Cankers are wounds on living wood or dead areas on tree twigs, branches, and trunks. If you have an apple tree with cankers, the wounds may serve as overwintering spots for fungal spores and bacteria that cause diseases.
Anyone with apple trees in a home garden needs to learn about cankers in apple trees. Read on for information on apple cankers and tips for apple canker control.
Think of canker in apple trees as evidence of tree injury. The reasons for these cankers are many and varied. Cankers can be caused by fungi or bacteria that attack the trunk or branches. Injury from extremely hot or cold weather, hail, or a pruning cut can also result in cankers.
An apple tree with cankers will have areas of roughened or cracked bark that seem darker than the surrounding bark. They may look wrinkled or sunken. You may also see fungal spore structures in the area that look like dark or red pimples. In time, you may see white profusions growing from the bark that are wood decay fungi.
For an injury to become a canker, it must have an entry point. That is the danger of cankers, fungal spores or bacteria enter the tree through the wound and overwinter there. During the growing season they develop and cause diseases.
For example, if the pathogen Nectria galligena overwinters in cankers, the apple tree will develop a disease called European canker. The Delicious variety of apple tree is the most susceptible to European canker, but Gravenstein and Rome Beauty trees are also vulnerable.
Other pathogens result in other diseases. The Erwinia amylovora pathogen causes fire blight, Botryosphaeria obtuse causes black rot canker, and Botryosphaeria dothidea causes white rot canker. Most canker pathogens are fungi, although fire blight pathogens are bacteria.
Many gardeners wonder how to treat apple canker. The mainstay of apple canker control is pruning out the cankers. If the canker pathogen is a fungus, prune off the cankers in early summer. After that, spray the area with a Bordeaux mixture or approved fixed copper materials.
Since fungal cankers only attack apple trees suffering from drought or other cultural stress, you may be able to prevent these cankers by taking excellent care of the trees. However, the fire blight pathogen is a bacteria that attacks even heathy trees. Apple canker control in this case is more difficult.
With fire blight, wait until winter to do pruning. Since older wood is not as vulnerable to fire blight, prune deep – 6 to 12 inches (15-31 cm.) – into wood that is at least two years old. Burn all of the tree tissue you remove in order to destroy the pathogen.
This deep pruning will prove more difficult in smaller, younger trees. Experts suggest that if the fire blight has attacked the trunk of a tree or if the tree attacked is young, opt to remove the entire tree instead of attempting treatment.
Photo by AJ Cespedes / Shutterstock.com
Powdery mildew is a common fungus that affects a wide variety of plants. It is easily identified and appears as light grey or white powdery spots usually found on infected leaves, but can also be found underneath, or on stems, flowers, fruit or vegetables. The spots spread and will eventually cover most of the leaves on the plant, with new plant growth being most susceptible.
Powdery mildew thrives in warm, dry climates however, it also needs fairly high humidity — like the warm days and cool nights in late spring to early summer. Not enough sunlight and poor air circulation also contribute to conditions that encourage powdery mildew.
Although rarely fatal, if left unchecked it can eventually cause serious harm to your plants by robbing it of water and nutrients. Most infections cause minor damage such as leaves turning yellow or becoming withered or distorted, but plants can also become weak, bloom less, and grow slower.
Apple scab is caused by the fungus Venturia inaequalis. It infects crabapples and apples (Malus spp.), mountain ash (Sorbus spp.), pear (Pyrus communis) and Cotoneaster (Cotoneaster spp.).
The apple scab fungus has several host-specific strains that can cause disease on one type of plant but not any other. For example, the strain of V. inaequalis that infects mountain ash will only infect other mountain ash trees and will not infect crabapple trees. Apple and crabapple trees are infected by the same strain of the apple scab fungus because the trees are in the same genus.
Planting disease resistant varieties is the best way to prevent apple scab. Many varieties of apple and crabapple trees are resistant or completely immune to apple scab.
Apple trees susceptible to apple scab in Minnesota
Apple scab resistant crabapples hardy in Minnesota
The following list highlights crabapple varieties that have shown strong scab resistance and are tolerant of Minnesota's low winter temperatures.
Remove fallen leaves in fall to get rid of places where the fungus can survive the winter to re-infect trees the next year. Even with good fall clean up of fallen leaves, spores from nearby apple trees could travel to your property, starting the infection cycle again.
