BLACK ROOT ROT OF
HOLLY
Photo courtesy of the Dept. of Plant Pathology, North Carolina State
University
The fungus can spread through the soil as mycelium, but its primary means of dispersal is by spores which are carried by water, wind, equipment and infected transplants. The fungus primarily affects the root system and reduces plant vigor. In more advanced stages, above-ground symptoms may include stunting of terminal growth, shortening of internodes and interveinal chlorosis. Infected roots are dark brown to black, usually starting at the root tips and progressing to the larger roots. These dark areas are caused by the presence of the fungus growing in and on the root tissue and by the decay of root cells.
The fungus produces two types of spores, endoconidia, which can germinate immediately and cause infection, and chlamydospores, thick-walled resting spores which can persist in the soil for extended periods of time. The root system of infected plants can be greatly reduced by the rotting of the roots caused by this fungus. Diseased plants usually decline over a period of months and frequently die during or following dry periods.
In landscape plantings where losses of Japanese holly have resulted from the presence of the black root rot fungus, resistant types of holly or other shrubs should be planted to prevent additional losses to black root rot. Losses due to black root rot can be reduced by using raised beds to provide good draining and by avoiding poorly drained sites.
| Highly Susceptible | |
|---|---|
| Ilex crenata | (Japanese) |
| Ilex hybrid | (Blue holly) |
| Moderately Resistant | Ilex vomitoria | (yaupon) |
| Ilex opaca | (American) |
| Highly Resistant | Ilex cornuta | (Chinese) |
| Ilex aquifolium | (English) |
Nematodes. Nematodes are one of the most destructive groups of pests causing decline of established ornamental plants in North Carolina. These microscopic roundworms feed on plant roots, causing various types of damage. Nematode-damaged roots often are further destroyed by fungi and bacteria.
Several plant-parasitic nematodes such as root-knot, stunt, ring, sting, lance, lesion, stubby root, dagger and spiral have been associated with decline of ornamentals in North Carolina. Hollies, particularly Japanese holly cultivars, are damaged by root-knot nematodes. The response of several cultivars of holly to nematodes is listed above.
Plant damage from nematodes is progressive and often results in poor growth, low vigor, yellowing or bronzing of the foliage, loss of leaves, stem die-back, failure to respond to fertilizer, and eventually death.
Symptoms of nematode damage usually are most apparent during late summer and fall or during extended dry periods. Affected plants usually decline and die over a period of several years, but occasionally plants may die suddenly. Other problems such as root rot or too much fertilizer can cause symptoms similar to those produced by nematodes.
See Pest Identification: Diseases and Nematodes for a discussion of nematode control measures.
Phytophthora root rot. Phytophthora cinnamomi causes significant damage to Inkberry holly (Ilex glabra), but otherwise does not occur on young Japanese hollies unless plants have been installed in very poorly drained sites or wet areas.
Root rot caused by the mushroom root rot fungus, Armillariella mellea, occurs on older holly plants under environmental stress. Affected plants die suddenly. Areas where this disease has occurred should be replanted with grass or a ground cover plant such as liriope, ajuga, or ivy.
Drought. Japanese holly, particularly 'Helleri', is not very tolerant to low soil moisture, particularly for the first several years after transplanting.
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