Fall Tree Care: Identifying Common Tree Diseases in Fall, Part 7
This is the seventh part of a series on fall tree diseases and disorders. This article examines leaf scorch and lophodermium needle cast.
During fall, the environment undergoes a drastic shift, with cool temperatures prevailing, and inclement weather often abundant. In this climate, many plant diseases flourish. The following describes some of the most common plant diseases to occur in fall, and how they affect their hosts.
Leaf scorch is a physiological disorder of plants. It can affect most plants if the weather conditions are favorable. Extreme temperatures, heavy winds, periods of drought, and low soil moisture all contribute to leaf scorch. Scorching develops when water evaporates from a plant’s leaf surfaces, and is unable to be replenished through the root system. As a result, the leaf tissue becomes scorched, eventually turning necrotic. Plants that suffer from extreme leaf scorch may be defoliated, with many experiencing a decline in plant vigor.
Leaf scorch impacts a multitude of plants. Plants that are prone to leaf scorch include alpine currant, ash, beech, birch, flowering dogwood, horse chestnut, Japanese maple, Norway maple, sugar maple, linden, and white pine. Evergreens are also commonly affected by leaf scorch. The most frequently affected are arbovitae, azalea, fir, laurel, Japanese andromeda, hemlock, pine, rhododendron, southern magnolia, spruce, and yew. Young trees, diseased trees, or trees that have been infested with insects are more susceptible to leaf scorch.
Leaf scorch symptoms differ between plant species. On many deciduous plants, symptoms of leaf scorch generally appear between July and August. Leaves that have been scorched become discolored. Initially, a subtle yellowing between the leaf veins and along the leaf margins occurs, followed by a browning of the leaf tips. Eventually, the yellowing progresses, with tissue rapidly becoming necrotic at the leaf margins, and between the leaf veins. Some leaves may become entirely necrotic, without exhibiting any previous yellowing. Leaves that have been severely scorched may curl and wither. Scorched leaves are usually most abundant on the side of the plant exposed to direct sunlight, and prevailing winds. Leaf scorch may be confined to a single branch, or uniformly affect an entire plant.
Winter leaf scorch impacts many broad-leaved and narrow-leaved evergreens. Scorch injury on evergreens generally results from persistent exposure to heavy winds. On broad-leaved evergreens, such as azalea, euonymus, and holly, two long brown areas will form parallel to the main leaf vein, and enlarge as the plant becomes increasingly stressed. On narrow-leaved evergreens, such as arbovitae, fir, hemlock, pine, spruce, and yew, a browning develops at the needle tip, and progresses inward. As the plant declines, more needles will turn brown. Eventually, all of the needles may become necrotic.
- While leaf scorch can defoliate plants, it seldom kills them altogether. To maintain plant vigor, and reduce the incidence of leaf scorch, use sound cultural practices. Ensure that plants are sufficiently watered, especially during periods of extended drought. A thorough drenching of the soil is most effective.
- Apply a layer of organic mulch around the base of plants in early spring. Proper mulching will help to improve the soil quality, conserve soil moisture, and moderate the soil temperature.
- Test the soil around scorched plants to determine if there are any nutrient imbalances.
- Apply fertilizers in early spring or late fall, following leaf drop, to minimize the potential of root injury. Avoid excessive fertilizer applications during summer, when the soil is dry. Do not use fertilizers that contain an abundance of nitrogen.
- Prune dead, diseased, and encroaching branches to promote air circulation throughout the crown, and reduce the amount of foliage the root system must support.
- If de-icing salts or fertilizer have been used around scorched plants, leach the soil with a slow trickle of water for twenty four hours.
- Protective screens can be erected around plants not suited to a particular climate or location. Screens can reduce a plant’s exposure to the wind.
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.
Photo courtesy of Irene Andersson CC-by-3.0