I. Introduction:  Leucostoma canker, also called perennial canker, Cytospora canker, and Valsa canker, is an important disease of peach, nectarines, and sweet cherries in cooler portions of the regions suitable for production of stone fruits, including Canada and the northeastern United States. Leucostoma canker is associated also with the complex set of factors that result in the peach tree short life syndrome in the southeastern United States, and is important on other stone fruits including prune and plum in California and Idaho, and cherry in the Pacific northwest. In Europe, the disease is important on apricot, peach, sweet cherry, and is part of the disease complex of stone fruits called 'apoplexy'. Leucostoma canker also occurs in South America and Japan. There are two causal organisms of similar host range, and they may be found on species within and outside the family Rosaceae, including apple; apricot; Sitka mountain ash; blackthorn; black, Japanese, flowering, pin, sour, and sweet cherries; chokecherry; nectarine; Russian olive; peach; pear; common, Damson, and wild plums; prune; serviceberry; and golden willow. The first observations of peach tree cankers caused by Leucostoma spp. were made in 1900 in western New York, and a few years later in Missouri. The disease was reported in southern Ontario in 1912. Leucostoma canker reduces the bearing surface of productive trees and shortens tree longevity.

II. Symptoms:  The physical appearance of Leucostoma canker depends on the part of the tree infected. Infections of small twigs appear as sunken, discolored areas, often with alternating zonation lines, usually around winter-killed buds or leaf scars from the previous year's foliage. Nodal infections are easily observed 2 to 4 weeks after bud break. The infected tissues become darker with time and an amber gum may ooze from the infected tissue unless the twig is killed entirely. One-year-old shoots that develop in the center of the tree are especially susceptible to nodal infections and, if left untreated, rapid invasion of scaffold limbs and large branches to which these shoots are attached may follow (Fig. 1). Branch cankers that result from such of infection will have dead twigs or twig stubs at the center of the canker (Fig. 2).

Cankers that form on the main trunk, branch crotches, scaffold limbs, and older branches are the most conspicuous expression of fungal infections. Cankers appear elliptical along the length of the stem. Usually the first external symptom of such cankers is the copious quantity of amber-colored gum. Gum production is a natural response of the tree to irritation, but that due to infection by Leucostoma spp. is excessive. As the canker ages, the gum becomes dark brown, the infected bark dries and cracks open, exposing blackened tissue beneath. Leucostoma canker can occur on weakened branches without the production of copious gum, especially if winter injury precedes infection.

Beginning in late spring and continuing through summer, the tree grows rapidly and resists further penetration of the fungus into healthy tissues. During this time, the tree may form a callus ring around the canker but the fungus usually invades this tissue again in late fall or early spring when the tree is dormant and cannot actively resist penetration. The yearly alternation of callus production and canker extension produces a canker with concentric callus rings. When tree defenses are compromised by environmental stresses, callus production may be inhibited and cankers may appear more diffuse.

Branch or twig infections may produce leaf symptoms during the growing season. Leaves on an infected branch often turn yellow, droop, and may wilt and die. Dead twigs and branches are usually covered with a multitude of pin-head-sized black structures erupting through the dead bark, which are the reproductive structures of the pathogen.

III. Causal Organisms:  The fungi which cause Leucostoma cankers on stone fruits, Leucostoma cincta (Fr. ex Fr.) Höhn. [= Valsa cincta (Fr. ex Fr.) Fr.] and L. persoonii Höhn. [= V. leucostoma (Pers. ex Fr.) Fr.], are ascomycetes in the order Diaporthales, family Valsaceae. The imperfect stages of these fungi, Leucocytospora cincta (Sacc.) Höhn. [= Cytospora cincta Sacc.] and L. leucostoma (Pers.) Höhn.) [= C. leucostoma Sacc.], respectively, are commonly encountered in the field. The duality in nomenclature is encountered frequently in the literature and stems from a taxonomic revision of the Diaporthales by von Höhnel in 1917 in which the genus Valsa was split into two genera, Valsa and Leucostoma. The imperfect stages of the two fungi were placed in the two genera Cytospora and Leucocytospora, respectively. These taxonomic revisions have been adopted only slowly in North America where the names Valsa and Cytospora prevail.

