Holly Diseases and Their Control in the Landscape
Ornamental Disease Note No. 7
R.K. Jones, Plant Pathologist (retired)
Hollies are some of the most commonly used plants in the landscape in North Carolina. Japanese holly (Ilex crenata) cultivars such as 'Helleri', 'Rotundifolia', and 'Convexa' are the most widely planted, but unfortunately they have the most disease problems of all hollies.
Black Root Rot
Black root rot, caused by the fungus Thielaviopsis basicola, was first reported on Japanese holly in North Carolina in 1977. The disease has since been found in numerous other locations, both in nursery and landscape plantings throughout the state. This disease has the potential of causing extensive economic and aesthetic losses to this important evergreen shrub in both landscapes and nurseries.
The black root rot fungus has the ability to persist in the soil for many years, even in the absence of susceptible plants. Development of black root rot is favored by high soil moisture and low soil temperatures. 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.
Symptoms and Control
The black root rot 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 due to the presence of the fungus growing in and on the root tissue and to 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 have the ability to 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. The use of raised beds in landscape plantings to provide good drainage and avoidance of poorly drained planting sites may help to reduce losses due to black root rot.
1. Resistance levels for some holly species.
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. For example, root-knot nematodes cause swellings or galls on roots of susceptible plants. Other types of nematodes cause plant roots to be stubby and branch abnormally. 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 (Table 2).
Damage to plants from these root-feeding 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 because of root damage 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. A laboratory examination may be necessary to determine the cause of plant decline. For laboratory examination, collect at least 1 pint of soil, plus some small fibrous roots, from several spots beneath affected plants and place in a plastic bag be sure to collect this sample from declining, but still living plants. The soil plus roots and some of the affected stems and leaves should be taken to your county extension office.
Nematode Control in the Landscape
Presently, there are no effective chemicals registered for control of nematodes on existing landscape plants. Control of nematodes in the landscape must be achieved by careful planning before planting.
For existing landscape plants with nematode problems, it may help to mulch the plants, to apply adequate water during dry periods, to fertilize and lime properly, and to prune out any dead branches. Plants already showing advanced stages of decline due to nematodes - (50% or more of above ground portions of plant) should be replaced with less susceptible shrubs or turf.
For new landscape plantings or replanting, nematode problems must be avoided rather than attempting to correct after planting. Where possible, it is best to avoid using highly susceptible plants (Table 2) on sites where damaging nematodes are known to occur. It is also very important to purchase plants free of damaging nematodes.
To determine if damaging nematodes are present in the soil before planting, collect a representative soil sample (1 quart) in a plastic bag from the area to be planted and take it to your county extension office. Check with your county extension agent for more details on the procedures, forms, fees, etc.
If damaging nematodes are known to occur in the planting site and highly susceptible plants must be used, the entire area may be treated with methyl bromide (restricted use pesticide), or SMDC (Vapam) before planting. These chemicals cannot be used between existing plants in a bed. Chemical treatment of planting sites is usually performed by trained professional applicators.
2. Response of Selected Woody Ornamentals to Nematodes.
HS - Plant highly susceptible (severe stunting, branch dieback and death); S - Plants susceptible (some stunting); T - Tolerant plants, will grow satisfactorily; O - Have not been tested
Diseases of Minor Importance
Several leaf spotting fungi have been reported on hollies, but they seldom cause damage on otherwise healthy plants. Leaf spot diseases are more common on American holly, but usually appear during the winter and spring on old leaves. The damage is usually not significant even though heavy defoliation may occur before new growth starts in the spring.
Phytophthora root rot, caused by the water mold fungus Phytophthora cinnamomi, is reported on holly, but only occurs on very poorly drained sites or wet areas on young Japanese holly primarily during nursery propagation. Avoid planting Japanese holly in very poorly-drained sites. Root rot caused by the mushroom root rot fungus, Armillariella mellea, occurs on older holly plants under environmental stress. Affected plants die suddenly. In areas where this disease has occurred, replant with grass or a ground cover plant such as liriope, ajuga, or ivy.
holly, particularly 'Helleri', is not very tolerant to low soil moisture,
particularly for the first several years after transplanting.
For assistance with a specific problem, contact your local North Carolina Cooperative Extension Service
for the use of chemicals are included in this publication as a convenience
to the reader. The use of brand names and any mention or listing of commercial
products or services in this publication does not imply endorsement by
the North Carolina Cooperative Extension Service nor discrimination against
similar products or services not mentioned. Individuals who use chemicals
are responsible for ensuring that the intended use complies with current
regulations and conforms to the product label. Be sure to obtain current
information about usage and examine a current product label before applying
any chemical. For assistance, contact your county North Carolina Cooperative
Extension Service agent. 04/91/1000
Published by North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age, or disability. North Carolina State University at Raleigh, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Reformatted Dec. 2000 by Plant Disease and Insect Clinic