I. Introduction: Leucostoma canker, also
known as perennial canker, peach canker, Cytospora canker, and Valsa canker, is one of the
most destructive diseases of stone fruits (peach, nectarine, apricot, sweet cherry, and
plum) in the mid-Atlantic region. The disease is most damaging to young orchards, where it
may cause tree death. In older orchards, trees gradually lose productivity and slowly
decline as individual scaffold limbs are killed.
II. Symptoms: The fungus attacks the woody parts of
stone fruit trees through any injury to the bark, pruning cuts, and dead shoots and buds.
The first visible symptom is the exudation of gum at the point of infection. The canker
starts from a small necrotic center that slowly enlarges with the collapse of the inner
bark tissue. The canker enlarges more along the length than the width of the branch. Older
cankers are therefore oval to elongate in outline (photo 2-64).
In new cankers, the outer bark usually remains intact
except at the points of gumming. In older cankers the bark in the center of the canker
becomes torn. The gum turns black due to alternate wetting and drying and the presence of
saprophytic fungi. Older cankers are surrounded by a roll of callus tissue. Each year the
canker enlarges by repeated invasion of healthy tissue. With renewed growth in the spring,
the tree forms a callus ring around the canker as a defense mechanism. This can be a very
effective defense except when the lesser Peachtree borer breaks the callus ring by
burrowing through the callus into healthy tissue.
III. Disease Cycle: The fungus which causes the disease
overwinters in cankers and dead twigs. Small black fruiting bodies containing spores of
the fungus are produced on the smooth bark covering diseased areas on dead wood. These
spores are washed from the fruiting structures during wet weather. The optimum
temperatures for growth of the fungus are 77 to 86'F (25-30'C). Spores are produced
anytime the temperatures are above freezing. Most infections occur during the fall, early
spring, and winter months when the trees are not growing vigorously.
The fungus cannot penetrate healthy bark or buds. Cold
injured buds or wood and pruning cuts are the most important sites of infection. The
fungus can also penetrate brown rot cankers, oriental fruit moth damage, sunscald wounds,
hail injury, leaf scars, and mechanical wounds. The fungus becomes established in the wood
and forms a canker by invading the surrounding healthy tissue.
IV. Monitoring: After shuck fall, monitor all trees
in the orchard for cankers (photo 2-64). Cankers can be very small or can girdle the
entire limb or trunk. Remove cankers surgically if possible or prune out the entire
diseased area. Monitoring for and removal of cankers is best done at the same time.
V. Management (HTML/JPEG slide show): 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
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.
Ellis, M.A. 1997. Peach Canker. Factsheet HYG-3005-94, The Ohio State
University Cooperative Extension, Columbus, OH.
Text prepared by A. R. Biggs, J. W. Travis,
and J. L. Rytter
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