Canker diseases and root rots
Excerpt from Publication 310, Integrated Pest Management
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Table of Contents
- European canker
- Anthracnose canker
- Phytophthora root rot
There are several fungal pathogens that cause cankers on apple trees. Canker refers to the symptom of sunken lesions on twigs, branches, crowns, stems or trunks, surrounded by living tissue. The bacterial pathogen causing fire blight and the fungal pathogen causing black rot both cause cankers on apples and are described in detail in a separate section. This section describes two other canker diseases observed on apple trees grown in Ontario.
Cankers form when a pathogen infects and colonizes the cambium tissue under the bark of twigs, branches, trunks or crowns of trees. The cambium tissue eventually dies and the bark adjacent to the infected cambium becomes sunken, discoloured, cracked or sloughed off. The impact of a canker on the health and productivity of a tree depends on the location, number and size of the canker. A canker on the main trunk of a tree results in the death of the tree if it expands or girdles the trunk. A canker on a branch causes the rest of the branch beyond the canker to die or become much less productive, without affecting the rest of the tree. While some cankers such as European canker expand year after year, others such as Anthracnose canker are contained by the tree's defense system and remain the same size after the initial infection and development, but produce and release spores that infect and cause new cankers on another branch of the same tree or spread to another tree.
It is difficult to assess the amount of damage caused by cankers in an orchard. They reduce the growth and yield of individual trees and may lead to their death. It takes only a few cankers left uncontrolled in an orchard for the disease to spread rapidly under favourable environmental conditions, and infect and weaken a high percentage of trees in a block. Cankers are often colonized by other pathogens or attract insect pests which further reduce the vigour and productivity of the tree.
In general, apple cultivars do not display much difference in their susceptibility to cankers. Organisms causing cankers infect all apple varieties, and many other tree species. The following text describes the symptoms of two common canker diseases and the conditions favouring their development.
European canker, formerly referred to as nectria canker or apple canker, is caused by the fungal pathogen Neonectria ditissima (formerly Nectria galligena).
The fungus invades injuries such as those caused by old pruning wounds or leaf and fruit scars. European cankers form a rough collar around a wound site, and this collar often grows larger each year. Bright orange fruiting bodies containing spores are produced on older cankers during winter (Figure 4-138). The result is a large canker, showing rings of increasing size. Cankers often girdle a large limb or trunk, killing all branches beyond that point. Trees infected in the nursery exhibit poor growth and eventual death (Figure 4-139).
Figure 4-138. Older European canker produces bright orange fruiting bodies during winter
Figure 4-139. A young tree complete girdles by European canker
Most infections occur when temperatures are 10-16oC and moisture is present. Orchards with poor drainage are more prone to European canker. Excessive vegetative growth also makes trees more susceptible. Ragged pruning cuts and branch stubs increase the risk of cankers developing at these wound sites. A fruit rot caused by N. ditissima has been reported in other apple growing regions but has not been observed in Ontario. Many other woody plants such as maple, birch, hickory, poplar, beech and hawthorn are alternative hosts to this pathogen.
Monitoring and management
Monitor trees for cankers throughout the season. Mark infected trees with bright coloured flagging tape or spray paint to make it easier to find them and prune out infected limbs during the dormant season. Although fungicides reduce the spread of cankers, they are not effective at curing existing infections. It is best to prevent the establishment of cankers by following these good horticultural practices.
- Keep pruning tools sharp to avoid ragged cuts or torn bark at
- Prune during the dormant season in the winter before sap flow
begins and when temperatures are too cold for the pathogen to
be active. This minimizes the chance of disease organisms being
spread on pruning tools.
- Prune branches back to the collar, leaving the collar intact
to heal the wound, but avoid leaving stubs.
- Prune out all dead wood and remove fruit mummies as they both
serve as reservoirs for disease organisms.
- Remove cankers promptly by pruning the branch several centimetres
below the canker. Where cankers occur on the main trunk, it is
sometimes possible to save the tree by cutting away diseased tissue
and encouraging the bark to heal around the wound. Surgically
removing cankers on main trunks of infected trees is time consuming
and often not completely successful.
- Remove other alternative hosts such as hardwood trees with cankers
in the vicinity of the orchard.
- Burn prunings or remove them from the orchard promptly. If this
is not possible, chopping prunings hastens their breakdown and
reduces the risk of disease spread.
- Inspect nursery stock carefully before planting and discard any trees with cankers.
Anthracnose canker, also called fiddlestring canker, is caused by the fungal pathogen Pezicula malicortici.
Infections on limbs and trunks in the fall develop into small circular reddish or purple spots that become elliptical and sunken the following spring. As anthracnose cankers mature, cracks develop separating the diseased tissue from the healthy bark as the tree compartmentalizes the infection limiting the lesion expansion and begins to heal itself. As the diseased tissue begins to crack and slough off, long fibers are exposed which give the canker a "fiddle string" appearance (Figure 4-140). Although the anthracnose canker does not expand after the first year of infection, it can produce spores on dead bark for several years. The spores also infect fruit and develop into a bull's-eye rot later on in storage.
