Fall Tree Care: Identifying Common Tree Diseases in Fall, Part 6
This is the sixth part of a series on fall tree diseases and disorders. This article examines diplodia tip blight and Dutch elm disease.
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.
Diplodia Tip Blight (Diplodia pinea and Diplodia scrobiculata)
Diplodia tip blight, also known as Sphaeropsis tip blight, is a fungal disease caused by two species of Diplodia: Diplodia pinea and Diplodia scrobiculata. Diplodia pinea is the more aggressive species, but both fungi are capable of causing disease. Diplodia tip blight is most common in pines that have two to three needles per bundle. Austrian pine is the most susceptible species. Other conifers, such as spruce, fir, larch and arbovitae may occasionally become infected. Pines that have five needles per bundle are highly resistant to the disease.
The fungus infects more than twenty pine species. It is commonly found on Austrian pine, but is also frequently reported on Scots, Jack, red, ponderosa, and Mugo pines. Infections occur on Monterey pine in California.
Brown, stunted, or curled new shoots, with short, brown needles may signify the presence of Diplodia tip blight. Symptoms are characterized by resin droplets, which indicate that a new shoot has been infected. Entire new shoots are killed rapidly by the fungus. Needles on infected shoots often turn tan or brown while still encased in their fascicle sheaths. Infection varies among major branches. Damage is first evident in the lower crown, before progressing upwards. Diplodia tip blight can form perennial cankers on mature trees and saplings, inducing sudden branch death. After two or three successive years of infection, the tree’s crown may incur significant damage.
Infection is most common in mature pines. Damage from Diplodia tip blight is generally confined to new shoots; however, the fungus may infect older tissues through damage resulting from wind and hail, or perforations in the wood created by insects. Tissues wounded during pruning or shearing operations may also become infected. Stressed or injured trees remain vulnerable to infection for several days, especially during warm, dry periods. Symptoms on new shoots can be detected in late May. The extent of the infection can be effectively determined in late June or July.
- Infection of new shoots can be reduced by applying a fungicide to pines. Perform the first application as buds begin to swell, followed by a second application two weeks later. Fungicides containing potassium bicarbonate or benomyl as the active ingredient, or the 4-4-50 Bordeaux mixture are most effective. Fungicides applied during late April and early May will not prevent infection of seed cones.
- Prune and dispose of blighted needles, twigs, and cones. Use rubbing alcohol, or a solution comprised of one part bleach and nine parts water to disinfect pruning tools before each cut. This will help prevent the fungus from being spread to other trees or branches.
- Forego pruning or shearing of pines during periods that are favorable for infection.
- When planting, avoid selecting vulnerable conifer species in areas where infection is common. Native species are more susceptible when planted on poor soil sites, or locations prone to environmental stressors.
- Ensure trees are sufficiently watered, especially during periods of extreme heat.
- Fertilizing trees in spring or summer will improve tree vigor; avoid using fertilizers containing high quantities of nitrogen.
- Maintain a layer of mulch around the base of the tree to improve soil quality, and conserve soil moisture.
- Avoid mechanical wounding of pines.
Dutch Elm Disease
Dutch elm disease is a fungal infection that affects many species of elm. It is one of the most destructive tree diseases in North America and Europe. The disease can be caused by two similar species of fungi: Ophiostoma ulmi and Ophiostoma novo-ulmi. Dutch elm disease spreads through infected trees, killing individual branches. Prolonged infections often culminate in the death of the tree.
The native species of American elm vary in their susceptibility to Dutch elm disease. American elm is highly susceptible. September elm, slippery elm, rock elm, and cedar elm range from susceptible to moderately resistant. No native elms are completely immune to the disease, but some individuals or cultivars express the ability to recover from infection. European and Asiatic elms exhibit a greater resistance to the disease than American elm.
Numerous physical factors influence tree susceptibility. These factors include seasonal changes, climatic conditions, and the vitality of the tree. Dutch elm disease requires moist conditions to thrive. As such, trees are most susceptible to infection during spring and summer, when rainstorms are most prevalent. Trees are not as vulnerable to infection during periods of drought. Trees that develop at a slow rate are less prone to infection than more vigorous trees.
The initial symptoms of Dutch elm disease are most apparent in the tree’s crown. The leaves of infected trees begin to wilt, assuming a distorted appearance. The leaves soon become discolored, turning a pale yellow before transitioning to brown. The pattern of symptoms within the crown varies depending on where the infection first enters the tree. If the fungus invades through the upper crown, symptoms often appear at the terminal point of an individual branch. This process is referred to as flagging. As the infection spreads, symptoms will progress downwards throughout the rest of the crown. Multiple branches may be individually infected, resulting in symptoms developing at multiple points in the crown. If the fungus is transmitted into the tree through a root graft, symptoms may begin in the lower crown, on the side closest to the graft. Symptoms typically begin in late spring, but they can develop at any point during the growing season.
In newly infected branches, dark brown streaks will appear in the sapwood. To detect this staining, strip the bark from a dying branch to expose the outer layer of wood. The staining may be overlaid by seemingly healthy wood later in the season. If infection occurred the previous year, the current sapwood may not be discolored.
- When planting, consider selecting disease resistant elm cultivars or other tree species.
- The most effective method for preventing Dutch elm disease is to limit bark beetle populations. In areas where the native elm bark beetle is present, a registered insecticide should be applied to the lower stem of healthy elms in late summer or early fall. In locations where the European elm bark beetle predominates, insecticide applications should be performed in early spring to prevent adults from feeding in twig crotches and bark.
- Fungicides can be utilized to control disease outbreaks. Applications must be performed at all potential points of infection. Macroinjections are ideal, as they provide thorough distribution of the chemical into the crown. Macroinjections should be made soon after budburst. Flush the holes created from each injection with water to reduce damage to the cambium. Applications should be repeated every one to three seasons, depending on the chemical used. Fungicide applications can produce undesirable side effects, including occasional leaf scorching or leaf loss. This damage is not permanent; elms will generally replenish their foliage in summer.
- Systemic fungistat treatments can be administered to healthy trees that are not symptomatic. Fungistats are compounds that inhibit fungal growth or sporulation. Fungistats are infused into the root flare of the tree. This enables the fungistat to be thoroughly distributed throughout the tree’s crown. Fungistat treatments are a preventative method; they can protect a tree for up to two growing seasons.
- In settings where there is an abundance of elms, promptly remove diseased trees or branches. This will help reduce the number of potential breeding sites for elm bark beetles. Infected stems and branches should be removed and destroyed before bark beetles emerge. During the growing season, removal of infected trees should occur within two to three weeks of detection. If an infected tree is discovered in late fall or winter, removal should be completed before April, when the beetles emerge.
- Disrupt root grafts between diseased trees and healthy trees. This can be accomplished through the use of a vibratory plow or trenching machine. Root grafts should be broken before the infected trees are removed. Otherwise, the healthy trees will absorb the contents of the diseased tree’s root system, enabling the infection to spread.
- Pruning can help eliminate the fungus if less than 5% of the crown has been affected. The infection must be new, and only present in the upper crown.