The Dark Side of Black Root Rot in Ornamentals

Black root rot can affect a wide range of ornamentals in home and commercial landscapes, nurseries, and greenhouses. Black root rot is commonly observed on Japanese and blue hollies, inkberry, pansy, petunia, and vinca.

Black Root Rot Facts

  • Symptoms are first noticed on above-ground plant parts, which result from root system decay. Plants may exhibit poor vigor or stunting. Leaves may develop a yellow color, wilt, and die (Figure 1). Infected plants may collapse or die back, with severe infections leading to plant death.
  • Root symptoms begin as dark brown to black lesions often in middle portions of roots (Figure 2).
  • Disease is favored by wet soils with mild temperatures or a high pH. Stressed plants are also more prone to disease.
  • The pathogen can persist indefinitely in soils or survive on plant debris.
  • Contaminated soil, infested plant material, and water can spread black root rot.
  • Caused by the fungus Thielaviopsis basicola.

Figure 1: Plants affected by black root rot often wilt and die.  (Photo: Elizabeth Bush, Virginia Polytechnic Institute and State University, Bugwood.org)

Figure 2: Black root rot results in roots with dark brown to black lesions that contrast sharply with healthy white roots. (Photo: Elizabeth Bush, Virginia Polytechnic Institute and State University, Bugwood.org)

Management

Landscapes

  • Avoid planting susceptible plants. Refer to Table 1 for a partial listing of some hosts susceptible to black root rot.
  • Remove and destroy infected plants, including rootball and surrounding soil.
  • Avoid stressing plants by providing proper site selection, nutrition, and irrigation.
  • There are no effective fungicide drenches available for homeowner use. Landscape professionals may treat established plants with low levels of infection to suppress black root rot. Fungicides do not cure black root rot.

Table 1: Plants resistant and susceptible to black root rot. (Image: Plant Pathology Fact Sheet, Black Root Rot of Ornamentals, PPFS-OR-W-03)

Greenhouses and Nurseries

  • Inspect roots of plants prior to bringing them into production areas. Use only disease-free stock plants.
  • Maintain a strict sanitation program. Keep production floors and benches clean. Do not reuse soil.  All tools, equipment, and containers should be disinfested between uses. Disinfest production area surfaces between cropping cycles.
  • Monitor plants regularly for disease development. Dispose of diseased plants immediately when black root rot is detected.
  • Soil drench fungicides may be applied preventatively. Fungicides do not cure black root rot. Always follow label directions when utilizing fungicides.

Resources

  • Black Root Rot of Ornamentals (PPFS-OR-W-03)
  • Greenhouse Sanitation (PPFS-GH-04)
  • Landscape Sanitation (PPFS-GEN-04)
  • Woody Plant Disease Management Guide for Nurseries and Landscapes (ID-88)

 

By: Kim Leonberger, Plant Pathology Extension Associate and Nicole Gauthier, Plant Pathology Extension Specialist

 

Posted in Landscape Trees & Shrubs, Ornamentals