Figure 4-140. Anthracnose cankers begin to crack as they mature exposing long fibers which give a "fiddle string" appearance
Pezicula malicortici produces sexual ascospores in the spring and asexual conidia or spores (anamorph Cryptosporiopsis curvispora) in the fall. During prolonged periods of cool, wet weather in the fall, rain splashes spores from nearby cankers to trunk or scaffold limbs of adjacent trees where the pathogen overwinters.
Monitoring and management:
Management techniques are similar for all canker diseases and are dealt with in Monitoring and management.
Phytophthora root rot
Several pathogens are known to cause root rot in apples. The most common root rot pathogens of apples in Ontario are species of Phytophthora. These soil-borne pathogens are sometimes referred to as "water molds" although they are not technically molds. Phytophthora spp. are closely related to yellow-brown algae and although not considered true fungi, are fungal-like organisms that prefer very wet conditions. Some species of these pathogens attack specific crops while others species such as those infecting apple roots, have a much broader host range.
The disease symptoms caused by Phytophthora spp. vary depending on the part of the tree affected:
- collar rot affects the tree above the soil line usually around
- crown rot affects the tree below the soil, in the area where
the roots join the stem
- root rot affects the roots
Early symptoms of collar, crown and root rot are very difficult to detect. By the time foliar symptoms are observed, the disease has progressed too far in infected trees to control. The first symptoms on severely infected trees are often observed in spring. The buds of infected trees swell, break dormancy and appear to flush out normally in spring - but infected trees often wilt and collapse suddenly after bud break. Trees weakened by root rots are easily pushed over by heavy winds due to lack of support from rotten roots (Figure 4-141). As the disease progress into the crowns and collar, a purplish canker is often observed at the base of infected trees (Figure 4-142).
Figure 4-141. Severe crown and collar rot on apple tree cause trees to be blown over in high winds
Figure 4-142. Severe crown and collar rot on apple tree - note the dark cracked canker
To diagnose crown collar and root rot, remove soil around the crown and roots of declining or dead trees and scrape the bark away along the trunk at the base of the tree and roots (Figure 4-143). Orange to dark reddish brown canker or streaks along the cambium of the collar or crown at ground level or just under the epidermis of the roots are good indications of Phytophthora infection (Figure 4-144). The reddish orange canker is often limited by a dark or black margin separating it from the white healthy tissue. When these symptoms are found in orchards, consider sending a sample of the diseased roots or crown tissue to a qualified pest diagnostic laboratory for accurate identification.
Figure 4-143. To diagnose crown and collar rot, remove soil from the base of the tree
Figure 4-144. Scraping the bark away from the crown and collar of a fruit tree infected with Phytophthora reveals an orange-red canker limited by a dark margin separating it from the white healthy tissue
Most actively growing trees tolerate a certain amount of root and crown rot, and may limit the advancement of the disease for a short while. However, dormant young trees or trees growing slowly due to other stresses are most vulnerable particularly when the pathogen is still active. Infected trees decline slowly over several years or die within weeks of the first symptoms, depending on the size and health of the tree. As the disease advances, infected trees often produce yellow, chlorotic leaves that look similar to iron deficiency symptoms. The foliar symptoms as a result of crown and root rot are often confused with other disorders such as nutrient deficiency.
Monitoring and management
Monitor trees for reduced shoot growth and small fruit size - early
symptoms of Phytophthora crown and root rot. There is no cure once
Phytophthora has infected and become established in apples trees.
Some fungicides are registered for collar rot control in apples
but only on non-bearing apple trees as preventative treatments.
When planting a new orchard, select fields with good drainage and
light soils if possible. If the soil is heavy or retains water for
prolonged periods of time, consider installing sub-surface drainage
tiles. Since the pathogen infects when soils are saturated for long
periods of time, managing irrigation to avoid over watering reduces
potential infections and the spread of these pathogens. Select rootstocks
with some resistance to Phytophthora particularly when planting
into heavy poorly drained soils. Cornell University researchers
have developed rootstocks with resistance to Phytophthora (e.g.
CG.30, CG.6210 and G.16). M.9 and seedling rootstocks have some
resistance to Phytophthora root rot but M.26, M.7 and MM.106 are
considered moderately to very susceptible. Carefully inspect roots,
crown and scion of young trees to ensure only trees with healthy
crowns and roots are planted in the orchard. Inspect mesh used around
the collars of young trees to protect them from rodent damage in
the spring and remove debris that is stuck in the collar. Leaving
debris caught in between the mesh and the collar retains moisture
around the crown and collar and is conducive for Phytophthora development
and advancement. See the OMAFRA
Publication 360, Fruit Production Recommendations, for a list
of recommended products.
For more information:
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