European canker, or Nectria canker—Nectria galligena
European canker in California infects at least apple, California bay, dogwood, maple, and pear. Other Nectria species cause the same fungal damage and symptoms on numerous other hosts.
Identification
Discolored, sunken, often elliptical areas (cankers) commonly develop in bark and wood infected with N. galligena. Infection of branches and twigs cause leaves and shoots to wilt and die back during spring.
Fruiting bodies of the fungus erupt from cankered bark mostly during spring and fall. Two types of spore-producing bodies are produced. The fruiting bodies during fall (sporodochia) appear as small, white tufts on the cankers after rain; these tufts persist through spring. The second type is small, red, spherical fruiting structures (perithecia) that occur on the surface of older cankers. Each of these spherical orange to red perithecia (called coral spots) are about 1/50 to 1/16 inch (0.5 to 1.5 mm) in diameter.
Where these fungal cankers occur, cutting away bark reveals a margin separating dead (dark brown, necrotic) and healthy (cream-colored) wood. The green, living cambial layer found just beneath healthy bark is absent where cankers occur.
Note there are many other common causes of cankers, including other species of Nectria and other canker fungi. For example on apple and pear, Nectria cankers can be confused with those from fire blight.
Life cycle
Nectria infection generally occurs during fall but symptoms do not become obvious until spring. Plants are especially susceptible to infection where bark is wounded and the plants are unhealthy or stressed, such as from recent planting.
The fungus survives the summer as mycelia (vegetative fungal growth) in infected branch and shoot cankers. Wet weather during fall through spring stimulates the formation of fruiting bodies that produce fungal spores. The spores are moved by wind and splashing rain. The spores infect any fruit left on trees or the ground after harvest, fruit spurs, leaf scars on young shoots, and mechanical wounds such as from impact or pruning. Leaf scars are most susceptible to infection during the first hour after leaf fall, but some leaf scars remain susceptible for up to 28 days after leaves drop.
In years when leaf fall occurs over a long time, the incidence of European canker is greater the following spring. Once infection takes place, the fungus slowly penetrates bark tissue. Bark lesions become visible 2 or 3 months after infection. Successive thickened ridges of infected tissue are formed as the fungus invades further into the wood.
Cankers expand rapidly in early spring when they can encircle and kill small branches and twigs. When the weather warms, cankers may stop growing before they girdle a branch. In this case, the fungus ceases activity until temperatures drop. The tree then produces a ridge of new unaffected tissue around the margin of the canker. When conditions and temperatures again favor the fungus, canker development resumes.
Damage
In California, European canker is generally a minor disease. Plants infected when young may die, but European canker rarely, if ever, kills older plants. Callous tissue developing around wounds often limits canker spread and prevents girdling of limbs or the trunk.
Infections cause reddish brown lesions to develop in bark and wood. Bark over the diseased area cracks, the edges may take on a papery appearance, and bark can slough off. Cankered limbs are more susceptible to breakage and may become hazardous.
The main damage on apple and pear is that cankers reduce the abundance of fruiting wood, thereby reducing yield. Calyx rot of fruit can occur in years when rain precedes harvest, but in California this is uncommon because fruit harvest is generally completed before the rainy season.
Solutions
Proper cultural care and good growing conditions are the most important management methods. Plant only species well adapted to local conditions. Provide plants with a good growing environment and appropriate cultural care to keep them growing vigorously. Appropriate irrigation is especially important. Use drip irrigation or prevent sprinklers from wetting bark. Protect plants from injury and avoid creating wounds, especially if plants are growing poorly. Where branches become cankered, prune them off during dry summer weather. Make cuts in healthy wood so the diseased tissue is completely removed.
No fungicides cure Nectria infections. If plants are heavily infected or susceptible species are newly planted, the abundance of new infections can be reduced by thoroughly spraying bark with Bordeaux mixture or another copper fungicide just before the first fall rain. If leaf drop is prolonged, a second application may be warranted when three-fourths of leaves have dropped.
Adapted from Integrated Pest Management for Apples and Pears and Pest Management Guidelines: Pears, University of California Statewide Integrated Pest Management Program (UC IPM). |
