Garden Variety: Know the signs, causes of tree decline
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Late summer is often the time of year when trees that are struggling to survive succumb to environmental stresses and/or pests. They may put on an early show of fall color before dropping their leaves, or they may simply drop their leaves or needles. This happens with trees in natural settings as well as the planted environment. Saving these declining trees at this point is unlikely, but identifying the cause of the decline could prevent spread or reoccurrence.
The first clue to determining the cause of the decline or death of a tree or shrub is to look for common environmental stressors. These can affect all species of plants. Knowing the tree or shrub species also helps in order to look for common pest problems associated with that particular species. Pests are often regional, though, so use caution with websites and printed resources that are not region-specific.
Some examples of common pests that kill trees and shrubs in the Lawrence area are:
• All species: environmental issues. These are the hardest to figure out but are the most common causes of stress and decline. Unfortunately, many common planting and care practices are the problems.
For young or newly planted trees and shrubs, always check to make sure the twine and burlap were removed from the base of the plant. Check planting depth (the flare root should be at the soil surface, but it often gets buried). Remove soil and mulch that cover the flare root. Mulch on the root zone is great, but mulch on the tree trunk can be detrimental.
After moving soil and mulch away if needed, try to lift the plant. Sometimes plants that were in containers too long will stay potbound in their planting hole and fail to root into the surrounding soil. If this is the case, use a shovel to cut into the root mass to get it to open up.
For older trees and shrubs, soil and mulch depth may still be problematic. Look for the root flare and remove excess material if needed. Compaction and root competition are often problems for large trees planted in lawns. Keep turfgrass away from the base of trees and shrubs as much as possible and water trees over extended dry periods.
Excess water can also be an issue in irrigated yards or if soil drains poorly in an area.
Damage from weedeaters and mowers is also a common cause of stress for trees and shrubs.
• Thin-barked trees such as maple: sun scald. Look for a wound or apparent damage on the trunk of the tree on the southwest side. Sun scald occurs in late winter on warm days. The sun shines on the trunk of the tree and warms it enough for sap to flow. Then, the sap freezes when the sun goes down and temperature drops. The freezing process ruptures plant cells and can cause substantial damage. Sun scald is also an entry point for wood decay fungi.
• Scotch and Austrian pines: pine wilt disease. Scotch pines are the most susceptible, but Austrian, ponderosa, mugo, and other pine species can also die from pine wilt. If the disease is suspected, send samples to a lab for confirmation and take appropriate control measures to prevent it from spreading. Samples can be sent through K-State Research and Extension — Douglas County, sent directly to the Kansas State University Plant Diagnostic Lab, or sent to other plant pathology labs.
• American elm: Dutch elm disease. The majority of American elms in the U.S. have already died from this disease over the last fifty years or so, but a few large, old specimens remain. Young American elms are also common in natural areas. Look for rapid death and brown streaking just below the surface of the bark on thin-barked branches and twigs.
• Ash, all species: borers, leaf spots, environmental stress. Emerald ash borer has gotten a lot of press for killing millions of ash trees across the U.S., and it is present in Lawrence and the surrounding area. However, some ash trees in the area are dying from infestation by native borer species, which are generally considered secondary pests. In these cases, the trees are probably growing in poor soil, receiving too much or too little water or enduring other sorts of environmental stress. Ash is also susceptible to a leaf spot disease called mycosphaerella that often causes trees to defoliate this time of year.
Ask an expert for help on ash problems. If the trees are stressed from the leaf spot disease or native borers, they should leaf back out next spring. If the trees are infested with emerald ash borer, they should be removed and the wood destroyed to prevent further spread of the insect.