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Posted on November 4th, 2021 in Forests and Street Trees, How To, Plants | No Comments »

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.

Image of roots showing black root rot

Figure 2. Clean roots showing the characteristic black root rot. Photo by Janna Beckerman.

Pansies, block root rot.

Figure 1. Pansies get a bad a rap for falling down in hot weather. Thielaviopsis is often the real culprit. Photo by Janna Beckerman.

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.

Image of black root under a microscope showing chlamydospores

Figure 3. Looking at sectioned black root reveals numerous chlamydospores. Photo by Janna Beckerman.

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).

Full Article >>>

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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