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Black Root Rot in Flowering Annuals

Michelle Grabowski, UMN Extension Educator

Photo 1: Petunias infected with black root rot T. Burnes, UMN Dept. of Plant Pathology.

With the weather warming up but the risk of frost not entirely past, many annual bedding plants are spending their days in pots. Gardeners are working to keep these plants healthy until they reach their permanent home in a hanging basket or in the front garden bed. Poor growth and yellowing leaves on these plants may be a sign of a root rot problem.

Black root rot, caused by the fungus Thielaviopsis basicola, has recently been identified in flowering annuals in Minnesota. Thielaviopsis can result in severe root rot in cool temperatures; 55-62F being ideal. Above ground symptoms are often mistaken for nutrient deficiency. Plants are slow growing or stunted. Leaves are yellow and leaf veins may remain green. The symptoms that give this disease its name are to be found below ground. Dark sunken black lesions occur on roots of all sizes. Under the microscope dark brown cylindrical spores with several distinct sections can be seen within infected roots.

Many different plants are susceptible to black root rot. Common hosts include Geranium, Phlox, Petunia, Pachysandra, cyclamen, lupine, fox glove, Gerbera daisy and gaillardia.

Photo 2: Dark lesions on infected roots T. Burnes, UMN Dept. of Plant Pathology.

Spores of Thielaviopsis can survive for several years in soil or infested pots. Plants that are grown with unsterilized garden soil as part of the potting mix are often infected. Reusing dirty pots can also be a source of disease.

Simple sanitation is the best way to prevent problems with black root rot.

  • Use only new pots or clean old pots with a 10% bleach solution.
  • Do not use garden soil as part of the potting mix as it may contain fungal spores and is not easily sterilized.
  • Use sterile potting soil for starting new seeds and in hanging basket or other large planters.
  • Clean tools with a 10% bleach solution, Lysol or Listerine before working with annual bedding plants.

Photo 3: Thielaviopsis spores T. Burnes UMN Dept. of Plant Pathology.
  • Purchase only healthy vigorously growing transplants.
  • Examine roots for dark lesions before planting.
  • Discard infected plants to prevent spread of disease to the other plants in the garden bed or hanging basket.

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