We have been receiving a lot of samples and inquiries about maples – red maples in particular. The concern is that the branches are dying or the tree is collapsing. When lawn care companies or arborists have been consulted, the diagnosis is Verticillium and the client is told the tree will have to be cut down.

Well…not so fast.

Verticillium is a soil-borne fungus that survives in the soil or debris from infected plants as tiny (smaller than a pin head) structures that are relatively resistant to environmental degradation. When a juicy root comes in the vicinity of one of these structures, it grows a slender fungal thread that can infect the root and become established in the cells that move water up the plant. The fungus eventually produces spores that clog these vessels, and also produces toxins, all of which contribute to branch dieback. Infected maples nearly always have greenish streaking in the wood of an infected branch. This can be seen when you cut the branch into cross-section.

Maple with Verticillium

Discoloration can be seen around the perimeter, and also with the longitudinal cut.

Japanese maple

Green discoloration in the wood due to Verticillium. Reddish discoloration due to saw marks.

Verticillium is common in maples, but it is not the only problem that these trees experience. Maples have thin bark, and especially when planted near pavement, the trunks of young trees can be injured by the heat of the sun during winter on sunny days. This results in death of and splitting of the bark, which then exposes the underlying wood. Damage is usually on the south- or west-facing side of the trunk. The wound thus created interrupts water flow up from the roots to the leaves, and there may be branch dieback on the side of the tree with the injured bark. In this situation there will NOT be greenish streaking in the wood of affected branches, because there is no real infection: the branch died from lack of water. The same effect occurs when the base of the tree has been injured by string weeders.

A second possibility is that the tree has an infection by the fungus-like organism Phytophthora. These infections often occur on the trunk, resulting in dark, bleeding areas of the bark. The Phytophthora kills the cells under the bark, forming what is known as a canker (a discolored area of dead tissue). Since the tissue on the trunk is killed, the branches above the affected area may also die.

Maple in Medford parking lot

Maple in a parking lot in Medford showing severe damage to the trunk from excess heat. The wounds face west, the direction of the setting sun. Note the extensive pavement around the tree, which intensifies the heat by radiation off the blacktop.

Maple grow site

The dark bleeding areas are cankers on this maple from an above-ground infection by Phytophthora.

Maple grow site

This is the canopy of the tree with the bleeding cankers. The infection was mostly on the left side of the trunk. Note that the foliage on left side is thinner than on the right side, which had a less-severe infection. The tree will eventually decline.

A third possibility is girdling roots. This occurs when the plant was in the nursery and the roots grew more than was accommodated by the pot it was in. When this happens, the roots will begin to circle inside the perimeter of the pot. If the roots are not straightened out when the tree is planted, the encircling roots will continue to grow in size and can eventually choke the trunk at the soil line or just above. The result is a tree that has dying branches, a thinner complement of leaves, or leaves that turn Fall colors early.

Girdling root at the base of a maple

An example of girdling root at the base of a maple.

Compacted soils, which have limited ability to allow gas exchange of the soil atmosphere and the roots; saturated soils from poor drainage; insufficient irrigation; or poor healing of wounds from improper pruning or torn branches can all contribute to maple branches dying back.

So although Verticillium CAN be the problem, don’t immediately assume it IS the problem.

If you notice branches dying back:

  • Examine the trunk for areas that are bleeding, split, or wounded.
  • Look for evidence of girdling (strangling) roots.
  • If the trunk looks great and there are no girdling roots, then check for the streaking in the wood of an affected branch. The branch has to be at least as large around as your thumb – checking pencil-diameter branches is insufficient. If you DO see the streaking, then you may want to have confirmation by a diagnostic laboratory before taking the drastic step of removing the tree.

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