The apple scab fungus needs moisture on the leaves to start a new infection. A well pruned tree with an open canopy allows air to move through the tree and dry the leaves quickly. This can help reduce the severity of apple scab in a tree. For proper pruning of apples, see Growing apples in the home garden.
The type of fungicides that can be used and the timing of the sprays depends on whether you intend to eat fruit from the tree or if the tree is only ornamental.
Fungicides do not cure leaf spots but will protect healthy leaves from becoming infected. For ornamental crabapple trees, fungicide sprays must be timed to protect new leaves as they emerge in spring.
Fungicides only protect healthy trees from becoming infected. Once leaf spots appear in the tree, fungicides will not control the disease.
If your tree is already infected with apple scab this year:
Small trees can be treated by a home gardener if all instructions on the fungicide label are read and followed. Contact a certified arborist to apply fungicides to large trees.
This section applies only to ornamental crabapples, NOT EDIBLE APPLES OR CRABAPPLES.
*Burning of plant tissue may be observed especially in times of high heat.
**Russeting (rough corky skin) may form on fruit with use of copper products.
Apples and edible crabapple trees that become severely infected with apple scab have poor quality fruit and reduced health as a result of leaf loss. Fungicides can be used to protect healthy trees from apple scab, but will not cure an infected tree.
Check fungicide labels for the recommended spray interval. Most labels offer a range of days to wait before spraying again, such as "7 - 10 days after spraying, you will need to spray again."
In mid-June, examine the leaves on your trees for scab lesions. Be very thorough, checking upper and lower leaf surfaces, leaves on the interior and exterior of the canopy, leaves close to the ground and those higher in the tree.
If you find no or very few apple scab leaf spots:
If you find scab leaf spots, or if there are scab infected apple or crabapple trees nearby with scab lesions:
If scab has been a problem in your apple planting, it may take a year or two to get it under control. You should find in the 2nd or 3rd year that you only need to spray from ½” green tip to mid-June, if you:
Materials available to home growers for scab control in edible apples and crabapples include captan, lime-sulfur and powdered or wettable sulfur.
Applications of lime-sulfur closely following captan sprays can damage leaves and flower buds, so use caution when rotating these two materials.
All-purpose sprays, contain combinations of fungicides, insecticides and sometimes miticides.
NEVER USE AN ALL-PURPOSE FRUIT SPRAY DURING BLOOM. These sprays kill pollinators, including honeybees, bumblebees, solitary bees and many other beneficial insects, because they include an insecticide.
The name of the plant being treated MUST BE LISTED on the fungicide label or the product cannot be used. Some products are registered for use on ornamental crabapples but are not safe to use on crabapple or apple fruit intended for eating. Always completely read and follow all instructions on the fungicide label.
Rebecca Koetter and Michelle Grabowski, Extension educator
Because doctors and scientists don't know exactly what causes canker sores, it's difficult to say exactly what will cure them. Most of the canker sores natural remedies focus on topical relief, such as salt water rinses, as well as immune-system boosting herbs. Adequate rest, nutrition and overall good health may reduce the frequency of outbreak.
Most of the herbs recommended are used as rinses and gargles, so unless you're allergic to them they should be relatively safe. Pregnant and nursing women may want to hold off taking any herbs since few studies demonstrate conclusively how they may affect unborn children or infants salt water gargles should be safe to use (just don't swallow it). If any adverse reactions develop, stop taking them and see a doctor. You may also want to tell your doctor about frequent canker sore outbreaks. This can be a sign of a weakened immune system, which can be a symptom of another disease or condition. But only your doctor can diagnose such a problem.
Apple canker is an economically important disease of apple and is one of the most important diseases in the UK and recognised as a serious problem as early as 1710.
