Landscape and Nursery - Black Root Rot of Holly

The following is a good article on black root rot of Holly from the University of Kentucky.

Black root rot, caused by the fungus Thielaviopsis basicola can do significant damage to hollies in landscape beds. Black root rot is most frequently observed on Japanese holly, blue holly, and inkberry. Susceptible blue holly cultivars include: Blue Angel, Blue Maid, Blue Prince, Blue Princess, Blue Stallion, China Boy, China Girl and Dragon Lady. While English and Chinese hollies are reportedly resistant, American and Yaupon hollies are considered to be only moderately resistant. Other ornamentals known to be susceptible include begonia, cyclamen, geranium, gloxinia, oxalis, petunia, phlox, poinsettia, sweet pea, verbena, and viola (pansy). Black root rot may also affect alfalfa, cotton, cowpea, eggplant, peanut, snapbean, soybean, tobacco, and tomato.

Symptoms. The first symptoms of black root rot include yellowing and marginal scorch of the foliage and shoot dieback. Later, twigs or stems may die back and eventually the entire plant may die. The root system of the declining plant is stunted and decayed. These symptoms could be confused with Phytophthora root rot which was also widespread in landscapes this year. Unlike Phytophthora, black root rot causes black lesions on the infected roots which, in the early stages of disease, contrast sharply with the adjacent healthy white portions. Lesions may appear on the tips of feeder roots or elsewhere along the root. Diagnosis can be confirmed by microscopic analysis which reveals the characteristic chlamydospores of the fungus embedded in the root tissues.

Disease Management.

Landscapers and nursery growers need to be aware that the black root rot fungus can persist indefinitely in the soil or it can survive as a saprophyte on plant debris, so once a landscape or nursery bed is contaminated with the fungus, it is difficult to remove.

Plant only disease-free plants in the landscape. Sometimes diseased, but well-watered and fertilized, nursery-grown hollies or bedding plants will appear to be healthy but, after they are placed in the landscape, they may decline due to more stressful growing conditions. This means it is very important to examine root systems prior to planting. If blackened roots are evident, plants should be rejected.

Avoid planting susceptible plants in soils known to be infested with the fungus. While the fungus is widespread, it may be present in higher levels in soils where black root rot was previously a problem on other plants such as petunia or pansy. Occasionally, when old agricultural lands are developed for housing, homeowners may find they have also purchased a black root rot problem from a former tobacco or alfalfa field as well.

In the landscape, badly infected plants should be removed and the site replanted with a non-susceptible host.

There are no effective fungicide drenches available for controlling black root rot in the landscape. Steam pasteurization or chemical fumigation will eradicate the fungus from propagation and growing media in nurseries. The fungicide Medallion is registered for managing this disease in greenhouses.

Information from "Holly Black Root Rot is Active" By John Hartman in the current edition of the Kentucky Pest News http://www.uky.edu/Ag/kpn/kpn_09/pn_091027.html#Corn3