Tuesday, June 24, 2008

Landscape - Sudden Death of Maples

Over the years we have seen landscape maple trees suddenly dying. The problem most often involves well-established Norway, sugar, and red maples. Homeowners usually report the sudden wilting and death of their trees; in some cases this is true, but in others, twig growth and tree ring analysis suggest that many of the dying maple trees have not been growing well for some years. The following is information on why maples may die.

Although there does not seem to be a single cause for the decline and death of landscape maples, most of the causes would be exacerbated by the 2007 drought. In addition to drought, there have been a number of factors observed that have caused death or triggered decline and death of maples, including:

• Girdling roots are probably the leading cause of decline, especially among Norway maples. Offending roots may not be visible above ground, but if the tree trunk does not have the normal buttress root flare at the base, and instead, goes straight into the ground like a telephone pole, self-girdling roots can be suspected. Trees with girdling roots may decline over a period of years, but then may collapse suddenly. Girdling roots are often a response to too-deep planting, often two or three decades or more after the tree was transplanted.

• Verticillium wilt may infect all types of maples, and can also cause disease in tuliptrees, catalpas, golden-rain trees, and redbuds. Often developing on branches on one side of the tree first, leaves progressively wilt and die throughout the tree during the growing season. Where infections occurred late in the previous season, trees may not have even leafed out this year, or if they did, they immediately died. The Verticillium fungus is often more active in stressed trees. Large trees may die over a year or two, but small trees can suddenly collapse throughout from Verticillium wilt.

• Bacterial leaf scorch can affect many kinds of maples, but appears to be most common on red maples. This chronically infectious disease could weaken trees and make them vulnerable to other stresses.

• Canker and collar rot. We have diagnosed some cases of Phytophthora bleeding canker and collar rot on maples in past years. Trunks of affected trees have water-soaked bark spots. Collar rot, causing bark decay and wood staining, if well developed, can cause death of the top of the tree. Usually, collar rots and bleeding cankers lead to gradual decline of infected trees. The microbe that causes the disease, Phytophthora, is favored by high soil moisture levels, especially temporary flooding.

• Restricted rooting space. Sugar maples planted as street trees sometimes lack space for their roots to exploit. Such trees with inadequate root systems would be especially vulnerable to drought and temporary flooding stresses. Trees growing where there is plenty of open space but on shallow soils with bedrock near the surface will be vulnerable to drought.

• Soil compaction from foot traffic, construction, or other activities crushes small roots and makes soils impervious to invasion by new roots. Affected maples may decline.

• De-icing salts used the previous winters can sometimes be a factor in tree decline.

• Mechanical injuries. Construction such as laying utilities severs roots and triggers decline. Wounds to the trunk or large branches can also have negative effects on maple tree health.

• Opportunistic fungi. Root, butt, and trunk rotters such as Armillaria mellea and Ganoderma lucidum are found on some declining trees. In addition, canker and canker-rot fungi such as Botryosphaeria obtusa, Nectria cinnabarina, Cerrena unicolor, and Stegonosporium pyriforme are capable of invading weakened trees and causing branch dieback.

Adapted from "SUDDEN DEATH OF MAPLES IN THE LANDSCAPE" By John Hartman in the Kentucky Pest News from the University of Kentucky.

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