Left photo: An uneven stand of petunias with black root rot. Right photo: Stunted vinca with stunted root system due to black root rot.
Photos courtesy of Margery Daughtrey
These poorly rooted calibrachoa plugs show signs of black root rot.
Photo courtesy of Margery Daughtrey
Purpling of pansy foliage associated with Thielaviopsis infection.
Photo courtesy of Margery Daughtrey

Thielaviopsis root rot is also called black root rot and it has a long history on tobacco plants and hollies in addition to bedding and flowering potted plants. In the greenhouse, we see it most often on calibrachoas, pansies, vincas and petunias. Infected plants usually are stunted. Leaves turn yellow or purplish because of dysfunctional roots — iron applications won’t help. To get the right diagnosis, look at the big picture. If a crop is uniformly yellowed, it indicates iron deficiency from too high a pH. However, small plants with purple or yellow leaves are scattered through a crop, rinse and examine the roots with a microscope or send a sample to a diagnostic lab to check for the Thielaviopsis fungus. Always check the roots of plugs as they come into your greenhouse to make sure you aren’t inviting in black root rot. Never reuse trays after an outbreak without disinfection.

Margery is a plant pathologist specializing in ornamentals at Cornell University’s Long Island Horticultural Research & Extension Center. She can be reached at mld9@cornell.edu