Summer Tree Care: Identifying Common Tree Diseases in Summer, Part 8
This is the eighth part of a series on summer tree diseases. This article examines Elytroderma needle cast, and honey fungus.
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.
Elytroderma Needle Cast (Elytroderma deformas)
Elytroderma needle cast, also known as Elytroderma disease, is a fungal disease of pine trees. It is especially common on ponderosa pine. Elytroderma needle cast is caused by the fungus, Elytroderma deformas. The fungus infects and kills the foliage of susceptible trees. The disease seldom culminates in tree mortality. However, successive defoliations can weaken trees, and render them more prone to failure.
Ponderosa pine is the preferred host of Elytroderma deformans, but infections may also occur on lodgepole pine, jack pine, and pinyon pine.
Symptoms of Infection
Persistent infection of vigorous branches often results in the formation of small to large brooms with upward turning branches, and an abundance of dead needles. The brooms caused by Elytroderma needle cast are compact, and rounded, which differentiates them from witches’ broom. The brooms contain discolored needles, many of which are shed by fall. The infected needles initially turn a reddish-brown, but become paler during the summer. The basal portion of the infected needles will remain green. By the following spring, the infected needles will be straw-colored. Branches that have been infected for three or more years tend to develop small patches of dead tissue beneath the inner bark. Brown, resinous legions may form in the phloem of infected twigs. The black fruiting bodies appear near the base of infected needles in spring. Elytroderma needle cast infections can significantly defoliate trees. Small trees may be permanently deformed. When an infection is severe, tree growth is inhibited, and the entire crown of the tree may die back. In these situations, the tree generally succumbs to the infection. Infected trees are vulnerable to infestation by bark beetles, and other insects.
- Thin the crown of susceptible trees to improve air circulation, and promote the rapid drying of foliage.
- Selectively cull moderately, and severely infected trees. Remove trees that have been extensively defoliated.
- When planting, select resistant tree species, such as Douglas-fir.
- Infected branches can be pruned to remove sources of inoculum.
Honey Fungus (Armillaria)
Honey fungus, or Armillaria, comprises a genus of parasitic fungi that infect a variety of plants. It is one of the most destructive fungal diseases in the world. The pathogenicity of Armillaria species ranges from mild to severe. Of the species that cause honey fungus, Armillaria mellea and Armillaria ostoyae are the most aggressive. Armillaria gallica frequently infects plants that are suffering from environmental stress, or other infections. Honey fungus infects and kills the roots of vulnerable plants. Affected plants will eventually succumb to infection. The disease is notable for the distinct clumps of honey-colored toadstools that appear briefly on infected stumps from late summer to early winter.
Honey fungus infects a bevy of plants. Apple, ash, beech, birch, black locust, buckeye, butterfly bush, camellia, cedar, cotoneaster, cupressus, currant, dogwood, eucalyptus, forsythia, golden chain, hawthorn, holly, hydrangea, leyland cypress, lilac, magnolia, maple, mountain-ash, oak, paenoia, paperplant, pear, photinia, privet, prunus, rhododendron, rose, viburnum, weigela, willow, and witch-hazel are among the plants most prone to infection.
Symptoms of Infection
Honey fungus infections result in the decaying of a plant’s root system. The most characteristic symptom of infection is the white fungal growth that forms between the bark and wood, typically at soil level. These sheets of fungal material are slightly luminescent, and emit a strong, sweet odor. Infected plants gradually experience leaf discoloration and wilting. Infected leaves will assume a malformed appearance. This is followed by the dieback of the upper portion of the plant. Dieback generally occurs over several years. Dieback is hastened by extended periods of drought.
Cracks often form on the bark of infected plants, particularly on conifers. Sap may seep from the cracks, and trickle down the main stem. A profusion of flowers or fruit generally precede plant mortality. The rhizomorphs produced by most species of Armillaria dwell too deep in the soil to be easily detected. However, Armillaria gallica establishes large rhizomorphs that are easily visible. Some species of Armillaria, such as Armillaria mellea, are bioluminescent. This causes a phenomenon referred to as foxfire, a bioluminescence that can be observed on decaying wood at night.
Yellow-brown fruiting bodies sometimes develop on infected plants from late summer to fall. These fruiting bodies produce spores, which can infect nearby plants that have incurred bark wounds. Infection through sporulation is rare. Clumps of honey-colored toadstools will often appear briefly on infected stumps from late summer to early winter. The toadstools produced by Armillaria ostoyae develop pinkish caps. These toadstools linger for a few days before collapsing. A lack of toadstools in an infected area indicates that the fungus is inactive in the soil.
- No chemical control is currently available for the eradication of honey fungus. If honey fungus is identified, diseased trees should be culled. Dispose of any infected root and stump material. Avoid using the material as compost, as rhizomorphs may continue to form.
- A physical barrier can be erected in the soil to prevent the spread of the rhizomorphs. Use a deep vertical strip of pond lining, or a heavy-duty plastic sheet. Bury the material in the soil, allowing it to protrude about 1 inch above the soil surface.
- Deep cultivation may be performed to disrupt the rhizomorphs, and limit their spread.
- When planting, select trees and shrubs that exhibit an increased resistance to honey fungus. Some of the most resistant plants include bamboo, black walnut, blueberry, boxelder maple, common box, common hornbeam, flannel bush, gold flower, heath, Japenese quince, jasmine, tamarisk, and tupelo.
- Avoid planting at infected sites for at least twelve months.
- Pruning mildly infected roots may allow for the salvaging of important specimens.
- Soil sterilization with commercial products can control the disease.
- Systemic applications of fungicide into the woody tissue of diseased trees and shrubs is usually ineffective.
- Prevent plants from sustaining mechanical injuries, or damage from pests.
Photo courtesy of Royal Horticultural Society