Phytophthora Root Rot
Both English (Buxus sempervirens cv. 'Suffruticosa') and American boxwood (B. sempervirens cv. 'Arborescens') are susceptible to this disease, which is caused by the fungus Phytophthora parasitica. The disease has also been observed in littleleaf boxwood (Buxus microphylla) in Virginia. Aboveground symptoms include poor growth and off-color foliage. Leaves are at first light green and may turn yellow, bronze, or straw-colored. Leaves turn upward and lateral leaf margins roll inward. Leaf symptoms may appear on just a few branches or on the entire plant, depending on the extent of infection of the roots. Usually, the bark at the base of the infected plant dies and can be easily separated from the wood. By the time foliar symptoms are observed, roots are few in number and many are brown in color. The lack of functioning roots precedes the yellowing and death of the top of the plant.
Plants growing in soils that have become water-logged following overwatering or heavy rains in summer are predisposed to infection by Phytophthora parasitica. Abundant moisture allows motile spores of the fungus to move in the soil, infecting new roots on the same or adjacent plants. New plantings should always be made with healthy appearing plants in well-drained soil. Avoid planting a susceptible plant in infested soil unless drainage can be improved prior to planting. Planting on raised beds may help improve drainage around plants.
Soil drench fungicides can be used to prevent disease on plants that do not yet have severe root rot. After severely affected plants are removed, soil around remaining plants or new transplants can be drenched with mefenoxam (e.g. Subdue MAXX), metalaxyl (e.g. Subdue), fosetyl-Al (e.g. Aliette), or etridiazole + thiophanate methyl (e.g. Banrot) at 4-week intervals. Consult the product label or the current Virginia Pest Management Guide for Horticultural and Forest Crops (VCE Publication 456-017), http://www.ext.vt.edu/pubs/pmg/, for details on application rates. For information on the proper use of pesticides and fungicides, refer to any current VCE pest management guide.
English Boxwood Decline
The disease called English boxwood decline can best be described as a slow but progressive decline occurring commonly in large plants 20 years or more in age. Decline symptoms resemble those of root rot caused by Phytophthora parasitica. However, Phytophthora root rot is primarily a problem in wet soils, whereas English boxwood decline often follows drought stress. A complex of fungi has been associated with English boxwood decline, but the fungus Paecilomyces buxi is believed to be the primary pathogen. Plant parasitic nematodes have also been recovered from the roots of dying plants, but not consistently enough to explain the disease.
External and internal stem discoloration usually accompanies the root rot phase of the disease. Plants dying from decline have vascular discoloration well up the main stem. The discoloration may be continuous or discontinuous in the stem. Sections of the foliage of infected plants turn a light green color. Later, foliage of infected plants turns yellow and then straw-colored. By the time foliar symptoms are observed, the root system has been severely impaired by root rot.
Every effort should be made to maintain plants in a high state
of vigor. Because drought stress is thought to be one of the main factors that
predisposes plants to disease, it is especially important to water plants deeply
and regularly during drought. Do not replant infested areas to English boxwood.
Both American boxwood and several cultivars of Buxus microphylla, tested
under field conditions, have been observed to be resistant to decline in field
tests and can be planted in areas where English boxwood decline has been
diagnosed. No fungicides have been found to be effective in controlling this
Damage to roots of both American and English boxwoods can occur from the feeding of several types of plant parasitic nematodes. The most common nematodes that feed on boxwood roots in Virginia are ring, lesion, and spiral nematodes. These nematodes obtain nutrients by inserting their syringe-like mouthparts into root cells and removing the contents. When populations of any of these nematodes are high in soil, their feeding can cause severe damage to roots and may also predispose roots to infection by fungi. Symptoms on roots include stunting and browning. Aboveground symptoms resemble those caused by root rot fungi: the plant undergoes a gradual decline characterized by yellowing and bronzing of the leaves and dieback of large sections of the plant.
There are no effective controls for nematodes on established ornamental plants. Providing good growing conditions, e.g. fertilizing and watering as necessary and providing good drainage, allows plants to tolerate some root injury due to nematode feeding. Adding organic matter to the soil can improve soil characteristics and encourage a diversity of microorganisms, some of which will be antagonistic to nematodes.
Chemical control involves use of soil fumigants, such as metam-sodium (e.g. Vapam) or dazomet (e.g. Basamid), which are phytotoxic and must be applied to soil before planting. Both of these materials must be applied under carefully monitored soil conditions in order to be effective. Vapam is a restricted-use pesticide and must be applied by a licensed applicator. Refer to the product label for detailed instructions on use.
A new biological control product called Deny is reported to control certain plant-feeding nematodes, including lesion and spiral nematodes. This product consists of a bacterium that is antagonistic to certain plant parasitic nematodes and fungi. Although specific studies with boxwood have not been conducted, Deny has a broad label and is registered for use on all ornamentals, as well as other crops, and may be an option for protection of new transplants where lesion or spiral nematode damage has been diagnosed. Research on other potential biological control agents is ongoing.
Foliar and Stem Diseases
Volutella Stem Blight
For several years, the role of the fungus Volutella buxi in the decline of boxwood has been open to question. This fungus is asso-ciated with wilt and canker, but its role as a primary pathogen has not been clearly established. Both English and American boxwood have been found to develop symptoms of Volutella stem blight.
In the spring, before the new growth appears, leaves on the tips of twigs turn orange or bronze, then straw-colored. Infected twigs die back for some distance. A dark brown to black canker is easily discernible after cutting bark away with a sharp knife. The Volutella fungus produces numerous clusters of conidia, which appear pink en masse. Winter injury causes foliar symptoms similar to those caused by Volutella buxi, and Volutella infection often follows winter or frost injury. If winter injury alone is the problem, new, healthy leaves will appear in spring and eventually hide the bronze-colored leaves.
Prune out infected branches back to healthy tissue. Discard all prunings. No fungicides have been shown to be effective in controlling this disease.
Macrophoma Leaf Spot
Boxwood leaves that die as a result of various root diseases or environmental stresses are frequently colonized by the fungus Macrophoma candollei. This fungus produces numerous black fruiting bodies, which can be seen as dark specks on dead leaves. It is a secondary colonizer of dead leaves and its presence indicates that the plant is stressed by some other factor. No controls for Macrophoma are recommmended; however, predisposing factors should be addressed.
Other Leaf Disorders
Two symptoms on boxwood leaves to which no cause has been attributed include a marginal yellowing on a normally green leaf, and an etching of leaves with scattered, sunken tan spots. These symptoms occur sporadically and do not appear to be a cause for concern.
Author: Mary Ann Hansen, Extension Plant Pathologist, Department of Plant Pathology, Physiology and Weed Science, Virginia Tech