Japanese holly (Ilex crenata) is an important evergreen shrub grown in nurseries and landscapes in Virginia. It is widely used in foundation plantings, hedges, and in mass plantings. In 1976 the disease, black root rot, caused by the fungus Thielaviopsis basicola, was detected in nursery containers of Japanese holly showing severe decline. Since that time, black root rot has been detected in numerous nursery and landscape plantings in Virginia and has become the major disease problem in Japanese hollies.
The disease is named for the black lesions that commonly occur on infected feeder roots (Fig. 1.). Symptoms on Japanese holly include foliar chlorosis, leaf drop, and stunting. Although stems and leaves are not colonized by the fungus, plants suffer a gradual dieback as a result of root death (Fig. 2.). Young holly plants in the nursery can be killed within weeks as a result of severe root destruction by the fungus; however, mature plants decline more slowly. Thielaviopsis basicola produces reproductive structures, called conidia, and survival structures, called chlamydospores, on infected roots (Fig. 3.). The chlamydospores can survive in the soil for long periods of time in the absence of a host plant.
In container production, cuttings of Japanese and other susceptible hollies should be rooted in a soilless pine bark medium in new containers or flats. Recycled containers should be thoroughly rinsed with a 10% bleach solution before reuse to prevent survival of fungal chlamydospores. In field production, care should be taken to avoid planting Japanese and other susceptible hollies in fields formerly cropped to tobacco, beans, or peanuts in which black root rot was a problem.
In landscapes with a history of the disease and where the grower would prefer not to use fungicides on a continual basis, the best option is to choose other species as replacement plants for Japanese holly. Most other woody species and many other species of holly can be planted successfully in soil infested with Thielaviopsis basicola. Some plants that might serve as acceptable replacement plants include one of the many compact hybrids or cultivars of boxwood or the 'William Penn' cultivar of the hybrid barberry, Berberis x gladwynensis. Susceptible herbaceous plants, such as pansy, Madagascar periwinkle, and foxglove, should not be planted in beds where black root rot has been a problem.
Wills, W. H. and R.C. Lambe. 1978. Pathogenicity of Thielaviopsis basicola from Japanese holly (Ilex crenata) to some other host plants. Plant Disease Reporter 62(12):1102-1106.
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Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; Jewel E. Hairston, Administrator, 1890 Extension Program, Virginia State, Petersburg.
May 1, 2009