Got Nature? Blog

Purdue Landscape Report – Black Root Rot: Black root rot (BRR), caused by the fungus Thielaviopsis basicola,has a host range of several hundred plants. In the landscape, the most common hosts include pansies (Fig. 1), and vinca, along with calibrachoa and petunia. In perennial hosts, it often infects lavender, phlox, milkweed and butterfly-weed (Asclepius spp.). Even woody plants, like holly and elderflower (Sambucus spp.) can succumb.

Symptoms of black root rot are often mistaken for nutrient deficiencies.  Older leaves may develop symptoms that look like iron chlorosis, while young leaves become stunted.  Infected plants grow more slowly compared to healthy plants. Shoot length is reduced compared to healthy plants, and scattered dead shoots may be apparent. In pansy, one of the key symptoms of BRR is when seemingly ‘healthy’ plants may suddenly collapse when a brief hot spell occurs (Fig. 1).

Keep in mind that diagnosing any root rot requires observing the affected plant for symptoms, and collecting the infected tissue at the right time to culture out the pathogen or pathogens causing the problem.

Fortunately, the black root rot pathogen creates both symptoms and even signs that can be observed with a little patience and a good hand lens. Carefully excavate roots, and wash them. Roots will be intact, but have blackened areas that are easily observed(Fig. 2). Honking chlamydospores may even be visible with just a hand lens or dissecting scope (Fig. 3).

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Resources:
Purdue Landscape Report
Root Rot in Landscape Plants, The Education Store
Ask The Expert: Tree Inspection, Purdue Extension- FNR YouTube Channel
Ask The Expert: Tree Selection and Planting, Purdue Extension- FNR YouTube Channel

Janna Beckerman, Professor
Purdue Botany and Plant Pathology

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