Pycnidial stroma form in cankers and on killed twigs and branches distal to cankers, appearing as quickly as 2 to 3 weeks or up to 6 months after the death of the bark. Stromata are black on the surface, gray to grayish-brown internally, and underlain by a black layer. Development of stroma is first evident as pinhead-sized swellings, the top of which ruptures to expose the disc of the stroma. The disc of L. persoonii is of white or frosted appearance, whereas that of L. cincta is gray to brownish-gray. Mature pycnidia under moist conditions extrude a flesh- to orange-colored tendril, or cirrus (Fig. 3), containing thousands of conidia. Individual conidia of both species are hyaline and measure 5 - 10 x 1 - 2 m.

Perithecia form much later, often 2 to 3 years after the original formation of pycnidial stromata. They may be found within, or underneath, the pycnidial stroma and may occur on both living and dead branches. Viewed from above, the perithecial stromata of L. cincta are round (1.6 - 2.8 mm in diameter), very prominent, and delimited by a black, circumscribing conceptacle layer (30 - 80 m thick) which may be visible through the bark. The ectostromatic disc (0.6 - 1.1 mm in diameter) is pale gray to tan. Black ostioles of individual perithecia, 10 - 30 per perithecial stroma, usually are arranged circinately around a central pycnidium, often obscuring the outer portions of the convex disc. Individual perithecia are 200 - 350 m in diameter. Asci (45 - 80 x 7 - 12 m) are clavate, sessile to sub-sessile, and contain eight ascospores (15 - 30 x 4 - 8 m) which are hyaline, aseptate, and allantoid. The perithecial stromata of L. persoonii are round (2.0 - 3.0 mm in diameter), whitish and granular within, and possess a circumscribing conceptacle similar to that of L. cincta. Asci are fusoid-clavate (35 - 45 x 7 - 8 m), sessile to sub-sessile, and ascospores are aseptate, allantoid, and hyaline (10 - 18 x 2 - 5 m). Asci and ascospores of L. cincta generally are larger than those of L. persoonii. The perfect stage of L. persoonii is fairly common in nature on Prunus spp., whereas that of L. cincta is relatively rare on Prunus spp. and more common on Malus spp. For L. persoonii, the fungus outcrosses and probably is heterothallic, with several alleles controlling vegetative compatibility.

The morphological differences between the two species are often difficult to discern, therefore differentiation of species often is based on their cultural characters on potato-dextrose and Leonian's malt agar, as described by Willison in 1936. Differences include color of mycelium (white turning to buff or olive-buff for L. cincta; white turning to brown or darker for L. persoonii), and size and characters of pycnidia (large, 1 to 3 mm diam, white, felty, rarely if ever exuding cirri, though usually full of viable spores for L. cincta; compared to small pycnidia, 1 mm or less in diam, possessing a beak, usually dark, exuding cirri when mature, for L. persoonii). Variability of cultures in appearance and several physiological characteristics, including virulence, have been observed. Both fungi can be isolated easily by placing small pieces of tissue from the margin of cankers on malt agar.

IV. Disease Cycle and Epidemiology:   Leucostoma pathogens can invade the host only if preceded by a wound or dead tissue. Mechanical and winter injuries, and other stressing agents are important predisposing factors. In mature peach orchards where L. cincta is the primary pathogen, most new infections appear at the nodes on one-year-old shoots. The pathogen enters the node through leaf scars or dead buds during the dormant season. Small twigs killed by L. cincta are pathways by which the pathogen can reach older limbs and initiate cankers which then may result in death of large portions of the tree. Many new infections also take place at pruning wounds, and the fungus most often isolated at these sites is L. persoonii. Generally, pruning cuts are most susceptible if they are made early in the dormant season, but many pruning cuts made in the spring can become infected. At temperatures from 14 to 20 C, L. persoonii can initiate necrosis, however lesion expansion is limited by host defense reactions. At 2 to 8 C, initial lesion expansion is slower, however the total brach area colonized is larger and host reaction is more limited. In general, cankers will be smaller as the number of days above 10 C increases.

