Diagnosing Root Death in Woody Ornamentals: A Systematic Approach

Root death is a primary cause of decline in woody plants. When roots die, the plant cannot absorb sufficient water and nutrients to support growth, leading to visible above-ground symptoms that often appear long after root damage has begun. This delay makes full recovery difficult, but identifying the cause is essential—especially if replacement is planned. Diagnosis requires careful observation of symptoms, signs, and site history.

Symptoms of Root Death

1. Slowed growth: terminal bud scars from recent years are closer together than in previous years.
2. Leaves wilt, yellow, and fall prematurely.
3. Marginal leaf burn or scorch in summer.
4. Roots appear dark brown or black; few or no white root tips when washed. (Note: some healthy plants like azaleas and rhododendrons naturally have dark roots.)
5. Roots are limp, not brittle and crisp.
6. Few small side branches; side branches dead, only main branches alive.
7. Asymmetric or misshapen canopy due to major branch death.
8. Extensive cankering and dieback of small twigs and branches in winter.

Potential Causes and Diagnostic Clues

1. Root-rotting fungi: Look for conks, shelf fungi, mushrooms, or "deadman's fingers" on root flares or main roots (late summer/autumn). Fibrous roots may be black (Thielaviopsis). Fungal sheaths under bark: white (Armillaria), black (Xylaria). Dark brown "shoestrings" (Armillaria). Wood discoloration: black (Xylaria), black and resin-soaked (Verticicladiella), brown to reddish-brown (Phytophthora). In elms, butterscotch-yellow wood with wintergreen odor (elm yellows). Soft, decayed wood (Ganoderma, Polyporus, Inonotus).
2. Over-fertilization or deicing salt damage: Check site history; tissue analysis may reveal excessive chloride or other ions.
3. Flooding or drought: Examine drainage patterns and recent changes; check weather records.
4. Temperature extremes: Determine planting date and conditions before, during, and after planting if within a year.
5. Phytotoxicity from pesticides: Review applications from past 1-2 years; some herbicide damage appears 1-2 years post-treatment.
6. Girdling roots, ropes, or wires: Look for encircling roots or trunk flattening at soil line; check for leftover wires/ropes from transplanting.
7. Planting too deeply: Suspect if no root flare visible at base.
8. J-rooting in conifers: Excavate to reveal main root in J-shape, not growing downward.
9. Roots unable to penetrate native soil: Roots confined to original transplant hole due to compacted hole walls, compacted native soil (clay), or overly favorable backfill.
10. Plastic burlap not removed: Test with a match—plastic melts; natural fiber burns.
11. Natural gas or landfill gases: Displace oxygen; contact gas company to check for leaks.
12. Insect feeding: Excavate roots to find chewing/tunneling damage or larvae.

Before treatment, confirm the diagnosis. For fungi like Phytophthora and Thielaviopsis, fungicides can suppress but not eradicate the pathogen; repeated applications are needed. No fungicides effectively control Ganoderma, Inonotus, Xylaria, or Armillaria. Recovery may take years, as damage accumulated over multiple seasons.

Prepared by Gary W. Moorman, Professor of Plant Pathology