In This Section |
Canker Diseases and Root Rots
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Figure 2. Scarified canker two months after treatment. |
| Common Names | Nectria Canker, European Canker, Apple Canker |
Anthracnose Canker, Fiddlestring Canker, Bull's-eye Rot |
Black Rot Canker, Frog-eye Leaf Spot |
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| Scientific Names | Nectria galligena | Pezicula malicorticis, Cryptosporiopsis curvispora |
Physalospora obtusa, Botryosphaeria obtusa |
| Common Sites of Infection | Wounds, such as old pruning wounds, leaf and fruit scars, etc. | Healthy wood on trunk of scaffold limbs. | Commonly invades dead wood such as fireblight cankers and areas of winter injury; can also infect healthy wood. |
| Alternate Hosts | maple, birch, hickory, poplar, beech and hawthorn | pear, peach, apricot, cherry, serviceberry, hawthorn and mountain ash | pear and many hardwood trees |
| Symptoms | Nectria cankers form a rough collar around a wound site (Figures 3&4), and this collar often grows larger each year. The result is a large canker, showing rings of increasing size (Figure 5). 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 6) | Infections often occur in the fall, and by the following summer, they are visible as small circular spots that appear reddish when wet. As they enlarge, they become oval and slightly sunken. In the fall, the canker dries out and cracks at the margins (Figure 7). Often an oval-shaped piece of bark will fall off to reveal long thread-like fibres (fiddlestrings) beneath (Figure 8). Anthracnose cankers rarely get any bigger after the first year. Anthracnose is sometimes confused with oil injury (Figure 9). Fruit symptoms develop in storage and result in softened, external breakdown (Figure 10). | Black rot cankers are often difficult to see, as they do not cause distinct holes in the bark. Rather, they appear as a roughness or peeling of bark along a limb. The canker looks like oil injury but is more severe. It may extend for 50cm or more along a limb (Figure 11). Leaf symptoms are referred to as "frog-eye leafspot" (Figure 12). Fruit show symptoms in the field with rot developing near harvest or in storage (Figure 13). |
| Favourable Conditions | Most infections occur when temperatures are 10-16°C and moisture is present. Poorly drained orchards are more prone to Nectria 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. | Cool, wet autumn weather. | Fruit mummies that remain on the tree after thinning can become infected and spread the disease. Pruning cuts that leave a stub or close pruning that doesn't leave a collar increases the chance of black rot infection. Most infections occur in the spring and early summer, in periods of rain or high humidity. The optimum temperature for infection is 20-27°C, but infection can occur as low as 8°C. |
| Other Tree Parts Affected | A fruit rot associated with Nectria occurs elsewhere but has not been observed in Ontario. | Spores from the cankers can cause bull's-eye rot on the fruit before harvest or in storage. The fruit lesions are round, with a dark brown edge and a light brown centre. | This fungus also causes a fruit rot (black rot) and leaf spotting (frog-eye leaf spot). On the fruit, black rot begins early in the season as a small red speck. These spots remain small, 1mm until the fruit begins to mature, but then enlarge rapidly to form a series of concentric rings that alternate form black to brown. On the leaves, frog-eye leaf spot begins shortly after petal fall as small purple flecks. These enlarge to for circular lesions with dark margins and tan centres, resembling a frog's eye. In severe cases, this leaf spot can lead to early defoliation. |
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Figure 4. Cross-section of trunk showing entry of European canker. |
Figure 5. European canker - "included" bark at branch crotch |
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Figure 6. European canker on trunk of young tree. |
Figure 7. Early symptoms of anthracnose canker. |
Figure 8. Anthracnose cankers showing typical fiddle-string appearance |
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 Figure 9. Oil injury to limb. |
 Figure 10. Bull's-eye rot on fruit. |
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 Figure 12. Frog-eye leaf spot. |
Figure 13. Black rot on fruit. |
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While several organisms are known to cause root rot in apples, the most common ones in Ontario are various species of Phytophthora. The disease is referred to by various names depending on the part of the tree that is affected.
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Phytophthora is a soil-borne disease and the species that effect apple (mainly P.cactorum and P.syringar) can also live on other native and agricultural plants. While the fungus is present in many orchard soils, it usually only becomes a problem in areas where the soil remains saturated for periods of time. The disease does not spread through the air - dissemination takes place with water movement or on contaminated nursery stock. Most infections occur in the spring, though they can begin at any time of year.
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Phytophthora - affected trees show general symptoms of vascular dysfunction: growth is stunted, fruit are small, leaves often appear yellow and may turn purple in spring or fall (Figure 14). Trees may decline over several years before dying. The disease often affects several trees in one area of the orchard - especially low lying areas or poorly drained spots. A very wet fall or spring may result in an increase in the disease.
When the bark is removed from the lower trunk or the roots, the wood of Phytophthora - affected tissues will be reddish brown, instead of the greenish white of healthy wood. Usually there is no sign of any fungal growth.
Figure 14. Phytophthora root rot.
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Select planting sites and manage soil water so soil is not saturated for prolonged periods. Select rootstocks with some resistance to Phytophthora. Most resistant are M.9 and seedling rootstocks. M.26, M.7 and MM.106 are considered moderately to very susceptible. Fungicide treatments may be effective if applied on a preventative schedule. Once trees begin to show symptoms of Phytophthora, fungicides will not cure the problem.
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