The primary inoculum for new infections is conidia, although the potential role of ascospores remains to be determined. Conidia are most abundant under the cool, moist conditions of late fall and early spring (maxima from November through March) but are present throughout the year, depending upon rainfall. There are significant correlations between spore capture for L. persoonii and L. cincta and the number of hours that temperatures are between 10 and 15 C, the duration of wetness, and the length of time that relative humidity is above 90%. Conidia are resistant to desiccation when contained within the cirrus; however, upon release by water and subsequent drying, most of them die within six hours. The agents involved in dispersal of inoculum have not been established conclusively for these fungi. Short range dispersal of conidia indicates that rain or wind-driven rain may be responsible. Localized splash dispersal also may contribute. Ascospores are discharged forcibly into the air following rain showers at any time of the year when rainfall occurs, but they are most abundant in late spring and early autumn. Spread of L. persoonii may occur also via infested pruning tools, birds, and possibly by the shot-hole borer (Scolytus rugulosus), although these are minor compared to air, wind-blown rain, and local splash and washing.

Cardinal temperatures for spore germination and growth of L. cincta are lower than those for L. persoonii. L. persoonii grows best at 25 - 30 C and with a maximum of 32 C, whereas L. cincta grows at 4 - 30 C (optimum is 18 - 20 C). Ability to grow at 37 C has been used to differentiate the two species in culture. Spore germination requires a carbon source and free water or 100% relative humidity.

V. Disease Management: All attempts to control peach canker must take place within the framework of an integrated crop management strategy. All phases of orchard management from establishment of new plantings to care of bearing orchards are important. Management of cankers is based on preventative measures designed to decrease winter injury and insect damage, promote optimum plant health, and facilitate rapid wound healing. As with any other disease, once established in an orchard, new infections become increasingly difficult to control.

Proper site selection for new peach plantings is essential if young trees are to enter their productive years free of disease. The site should have deep, well-drained soil and good air drainage to minimize the chances for winter injury. Tile drainage systems should be installed where feasible and whenever natural drainage is impeded. New plantings should be reasonably isolated from sources of inoculum. Young trees should not be planted adjacent to older, heavily infected peach blocks and the down-wind side of older blocks should be avoided.

Nursery stock should be disease-free and not excessively large (greater that 11/16 caliper). Trees with small cankers on lateral branches may be planted if they are pruned so that at least 10 cm of healthy tissue below the canker is removed. Examine all trees closely. Plant trees immediately after receiving them from the nursery to avoid any additional stress. Protect trees from peach tree borer by dipping the roots and crown of new trees in an appropriate insecticide. Newly planted trees should be pruned when their buds begin to break and trees should be headed back to about 100-115 cm to promote wide-angled branching. Small trees can be pruned to whips, but four to six side branches on larger trees should be pruned to two or three nodes since trunk buds may not develop. Trees should be inspected after growth begins and any dead branches should be removed.

Control oriental fruit moth and peach tree borer even in the first few non-bearing years. These insects can cause serious damage and their feeding activity creates infection sites for Leucostoma spp. It is also important to control brown rot since twig infections by the brown rot fungus are often invaded then enlarged by Leucostoma spp.

Trees must be trained during the first season so that the tree branches develop the wide crotch angles that are necessary for long orchard life. Where narrow crotch angles form, the tissue in the crotch is susceptible to winter injury and invasion by borers. Also, portions of bark become included in narrow crotches where normally there should be solid wood, thus making the branch more likely to split when bearing a heavy crop. Wire spreaders or wooden spreaders with nails should be avoided because they injure the bark which may then become infected by Leucostoma spp.

Rodent damage should be prevented with wire or plastic guards. Plastic wrap-around guards should be removed each summer because they may delay hardening of the wood in late fall, they may harbor boring insects and interfere with trunk sprays for borer control. Latex paint with Thiram also discourages rodent feeding.

