Summer Tree Care: Identifying Common Tree Diseases in Summer, Part 9
This is the ninth part of a series on summer tree diseases. This article examines Lophodermium needle cast, and Nectria canker.
During the summer months, plants are in the midst of their development. While many plants flourish due to the warmer climate, others can be subject to infections from a slew of disease pathogens. The following describes some of the most common diseases to affect plants in summer, and how they impact their hosts.
Lophodermium Needle Cast (Lophodermium seditiosum)
Lophodermium needle cast is a fungal disease that affects two and three needled pines, as well as some five-needled pines. The disease is caused by the fungus, Lophodermium seditiosum. In spring, needles infected the previous year will develop small, brown spots, with yellow margins. By summer, the infected needles turn yellow, and then reddish-brown. Fruiting bodies form on the needles once they have expired. Infected needles are often shed from the tree. Significant defoliation can disfigure infected trees, and reduce tree vigor. Severe infections may culminate in tree mortality.
Lophodermium needle cast commonly infects Scots and red pines. Ponderosa, Monterey, Virginia, and Austrian pines have also been reported as hosts, albeit with less frequency.
Symptoms of Infection
In early spring, brown spots, or bands with yellow margins, appear on the previous year’s needles. As the spots or bands enlarge, the entire needle turns yellow, and then reddish-brown as the shoots become elongated. Severe infections will cause the crown of infected trees to appear scorched, with only tufts of green current-season needles remaining at the tips of branches. As the disease advances, infected needles generally drop from the tree. Defoliation often begins on the lower portion of the crown, before progressing upwards. In late summer, slightly raised, small, black fruiting bodies appear on the dead needles. The fruiting bodies are aligned length-wise on the needles. They feature a characteristic slit in their center, which splits open to release minute spores when conditions are sufficiently moist. These fruiting bodies may also be observed on cast needles under the tree. The current season’s growth will generally not exhibit symptoms of infection.
- When planting, select trees that are genetically resistant to the disease. Avoid planting susceptible trees in low areas with poor soil drainage. Adequately space trees to encourage the drying of foliage, and limit sporulation.
- Remove weeds growing around and under trees to improve air circulation, and the drying of foliage.
- In early spring, inspect needles for symptoms of infection. Severely infected limbs, and needle litter should be removed, and destroyed to prevent sporulation.
- Maintain tree vigor through sound cultural practices. Ensure that trees are sufficiently watered, especially during extended periods of drought. Apply a layer of organic mulch around the base to improve soil quality, moderate soil temperature, and maintain soil moisture.
- Registered fungicides can be applied three to four times during the growing season. Initial applications should be performed in early July, when the current year’s needles have fully expanded. Subsequent applications should be administered at three week intervals.
Nectria Canker (Nectria)
Nectria canker is one of the most serious diseases to afflict hardwood trees. The disease is caused by several species of Nectria. Nectria is a genus of Ascomycete fungi. The most widespread and damaging Nectria fungus is Nectria galligena. Nectria magnoliae, Nectria coccinea, and Nectria coccinea var. faginata occur less frequently, but can be just as problematic. Several Nectria fungi target specific tree species. Nectria magnoliae affects magnolias and yellow poplar. Nectria coccinea invades sugar maple. Nectria coccinea var. faginata attacks American beech. Nectria coccinea var. faginata works in conjunction with a scale insect to cause beech bark disease. Another member of the Nectria genus, Nectria cinnabarina, causes the disease Nectria dieback. Nectria dieback is also referred to as coral spot Nectria or Nectria canker.
Nectria canker occurs on over sixty species of trees and shrubs, including apple, ash, aspen, birch, dogwood, elm, holly, maple, pear, sassafras, sweet gum, and walnut. Trees that are frequently attacked by the disease include American beech, black walnut, red maple, sugar maple, sweet birch, paper birch, yellow birch, and largetooth aspen. White oak can be infected in certain localities. Nectria magnoliae infects members of the magnolia family. Nectria cinnabarina afflicts apple, ash, barberry, birch, boxwood, crabapple, elm, hickory, honey locust, linden, maple, pear, rose, and Japanese zelkova. Nectria fungi that cause cankers on one host species may readily infect other nearby tree species. Infections by proximity can occur on any susceptible tree, with the exception of magonlia, sassafras, and yellow poplar.
Symptoms of Infection
The first visible symptoms generally appear in spring, as the host plant breaks dormancy, and resumes development. Dark red, or blackened water-soaked patches appear on the bark near wounds at the base of dead twigs, and branches. As the disease progresses, further discoloration of the bark occurs. Eventually, the bark fissures at the margins, and sloughs away, revealing the canker. In some instances, dead bark may remain in place, and conceal the canker. During rainy periods, small, creamy white or red to reddish-orange fruiting structures may appear on infected areas. These fruiting structures produce spores, which are expelled into the air, and disseminated by the wind or rain to create new infections.
Cankers may extend vertically for several feet. The diameter of each canker is determined by the width of the host plant. As cankers enlarge, they become gaping wounds in the host plant’s wood. Irregularities, or bulges in the bark can be detected, and generally indicate the presence of a canker beneath the bark surface. On fruit trees, infected fruit may blacken, and rot.
Affected twigs, branches, or entire plants may become devoid of growth. New growth often wilts abruptly. Extensive cankers may girdle larger branches and small trees, causing them to decline rapidly. Birch and black walnut are particularly susceptible to girdling from cankers. Cankered branches are prone to breakages, which can distort the host plant, and stunt its growth. Plants that are stressed, or have incurred wounds are especially vulnerable to infection. Infections may be intensified in fall and winter, when the host plant has entered dormancy. On most hardwoods, excepting sweet birch and yellow birch, infections become less frequent as the tree matures.
- Maintain plant vigor through sound cultural practices. Ensure that plants are sufficiently watered, especially during extended periods of drought. Apply a layer of organic mulch around the base of plants to improve soil quality, moderate soil temperature, and retain soil moisture. Many trees and shrubs also benefit from periodic fertilization.
- Select trees and shrubs that are well adapted to the climate of the area. This will minimize the potential for infection due to environmental stressors.
- Prune, and dispose of branch cankers during dry periods. Disinfect pruning tools between each cut using a solution comprised of one part bleach, and nine parts water. Avoid pruning in spring during moist periods.
- Minimize wounding due to pruning, transplanting, or maintenance activities to reduce the number of infection sites.
- In urban settings, severely cankered trees should be culled to prevent additional infections from occurring.
Photo courtesy of Chad Behrendt