Summer Tree Care: Identifying Common Tree Diseases in Summer, Part 11
This is the final part of a series on summer tree diseases. This article examines Scleroderris canker, and sooty mold.
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.
Scleroderris Canker (Gremmeniella abietina-Scleroderris lagerbergii)
Scleroderris canker is a fungal disease that affects various conifers. It is caused by the fungus Gremmeniella abietina-Scleroderris lagerbergii. Scleroderris canker is one of the most significant diseases of conifers. It has caused extensive mortality to conifers in plantations and nurseries, as well as in landscape and forested settings.
Two strains of the fungus have been discovered across its range. The first is the North American strain, which primarily infects young and newly developing trees. Older trees may also be infected, though they seldom incur significant damage. The European strain is more aggressive, and can infect all species of pine. Both strains can decimate susceptible conifers, causing extensive foliar dieback, and tree mortality.
The North American strain generally infects jack, lodgepole, and red pine. Austrian, pitch, ponderosa, and white pine, as well as black, Norway, and white spruce are also infected, albeit with less frequency. The European strain infects all species of pine. It most commonly infects red and Scots pine.
Symptoms of Infection
The North American and European strains exhibit similar symptoms. The first symptom is typically the dieback of buds, followed by needle discoloration. The needles initially turn orange-brown at the base. By mid-summer, they become entirely necrotic. Infected needles often drop prematurely. With the North American strain, discoloration occurs in early May, approximately nine months after infection. Lesions may form under the bark of infected trees. As the infection deepens, the pycnidia will become increasingly conspicuous on the dead branches.
In late July, the European strain may cause a second dieback of needles that were infected in spring. Once the fungus has infected a tree, it rapidly spreads into the main stem. Cankers that form on infected trees may enlargen. If a canker forms on the main stem, it will often deform the tree. Younger trees can be girdled, and killed within a few months. Small cankers are more common on branches infected by the European strain. They are rarely found on branches infected with the North American strain.
- When planting, select resistant species. Balsam fir is immune to infection. Certain varieties of white pine and spruce exhibit an increased resistance to the fungus.
- Fungicide applications are effective at eradicating the disease. Treatments should begin in spring, during budburst. Applications should be performed at two week intervals until July, and then every four weeks until September. During years that experience heavy rainfall, additional applications may be required. Trees infected with the European strain may need to be sprayed until late October.
Sooty mold is a collective term applied to several species of dark fungi. The fungi grow on honeydew excreted by insects, or exudates from leaves of certain plants. Sooty mold growths are composed of fungal complexes consisting of ascomycetes, and fungi imperfecti. Some of the common genera include Aureobasidium, Antennamariella, Limacinula, and Capnodium.
Symptoms of Infection
Sooty molds grow on surfaces where honeydew deposits accumulate. Coatings of sooty mold reduce or block sunlight penetration, making photosynthesis less efficient. Without adequate sunlight, plant growth is stunted, causing leaves to wither and drop prematurely. Sooty mold growths can also develop on outdoor structures and furniture. Large masses of sooty mold create an unseemly appearance, and are often difficult to remove. Sooty molds have high allergenic potential, particularly the Cladosporium and Aureobasidium components found in sooty molds of the Eastern United States.
- Sooty mold can be managed by reducing populations of insects that excrete honeydew. Pressure washing can help dislodge insects from trees.
- An important biological consideration is ant management. Ants are attracted to, and use honeydew as a source of food. As such, they protect insects that produce honeydew from predators and parasites. If ants are eliminated, predators and parasites will become more prevalent. As their populations increase, they begin feeding on scale insects, aphids, psyllids, whiteflies, and mealybugs. Ant stakes and other baits can be placed under trees and shrubs to prevent ants from foraging. Sticky compounds are another effective deterrent; they may be placed around the base of trees.
- If insect populations fail to decline, horticultural oils, insecticide, fungicide, miticide, or insecticide soap can be applied to suppress insects; one or more applications may be required.
- Neem oil is an organic broad spectrum pesticide that can used to quash insect populations on house plants, flowers, vegetables, trees, shrubs, and fruit. It is a biodegradable substance, and has not been shown to be toxic to mammals, birds, bees, earthworms, or beneficial insects.
- Judicious pruning cuts should be applied to remove infested plant parts.
- Branches close to buildings or other access points should be trimmed back to prevent insects from invading the tree.
- Trees should be fertilized in late spring or early summer to maintain tree vigor.
- Ensure trees are sufficiently watered, especially during periods of extreme heat.
- Apply a layer of organic mulch around the base of trees to improve soil quality, moderate soil temperature, and maintain soil moisture.
- Outdoor furniture can be cleansed with water during periods of honeydew excretion, particularly during drought.
- Sooty mold fungi growth can be inhibited by preservatives used in treated wood.
- The following cleaning solution can be used to remove sooty mold from plastic or painted surfaces:
– 1/3 cup of powdered house detergent
– 2/3 cup of trisodium phosphate
– 1 quart of household liquid bleach
– 3 quarts of water
- Always wear rubber gloves when cleaning with this solution.
- A mixture of lukewarm water and mild soap can be used to remove sooty mold from fruits and vegetables. Fruits and vegetables covered with sooty mold remain edible.
Photo courtesy of the Royal Horticultural Society