effective if done before tree injury occurs. Clear away all trash and grass for a distance of 3 feet Fig. 30. Browsing deer have defoliated this young sweet around the trunks at all times. This will normally cherry tree and broken the branches (photo by Fred Scholes) reduce mouse injury and will make it easier to observe the presence of gophers. volves genetic factors or inherent incompatibilities, such as when pear is budded to apple. In some cases Rabbits may damage young trees by eating the bark a virus or mycoplasma may be involved and the term and twigs. The most effective control is to put up a then used is "induced incompatibility." This situation rabbit fence. occurs in some citrus diseases, in pear decline, and possibly in some cherry problems. Deer Damage In some areas young cherry trees are severely in­ After several years of investigation it was found jured or killed by leaf feeding, twig browsing, and tree that the Van variety did not do well on Mahaleb root­ breakage caused by deer (Fig. 30). Whole orchards stock. Many trees declined and died. Symptoms were may be destroyed or replants may be eliminated. Even much like girdling, although the decline was not so slight feeding causes severe delay and reduction in rapid. Growth slowed and stopped, extreme fruit set growth. Late maturity of the new shoots leads to occurred, and tops died back. The root system was winter injury. In an orchard near Sunnyside, Washing­ very poorly developed and continued to decline (Fig. ton, many young trees died in the spring the year after 31). One Mazzard line (Mz-570) used by nurserymen severe defoliation by deer. also is incompatible on Mahaleb. Viruses, so far as is known, are not involved in either case. With both Certain repellents are helpful, but the best solution Van and Mazzard-570, the decline is not uniform and is a deer fence. State funds may be available to par­ some trees maintain good growth, at least for many tially defray construction costs. Consult your local years. game protector. The use of Mahaleb rootstocks for Van is now dis­ Incompatibility couraged and the problem is being solved in new plant­ As used here, the term incompatibility means that ings. In the few cases tried, the inarching of Mazzard the stock and scion "do not get along" well. This in- seedlings to affected trees on Mahaleb was successful.

14 In several plantings where dwarfing of sweet cherry Arsenic Residues has been done with a Mahaleb root and a Montmoren­ From about 1920 to 1950, large quantities of lead cy trunk or framework, there has been severe (but arsenate were used over much of the Northwest to con­ variable) tree decline and death. In one case, the trol the codling moth in apples. When some of these Chinook variety showed extensive gumming of the orchards were removed and other crops were planted, main branches accompanied by tree decline. These the arsenic residues persisted in the soil and seriously cases seem to be degrees of incompatibility. Better tree affected plant growth. vigor has been noted when Mazzard rootstock has been used in these scion-root combinations, although Mont­ In peaches, apricots, prunes, and cherries, arsenic morency on Mahaleb has long been considered residue causes systemic arsenic toxicity. Symptoms standard. consist of leaf yellowing, interveinal necrosis, lace leaf condition, and defoliation. The older leaves are af­ Herbicide Damage fected first. Young apple trees grow poorly in "arsenic The use of certain weed killers has weakened, and soils." Cherries are probably the least affected. The in severe cases, killed trees. Usually the leaves become addition of organic materials and growing sweet clover yellow, perhaps deformed, and virus-like symptoms on affected soil will eventually improve it. The use of may appear. Care should be taken to apply the right virgin soil in setting out trees in helpful. materials at recommended rates and to avoid drift or contamination. Soil types may cause wide variations Fire Damage in the extent of damage. Sweet cherry bark is quite sensitive to fire injury. In some cases, trash around trees has accidentally been set on fire from weed burning. Less common, but just as serious, are cases where trash is ignited around or­ chard heaters.

If severely damaged by fire, the trees may start to grow but early in the summer the leaves turn yellow and later red and finally brown and die. Extensive gum occurs on the injured trunk and scaffold branches. If the damage is severe, the trees die about mid- or late summer.

Heating Oil Damage Young cherry trees or branches of old trees may be killed by seepage or leaks from oil heating pots plac­ ed near the tree trunks during the summer or winter. The leaves of affected trees turn a bronzed yellow and finally brown as the tree or branch dies. Heating oil in the soil is very toxic to cherry trees.

Spray Oil Damage Cherry trees of any age can be severely damaged or killed by poorly mixed oil spray combinations or Fig. 31. Pronounced incompatibility in Van on Mahaleb excessive application in the delayed dormant stage. root-stock. There is marked dwarfing of the roots and swel­ Die back and wood injury or bud killing may occur. ling of the bark tissue above the union.

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