This is the fourth part of a series on fall tree diseases and disorders. This article examines bacterial wetwood and black knot.
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.
Bacterial Wetwood (Slime Flux)
Bacterial wetwood, often referred to as slime flux, is a bole rot that afflicts hardwood trees. The disease is associated with numerous bacteria, which infect the inner sapwood, and outer heartwood. Bacterial infection generally occurs when a tree has been wounded, or is suffering from environmental stress. Once an infection has occurred, the bacteria feeds within the inner tissue, utilizing the sap as a nutritional resource. This causes slimy, water-soaked areas to appear in the trunk, branches, and roots of infected trees. Bacterial wetwood is not fatal to trees, but it is considered a chronic disease. Persistent infection can contribute to a general decline in tree vigor over time.
The trees that are most susceptible to bacterial wetwood include elm, apple, crabapple, London plane, redbud, aspen, dogwood, magnolia, Russian olive, beech fir, maple, sweet gum, sour gum, birch, hemlock, mountain ash, sycamore, boxelder, hickory, mulberry, butternut, horse chestnut, oak, tulip tree, cottonwood, linden, pine, black locust, poplar, willow, and walnut.
Some bacteria target specific hosts. One example is Enterobactor cloacae, which is the causal agent of bacterial wetwood in elm. Species of Clostridium, Bacillus, Klebsiella, and Psedomonas have been identified as the causal agents in various other trees.
The first external symptom of bacterial wetwood is a seeping or bleeding from the wounded tissue in branch crotches, pruning cuts, injection holes, and cracks in the trunk. A slimy, foul liquid is discharged from the tree, and trails down the bark, causing visible staining. Insects are attracted to the liquid, upon which they feed. When affected by bacterial wetwood, the branches of younger trees often wilt. Mature trees may experience a reduction in foliage.
Bacterial wetwood can also be observed internally. If an infected tree is removed, the heartwood will appear darker in color than the surrounding wood. Water-soaked patches that range from dark brown to black will also be identifiable in the tree’s trunk and limbs.
There is no effective remedy or preventative treatment for bacterial wetwood. The following methods may be used to help alleviate symptoms, and manage infections:
- When planting, select trees that are disease resistant. Plant trees in locations where there is little urban soil compaction.
- Fertilize stressed trees to stimulate growth and mitigate the severity of the disease. Refrain from over fertilizing healthy trees, as this may increase their susceptibility to the disease.
- Remove and dispose of any dead or weakened branches, as well as any loose or diseased bark. Disinfect tools with 70% rubbing alcohol prior to pruning, and apply clean cuts around the wound to facilitate healing.
- Avoid boring holes in trees. Boring will not enhance recovery. Instead, it will create additional wounds through which disease pathogens and insects can enter.
- Avoid using draining tubes to cleanse infected areas. Drainage tubes can inadvertently transmit the disease to other sections of the tree.
- Ensure that trees are sufficiently watered, especially during periods of dry weather.
- Apply a layer of organic mulch around the base of susceptible trees. This will help improve soil quality, retain soil moisture, and moderate soil temperature.
- Avoid inflicting any mechanical injuries on trees.
Black Knot (Apiosporina morbosa)
Black knot is a fungal disease that infects over 25 species of Prunus. It is caused by the fungus Apiosporina morbosa. Outbreaks of black knot may occur in landscape and forested settings. The knots that form on infected plants can cause extensive defoliation, and foliar dieback. Tree mortality is common in young or highly susceptible species of Prunus.
Black knot primarily affects plum and cherry trees. Over 25 species of Prunus are susceptible to infection, including American, Canadian, European, Japanese, and purple-leaf plum, as well as chokeberry, and European bird cherry. Infections also occur on amur, apricot, chokecherry, and flowering almond, as well as several other varieties of cherry, including Nanking, pin, Sargent, sour, and western sand cherry.
Symptoms of Infection
Knobby, irregular black swellings will grow parallel along the length of infected stems and branches. During the first year, the knots may be difficult to discern. As the galls increase in size, they become more conspicuous. They are most apparent in winter, when the hosts have shed their leaves. Once the galls mature, they harden and turn black. Infected branches may become malformed, assuming a bent or distorted appearance. Girdled branches are often bereft of healthy growth. Leaves that do emerge tend to be stunted, and will generally wilt before expiring in early summer. The canopies of infected plants may become laden with up to several hundred knots. The knots may ooze a sticky liquid, when can be observed streaming down the main trunk of the plant. Older knots may brighten in color, turning white or pink. This discoloration is caused by another fungal parasite, Trichothecium roseum.
- Landscape and forested sites that contain an abundance of Prunus species should be monitored consistently for infection symptoms.
- Prune and dispose of any knots that are present to reduce the incidence of infection. In late winter, remove all branches with swellings, cracks in the bark, or black knots. Diseased plant material should be disposed of thereafter.
- Knots that are cracked or oozing, especially those located on the main trunk of a plant, should be inspected by a certified arborist. Knots that have grown on or into the main trunk can disrupt the structural integrity of the plant, rendering it prone to failure.
- When planting, select trees that exhibit an increased resistance to black knot.
- Fungicides may be utilized to protect young or highly susceptible plants from infection. Initial applications should be performed in early spring, as bud expansion begins. Repeat applications at 7 to 14 day intervals until the shoots mature.