Black root rot is caused by the fungus Thielaviopsis basicola. The black root rot fungus can infect many different kinds of plants in the landscape besides holly. For example, bedding plant flowers such as phlox, petunia, pansy, geranium, and begonia, and garden vegetables such as beans, tomatoes, and eggplants are susceptible to black root rot. Infected plants often have black lesions on the roots which are sometimes followed by root decay. This fungus can persist indefinitely in the soil or it can survive as a saprophyte on plant debris. The black root rot fungus can be spread via cuttings, potting soil, and containers during nursery and greenhouse propagation, so the disease may be obtained on greenhouse transplants or nursery stock.
Therefore, a holly could become infected in the landscape after pansies were planted nearby in the same bed. In addition to being spread through infected and contaminated plants, the fungus may be moved in contaminated soil that is carried from one part of the garden to another. Before planting, growers should inspect hollies and bedding plants and reject those with dark root lesions.
