Why is my Maple tree dying?
Tom Creswell, Plant & Pest Diagnostic Lab Director, Purdue University
We hear that question frequently in the diagnostic lab. While there can be several possible causes for trees dying, ranging from disease to injury to drought, one of the most common disease problems on maple and several other trees is Verticillium wilt (Fig 1). Selecting the right sample to us to allow confirmation in the lab involves collecting branches about an inch or two in diameter that show streaking in the wood just below the bark. Figure 2 shows the ideal type of branch sample that has characteristic olive green to gray discoloration in streaks just below the bark. Note that the cross section of the stem also shows discoloration in partial rings deeper in the wood as well. Maple usually has streaking present in infected trees but some trees show only a few streaks and some, such as Tulip Poplar, show only a gray general discoloration.
We take small chips of wood showing the streaks, place it on an isolation medium (Fig 3) and wait for fungal growth in a about a week. The black dots in the medium are due to the formation of thick walled microsclerotia (Fig 4). These resting structures are resistant to heat, cold and drying out and allow the fungus to remain in the soil, even after a tree is removed. Figure 5 shows the spores of Verticillium in tiny whorls.
See these feature articles for more information on Verticillium wilt on trees and other plants:
Verticillium wilt - APSnet
Verticillium wilt - What's Hot at the P&PDL on
August 11, 2008
Click image to enlarge
Fig. 1. Maple tree showing Verticillium wilt symptoms
Fig. 2: Japanese maple branches showing typical streaking in stems
Fig. 3: Chips of wood from the streaks are placed on the isolation medium and allowed to grow. Photo credit: Jan Byrne, MI State.
Fig 4: A close up view under the microscope shows the dark resting structures (microsclerotia) of Verticillium dahliae on the isolation plate.
Figure 5: The fungus producing spores in culture on a wood chip.