Diagnosis of cankers
|Disease/problem||Canker description||Fruiting bodies||Canker location|
|Blossom wilt(Monilinia laxa f.sp. mali)||Brown/cracked, distinct light/dark zones of infection||Grey pustules in spring||Fruiting spur, base of fruiting spur, branch|
|Brown rot(Monilinia fructigena)||Brown/cracked, distinct light/dark zones of infection||Buff pustules in summer||Fruiting spur, base of fruiting spur, branch|
|Apple canker(Neonectria ditissima)||Distinct cankered areas. Initially sunken areas around bud, leaf scar, shoot base, or wound. Older cankers, flaky brown bark strips surrounded by swollen tissue. Sometimes papery bark on cankered young shoots.||White/creamy pustules especially on young cankers in summer. Red pin-head sized fruiting bodies in autumn and winter, which can be confused with eggs of fruit tree red spider mite.||Young shoots causing shoot dieback, shoot bases, branches of all ages, tree crotches, branch angles, main trunk especially of young trees, rootstock.|
|Fireblight(Erwinia amylovora)||Cankers indistinct, associated with dieback. Cankered area slightly sunken and darker than healthy tissue with separating crack. Internal tissue water-soaked with red/brown streaks.||Milky bacterial ooze.||Shoot dieback. Disease progression to branch|
|Coral spot(Nectria cinnabarina)||Cankers indistinct, associated with twig or branch dieback||Pinkish pustules in summer. Pinhead –sized red fruiting bodies in winter||Shoot /branch dieback. Often associated with pruning snags.|
|Papery bark(physiological)||Initially pale blister-like swellings which eventually develop into peeling papery bark.||On young shoots and older branches. Often associated with excessive soil moisture.|
|Silver leaf(Chondrostereum purpureum)||Associated with pruning wound. Blistering and papery bark near wound. Affected wood if cut is discoloured. Foliage on tree or tree part is silvered.||Small bracket fruiting bodies (creamy-coloured above and purple below) arise on affected tree parts once they die.||On large branches, associated with pruning wounds especially major tree restructuring.|
|Perennial (Neofabraea – formerly Gloeosporium) canker (Pezicula malicorticis)||Distinct cankers. Initially small circular brownish / purplish spots that develop into elliptical cankers separated from healthy tissue by crack. Bark in affected area sloughs off||Cream-coloured fruiting bodies develop on the cankers.||Associated with wounds, either pruning, frost cracks etc.|
Diagnosis of wilting dying blossoms
|Disease/Problem||Blossom symptom||Fruiting bodies||Smell||Othersymptoms|
|Blossom wilt(Monilinia laxa f.sp. mali)||Wilting/brown,Internal browning/necrosis||Grey pustules on infected parts||Fetid smell, similar to scent of sweet chestnut flowers||Disease progression into spur and branch forming cankers|
|Apple canker(Neonectria ditissima)||Wilting/brown, no internal browning||None||None||Nectria canker somewhere on branch with wilting blossoms|
|Fireblight(Erwinia amylovora)||Wilting/brown, internal browning/necrosis||Milky bacterial ooze on infected flower parts||None||Disease progression into spur and branch, possible further ooze|
|Bud moth(Spilonota ocellana)||Wilting/brown blossom. Hollow||None||None||Evidence of internal mining, caterpillar and frass|
Other problems that may be confused with apple canker
Many other fungi cause cankers on apple trees.
Neonectria fruit rot can be confused with rots caused by Neofabraea – formerly Gloeosporium spp. or Penicillium spp. These rots similarly occur at the stalk, cheek and calyx end of the fruit.
Neonectria can only infect through wounds such as those caused by pruning, mechanical damage, manganese toxicity, frost damage, woolly aphid, wood scab etc. or through natural openings such as bud scale, fruitlet scars, fruit scars and leaf scars.
The time of maximum ascospore release varies considerably between region, country and season. Neonectria spores and potential infection sites are available all year round, but there is considerable variation between countries with regard to the season considered the most important for infection.
Such patterns of infection however, are not rigid and are very much dependent on seasonal rainfall patterns. What is most important is an understanding of when apple trees are most susceptible. These timings are listed below:
Although spores can germinate at 0 o C and orchard infection can occur between 5-16 o C, most infection occurs between 10 and 16 o C.
Fruit infection occurs on the tree through the calyx, lenticel or stalk end and takes place between blossom and harvest.
Many factors affect the susceptibilty of the tree to canker. These include climate, variety, rootstock, soil type, water content, pruning and fertilizer regime.
All varieties are apparently susceptible to canker to some degree and a variety can vary in its degree of resistance between localities. Rootstock can also influence the susceptibility of the scion variety.
