PPDL Picture of the Week

September 28, 2015

Black Root Rot of Pansy

W. Garrett Owen, PhD Student and Floriculture Technician, Purdue University

Roberto G. Lopez, Ph.D., Associate Professor & Floriculture Extension Specialist, Purdue University

Black root rot (Thielaviopsis basicola) is a soil-borne fungal pathogen that often infects all growth stages of pansies during greenhouse production.

Diagnosis

Symptoms of black root rot will initially appear as black spots or bands on healthy white roots. Early infection of black root rot is often seen at the tips of secondary feeder root tips. As the disease progresses, the entire root system will turn black (Fig. 1) due to the development of fungal spores within the root. For severely infected plants, the entire root system will turn black, appear water-soaked, and rotten, resulting in plant death.

Above-ground, the pansies will often exhibit chlorosis (yellowing) of the older foliage, stunted growth, and wilting (Figs. 2 and 3). Wilting may or may not occur, but is often dependent on the plant species and the extent of infection.

Spread

In greenhouses, the black root rot is often introduced from infected plugs. Once in the greenhouse, the disease is typically spread by movement of contaminated water, by wind, and other infected plants. Additionally, black root rot favors conditions such as substrate pH above 5.5 to 6.0, moderate to high substrate moisture, excessively cool or high substrate temperatures (56 to 77 °F) and fertility with an excess of ammonia nitrogen.

Control

Preventing black root rot from entering the greenhouse environment is the first method of control. Always examine pansy plugs carefully for symptoms of infection. If you suspect infection, immediately submit a plant sample to a diagnostic clinic. If the diagnosis is black root rot, ask your plug producer if they reuse their plug trays. Reuse of pansy plug trays is not recommended as this can spread black root rot from season to season.

Controlling black root rot during production involves the removal of infected plants and associated substrate and growing containers. Greenhouse benches, concrete floors, or outdoor ground covers should be sanitized with a greenhouse labeled disinfectant. Spores can survive on benches and on plug trays.