March 2015 Plant Disease Update

Holly roots blackening from black root root
Blackening of holly roots caused by the black root rot fungus (photo by R. L. Lambe).

Although Japanese holly (Ilex crenata) is a popular broadleaf evergreen in Virginia landscape plantings, dieback is a common complaint. The most common cause of dieback in Japanese holly is the root disease, black root rot, which gets its name from the masses of black fungal spores that form in and on infected roots. Japanese holly is especially susceptible to this disease; however, some other hollies, such as Ilex glabra (inkberry), I. xmerserveae (Meserve holly), and I. opaca (American holly) can also be affected. Plants initially turn off-color, but sections of the plant gradually turn brown, and eventually the whole plant dies. 

The fungal pathogen is soil-borne, so replanting susceptible hollies in soil where the disease has been diagnosed before usually ends with the same result. Soil drench fungicides containing the active ingredient, thiophanate methyl, can be used to control the disease preventatively, but regular applications are necessary to hold the disease in check. Another option is to replant infested soil with a different species. 

Browning and dieback caused by the black root rot fungus
Browning and dieback due to black root rot on Japanese holly (photo by M. A. Hansen).

Other species of woody shrubs, including species of holly other than those listed above, are not susceptible to black root rot. Broadleaf evergreens that can be used as replacement plants include boxwood and barberry. Note that it is also advisable to avoid planting pansies, petunias, phlox, annual vinca, coral bells, and zinnias in soil where black root rot has been diagnosed because these herbaceous plants are also susceptible to the disease.