Susceptibility of some apple varieties to fungal diseases
|Variety||Scab||Powdery mildew||Nectria canker||Blossom wilt|
|Discovery||vl (pg)||vl (pg)||h||vl|
|Grenadier||vl (pg)||vl (pg)||vl||vl|
|Red Charles Ross||m||m||?||?|
|St. Edmunds Pippin||m||m||m||m|
vl = very low, pg = polygenic resistance, vf = major gene resistance
l = low susceptibility, m = moderate, h = high, vh = very high
Canker in new apple orchards
Canker on trees in newly planted apple orchards can arise from two sources. Either from the nursery of tree origin as symptomless infection, which can take up to 2 or 3 years to express itself or spread in from existing canker in an adjacent orchard.
The risk of Neonectria fruit rot in store can be estimated pre-harvest, based on the incidence of cankered trees in the orchard, the rot history taken from packhouse records and the rainfall between blossom and harvest.
Inspect orchards in the spring for cankered trees and estimate the % cankered trees.
Canker incidence Risk
Orchard canker risk
|Orchard canker risk||RainBlossom-harvest||Action|
|Low||average||Market pre-Christmas if no sprays applied in blossom|
The ADEM system is a PC-based system and contains a disease forecasting model for Neonectria canker and fruit rot. The disease models are driven by the following weather variables recorded on a logger and downloaded to a PC:
The models use the weather data to determine the favourability of the weather for Neonectria infection of fresh leaf scars and near-mature fruits and indicate the incidence of disease likely to occur at these two infection sites.
Forecast of cankers
The model assumes that the inoculum level is high and varietal susceptibility to Neonectria is high.
Forecast of fruit rot
Cankers – paints
Wounds do provide entry points for Neonectria and if used correctly paints can provide protection. Canker paints contain chemicals active against Neonectria and are applied to pruning wounds or pared back cankers to provide temporary protection against infection while the tree develops its own protective callus layer. The value of treating pruning wounds with paints is often questioned, for if they are not treated rapidly the paint can act as a seal on fungal infection that has already occurred.
Other coatings (e.g. BlocCade) contain no fungicides but provide a physical coating to protect wounds against infection by fungi such as Neonectria. To be effective, they must be applied immediately after pruning.
Canker – sprays
Fungicides with good activity against Neonectria are limited.
Control of Neonectria in orchards presents a particular challenge. Entry points for infection are available all year round, inoculum (either conidia or ascospores) is available all year round and the rain, essential for Neonectria sporulation and infection, often makes timely spraying impossible. Therefore the strategy for control, especially in cankered orchards, must be to protect at key times to limit infection.
Fruit rot – sprays
Recent inoculation experiments have shown that fruit is most susceptible to infection at blossom and petal fall. Therefore it is important to apply protectant sprays at this time.
The recent inoculation studies indicate a slight increase in fruit susceptibility to Neonectria ditissima pre-harvest, but it is not known whether there is any benefit from additional sprays at this time and there is the risk of fungicide residues in fruit from the late applications.
Avoiding fungicide resistance
Research has indicated that biological control of leaf scar infection is at least feasible. The tissues of leaf scars become invaded by various host-specific fungi, common primary saprophytes and bacteria.
Given how widespread canker is in UK apple orchards and the significant tree and fruit losses it causes every year, AHDB Horticulture funded Project TF 167 to compare the level of control achieved by a best practice control programme compared to a standard programme used by commercial apple growers. The project, led by East Malling Research used two commercial orchard sites, both planted with Gala on M9 rootstock. The effect of a full integrated progamme for canker control on the incidence of new cankers, Neonectria fruit rot and shoot growth, was compared with that of a standard fungicide programme with no addditional specific measures for control of canker.
Standard fungicide programme
Fully integrated programme
Standard fungicide programme + additional sprays for canker at key times as follows:
The results of the three year study clearly showed that application of an intensive integrated programme for canker control resulted in a significant reduction in numbers of new cankers and Neonectria fruit rot. However, such an intensive programme is costly and can result in detectable residues in fruit at harvest.
Swinburne T R 1975. European apple canker. Review of Plant Pathology, 54, 787-799. Compendium of Apple and Pear Diseases edited Jones and Aldwinkle. APS Press 1990.