What Do You Do if Your Million Bells Have Root Rot?

Million Bells

• Although million bells, which grow as perennials in U.S. Department of Agriculture plant hardiness zones 7 through 11, and petunias, which grow in USDA zones 10 and 11, are in the same plant family, million bells have botanical and physical differences that set them apart from petunias. Before 1985, million bells were grouped in the Petunia genus. When genetic research revealed they have four additional chromosomes that petunias lack, million bells claimed their own genus. Although this additional genetic material allows million bells to withstand heat and drought better than petunias, this perk is a double-edged sword. When these drought-tolerant plants get too much water, they are susceptible to black root rot.

Root Rot

• Black rot is caused by the fungal pathogen Thielviopsis basicola. This disease invades million bells through their roots and causes stunted growth, yellow leaves and sparse flowers. The roots turn black as the rot progresses and are eventually unable to sustain plant growth. Entire plants collapse and die. Black rot can spread to other susceptible plants through the soil or water. This fungus lives in garden soil, but it may also thrive in peat moss or peat-based potting mixes.

Treatment

• If your million bells become infected with black rot, there is no effective cure. Before they infect nearby plants, remove and discard them. The fungus lives in the soil or potting mixes where it can survive for prolonged periods, so if you plant new million bells plants in the same area where you removed infected plants, they may also contract this disease. Soil solarization is an effective, organic way to kill black rot fungal pathogens. Solarization uses heat from the sun to kill disease organisms in infected soil. Cover the infected garden bed with clear plastic sheeting, which is more effective than black plastic, and anchor the edges securely so the plastic maintains good contact with the soil. Leave the plastic in place for four to six weeks, preferably in the hottest part of the summer.

Management

• Managing the conditions that favor black root rot may prevent or eliminate this disease. Plant million bells only in well-draining soil, whether you grow them in a garden bed or container. When the roots sit in waterlogged soil, particularly in cooler weather, root rot is highly likely. Even if the soil drains well, you can still over-water these plants. Gary R. Bachman, Mississippi State University horticulturist, notes that million bells prosper on a bit of neglect. Bachman recommends watering them only when the soil surface is dry. A line of million bells, named Superbells, developed by plant company Proven Winners, has been bred with resistance to Thielviopsis root rot. These are suitable choices for moist climates and conditions that promote this disease.