Cause Armillaria mellea, a fungus, infects roots, killing the cambium and decaying the underlying xylem. Occasionally found on rose, this root pathogen is native to the Pacific Northwest where it occurs on the roots of many forest tree species, agronomic and ornamental hosts. The host range includes over 500 species of woody plants. The fungus spreads slowly, below ground, from root to root and can survive on woody host roots for long periods. When infected plants are removed, infected roots that remain below ground serve as a source of inoculum for other plants placed in the same location. Infection occurs when rose roots come in direct contact with partially decayed tree roots and are colonized by mycelium. Infection can also occur when roots contact rhizomorphs (black, shoestring-like fungal structures) that grow out from partially-decayed roots and through the soil.
Symptoms Aboveground symptoms are general and not specific. Diagnostic mycelial fans can be observed beneath the bark of the root crown of infected plants. Dig down about a foot below the soil line and use a pocketknife to remove thin layers of bark from the root collar. Mycelial fans are thick, white layers of fungus that adhere to the root bark and/or the wood beneath the bark. The fungus also makes black, shoestring-like structures called rhizomorphs, which are occasionally found within the bark and/or extending into surrounding soil. Rhizomorphs may look like roots on the outside but have an entirely different structure when cut open in cross-section.
Cultural control
- Roses that are vigorous, as a result of favorable climate and sound cultural practices, usually are more tolerant.
- Water deeply when watering is needed. Avoid surface watering, especially wetting the crown and trunk root area. If using drip irrigation, move drip-line emitters away from the crown and in between plants.
- Remove severely infected bushes and destroy them, being careful to remove small roots from the soil. Replant with resistant species such as Boxwood, Honeysuckle, or Hypericum (see the table "Plants Resistant or Susceptible to Armillaria Root Rot," in Section 3).
- Permanently removing soil around the crown and main root area has been effective in tree fruits grown in California and Australia and may be of benefit for managing infected bushes in the Pacific Northwest.
Reference Pscheidt, J. W., and Rodriguez, T. G. 2018. Diseases of Rose. In McGovern, R.J. and Elmer, W.H. (eds.) Handbook of Florists' Crops Diseases. Springer Int.