How to Manage Pests
UC Pest Management Guidelines
Thielaviopsis root rot is also called black root rot after one of the major symptoms. Plants are stunted and roots are badly rotted. Stems below ground may enlarge and develop black, rough, longitudinal cracks. Characteristic dark brown to black, thick-walled, barrel-shaped chlamydospores form in infected tissues and may be visible under magnification.
The fungus has a wide host range: 120 species in 15 families are known to be susceptible. Strains of the fungus are known that differ in pathogenicity and virulence. Important ornamental hosts include begonia, cyclamen, geranium, gerbera, kalanchoe, pansy, petunia, poinsettia, primula, snapdragon, sweet pea, verbena, and viola. The disease is favored by wet, cool soil and any condition that weakens plants; it is most severe from 55° to 61°F, while only a trace of disease develops at 86°F. Alkaline soil favors the disease, which can be prevented at pH 4.8 and greatly reduced at pH 5.5 or below. However, many plants do not grow well under such acidic conditions.
The fungus is soilborne and capable of prolonged survival in the absence of susceptible plants. Two kinds of spores are formed: barrel-shaped chlamydospores (resting spores) in short chains of 3 to 7 and rectangular-shaped endoconidia. The fungus can be spread in water, soil, by infected plants or vectored by fungus gnats and shore flies.
The use of pathogen-free plants, along with improved sanitation and cultural practices, has reduced the importance of this disease, which at one time was widespread, especially in poinsettias. The fungus can still be troublesome in field-grown flowers. The benzimidazole fungicides such as thiophanate-methyl are very active against the fungus and are used as soil treatments to control it.
To treat container media, steam (at 140°F for 30 minutes), or solarize (double-tent at 160°F for 30 minutes or 140°F for 1 hour). For flower production in open fields, solarization in warmer climates has been successful for control of most Thielaviopsis in many crops. Solarization and steaming are acceptable for organic production. For more information, see MANAGEMENT OF SOILBORNE PATHOGENS.
UC IPM Pest Management Guidelines: Floriculture and Ornamental Nurseries