Low temperature injury is always a potential problem with stone fruits. This injury occurs to buds, twigs, branches and branch crotches, and trunks. Cold temperatures can injure peach trees early in the winter before the trees are completely acclimated to the cold. Practices to avoid include excessive or late fertilization with nitrogen and late season cultivation. Nitrogen fertilizer should be applied in late winter or early spring to avoid inducing late, cold-susceptible growth in the fall. Foliage should show a healthy green color and terminal growth should be about 30 cm on bearing trees and 45-60 cm for non-bearing trees. Trees with pale, nitrogen deficient leaves are more susceptible to infection by Leucostoma spp. Balance nitrogen fertilizer application with an adequate supply of potassium. Use leaf analysis to determine fertilizer requirements. In clean cultivation management systems, cease cultivation and sow a cover crop within 3 weeks of early fruit drop. Sod management, as an alternative to annual clean cultivation, with trickle irrigation, in addition to maintaining tree growth and fruit size, has the added benefit of making trees more resistant to Leucostoma spp.

Southwest-injury or sunscald is caused by the warming of the bark by direct sunshine on the south and west exposures of the trunk and scaffold limbs and may occur even during relatively mild winters. This injury may be the most damaging since it occurs on trunks, scaffolds, and crotches. These sites are commonly infected by Leucostoma spp. To avoid southwest injury, trunks and scaffolds should be covered with white latex paint which can reduce bark temperatures on sunny winter days. Small mounds of soil or mulch that drain water away from the tree trunk may prevent direct cold temperature injury to the crown. In addition, the mulch prevents formation of ice collars which could cause physical injury. Do not use gravel to fill depressions around tree collars.

Infection at pruning cuts is less frequent when pruning is delayed until late in the spring. The faster a wound heals, the less risk there is for infection. Wound healing is temperature dependent, therefore pruning should be delayed until the first forecasts of warm, dry weather. Approximately 390 accumulated degree-days (base = 0 C) are required for complete wound healing. In general, any practice which promotes tree health encourages more rapid healing. Pruning should be well planned each year so that large cuts, which heal more slowly, will not be needed. When pruning, avoid leaving stubs which may become infected. When pruning side branches from larger limbs, the cut should be made just beyond the ridge of thickened bark where the smaller branch joins the larger limb. The branch bark ridge should not be removed because it is in this region where the most rapid wound healing occurs. On one-year-old wood, the ridge of thickened bark is slightly inset and it is difficult to make the proper cut. In this situation, cut as close as possible to the larger branch without injuring it or leaving a noticeable stub. Prune to open the center of trees to light penetration because shaded branches are weakened and more susceptible to winter injury and Leucostoma infection. Remove all dead and weakened wood.

Cankers should be removed from the tree and burned, buried, or moved out of the orchard. Cankers on trunks and large limbs can be removed surgically in mid-summer when trees heal most rapidly. Surgery should be performed in dry weather with a forecast of dry conditions for at least three days. During surgery, remove all diseased bark around the canker and about three and five centimeters of healthy tissue from the sides and ends, respectively. Disinfect cutting tools between cuts with an alcohol or bleach solution. The resulting wound when finished should have a smooth margin and be slightly rounded above and below to favor rapid wound closure.

The practice of covering pruning cuts in spring with a thiram-latex paint mixture provides some degree of protection against fungal infection. Sites of surgery heal best if left uncovered. Leaf scar infections by L. cincta take place as the tree defoliates in autumn. Fall or spring sprays applied for leaf curl control have been shown to reduce leaf scar infections. There are no fungicides registered specifically for control of Leucostoma spp.

Online references:Ellis, M.A. 1997. Peach Canker. Factsheet HYG-3005-94, The Ohio State University Cooperative Extension, Columbus, OH.

Grove, G. G. 1997. Leucostoma (Cytospora) canker of stone fruit.  Washington State University Cooperative Extension, Wenatchee, WA.

Text prepared by A. R. Biggs, West Virginia University.