Kelly Jackson
Christian County Extension Office
Declining Pansies This Fall
Black root rot is a pest which during recent years has been appearing more frequently on fall-planted pansies in flower beds in Kentucky. In many cases, the plants are already infected when you buy them. In order to protect your plants and your landscape from black root rot, it is important to know a little about it and the damage it causes.
Black root rot is caused by the fungus Thielaviopsis basicola. Infected pansy roots develop black lesions or blackened root tips. Symptoms are expressed by yellowing or stunted plants, dead areas on leaves, or wilting and death. However, symptoms may not show on plants provided with excellent growing conditions (temperature, water, drainage, fertility) despite having roots that are barely functional because of the black root rot fungus. Once a stress occurs that affects its growing conditions (i.e., poor drainage, lack of water), pansies decline rapidly.
Because black root rot fungus can produce specialized fungal spores capable of surviving many years in flower beds, this disease is extremely difficult to eliminate once established. Additionally, the fungus can be spread in water, in soil, by infected plants, or vectored by soil-inhabiting insects such as fungus gnats. Not only will you be unable to grow pansies in these beds but many other bedding plants are also susceptible to black root rot, including begonia, geranium, petunia, primula, snapdragon, sweet pea, verbena, viola and many others.
A combination of cultural practices and avoidance of susceptible plants will be needed to reduce black root rot. Special emphasis should be place on avoidance, starting with the purchase of pansies, because it is much harder to control black root rot than prevent.
• Inspect plants for symptoms before buying. Roots should be clean and white; blackened areas indicate disease.
• Establish annual flowers in beds where good growing conditions can be provided.
• Avoid unnecessary stresses in growing the plants.
• Grow plants in well-drained growing medium or soil that is slightly acid (pH 5.5-6.0).
• Avoid flower bed sites that are poorly drained, contain excess salts, or were subject to devastating diseases in prior years. Replace or leach out contaminated soil, if necessary (e.g., soils with a prior history of severe black root rot or with excess de-icing salts or other toxic contaminants).
• This fungus responds well to a two-year rotation to grass so where feasible in the landscape design, convert problem bed sites to grass for two years. Otherwise, rotate planting beds in the flower garden avoiding susceptible plants mentioned earlier. Some perennials and woody plants can are also susceptible and as such should not be planted. These include astilbe, catalpa, dianthus, elm, gaillardia, holly, lilac, black locust, peony, phlox, and vinca.
• There are no chemical "cures" for black root rot; chemical drenches in outdoor beds are mostly ineffective and impractical.
• At the end of the growing season, remove and destroy all plants for purposes of sanitation.
The Cooperative Extension Office can help you diagnosis declining pansies for black root rot. Contact our office for more information – 270.886.6328.