Phytophthora means plant destroyer, which gives an indication of how serious a problem this can be. There are about 40 species, but only one is important to boxwood, Phytophthora parasitica. This soil-borne fungus affects all cultivars of B. sempervirens at any age or size. It damages leaves, stems, and roots. Infection begins when the soil is wet and cool, about 58oF to 70oF (14oC to 21oC). This type of weather is common in the spring and fall. The disease will progress and cause injury only under higher soil temperatures. Damage occurs at 75oF (24oC), with the greatest effects occurring at 85oF (29oC). Boxwood infected with Phytophthora seldom survive. Phytophthora produces spores that move in a water film in the soil. As a result, Phytophthora is most damaging to boxwood growing in poorly drained soils.
What does it look like?
The leaves on an infected boxwood will first gradually turn yellow and then the edges will become wavy. Next the leaves will change to a bright straw color and remain attached to the twigs. This may happen to one branch, or several, or throughout the entire shrub simultaneously. The stem will have a dark brownish-black coloration of the vascular tissues under the bark at ground level to a few inches (7 cm to 18 cm) above. The dark color is a response of the vascular tissue to the fungus. The fungus can cause a partial, or even complete, blockage of nutrient and water movement in the stem. The roots will appear dull and dark brown in color. This is in contrast to healthy roots that will have a bright light tan color. The “bark” will be decayed and is easy to remove.
How can I correct the problem?
Avoid the initial infection:
adequate drainage and avoid sites with extended periods of excessive
moisture, particularly in the spring and fall. Avoid over watering boxwood
planted in the ground. For boxwood in pots, the container should not be
reused. New containers should be filled with a growing medium that drains well.
Boxwood weakened by Phytophthora:
Transplant the boxwood to a site that is well drained. If transplanting is not
practical, consider drains and grade changes to redirect water away from
the plant. Lifting smaller boxwood, even only one or two inches
(2.5 cm to 5 cm), may provide effective relief.
Boxwood killed by Phytophthora:
The boxwood plant, including the roots, should be removed from the site. The area should not be replanted until soil drainage is improved.
Cylindrocladium buxicola, boxwood blight, is a relatively new fungal disease of boxwood, first discovered in Great Britain in 1998. Spreading quickly through southern England, by 2001 it had spread to Ireland, Belgium, Italy, France, Holland and surprisingly, New Zealand. In 2004 it was identified in the United States near Philadelphia and has quickly spread to other regions.
As with all other fungal boxwood diseases, Cylindrocladium buxicola prefers high humidity, moderate temperatures, 77oF (25oC) and poor air circulation. This disease has proved particularly acute in England where boxwood is tightly and regularly sheared, resulting in dense foliage, and planted too close to other plants, both of which result in poor air circulation. This boxwood blight is primarily limited to Buxus sempervirens ‘Suffruticosa’.
What does it look like?
Cylindrocladium infects a broad range of plants in both nursery stock and landscape sites, typically causing root and crown rots. Cylindrocladium buxicola, however, is a leaf and twig blight. As it begins to infect the leaf it produces visible brown spots, the mycelium. They rapidly enlarge and coalesce, causing defoliation, and form black streaks on the bark. The black streaks appear to progress from the bottom to the top of plants. Patches of whitish or grayish fungus may be visible on the lower leaf surface under moist conditions.
Symptoms usually appear during the autumn or winter season, producing their damage to the twigs in spring. While only severe injury has been observed, chronic infection can result in a severe weakening, contributing to death of the boxwood.
What can I do about Cylindrocladium?
Like Macrophoma and Volutella, the most effective control for Cylindrocladium is avoiding the initial infection. The single most important technique to avoid and correct this blight is to encourage free circulation of air through the interior portions of the boxwood. Annual thinning of the dense foliage in fall will greatly improve the air circulation, providing an unfavorable site for the blight. Boxwood grown in sunny locations have more dense foliage, requiring more frequent and more aggressive thinning. Because shearing Buxus sempervirens ‘Suffruticosa’ produces dense foliage, it encourages the development of Cylindrocladium and is not recommended. Sanitation is an important preventative measure as infected foliage in the debris can cause reinfestation.
Improving the air circulation around the exterior portions of the boxwood is also important. Boxwood plants growing together in a hedge or specimens touching each other are two common situations that can limit air circulation. Even one boxwood planted on the inside corner of, or next to, a building, fence, or hedge can have poor air circulation. In these examples, reducing the overall density of the planting will help.
Macrophoma Leaf Spot
Macrophoma candollei, an imperfect fungus, attacks weakened or decaying branches of many cultivars of B. sempervirens, especially ‘Suffruticosa’. Fortunately, Macrophoma is only of minor concern as it is classified as a secondary invader or a weak parasite. Usually the fungus infects plants that have been weakened by poor culture, winterburn, or overly-thick foliage.
What does this leaf spot look like?
On close inspection, tiny black raised spots can be found on the upper or underside of the light green or, more commonly, tan-colored foliage. These are the round hollow fruiting bodies, or pycnidia, of the fungi. The large transparent single-celled conidia are produced in large quantities and ooze out of the pycnidium when placed in water. Water is the primary means of dispersal and movement for fungi. The fruiting bodies first appear on the oldest leaves inside the center of the boxwood. As the infestation progresses, it affects the younger leaves which will defoliate. Heavy infestations can cause entire branches to die in only a few weeks.
What can I do about Macrophoma?
Macrophoma leaf spot can most often be controlled by pruning out the infected branches when the pycnidia appear. In addition to pruning out the fungus, thinning the healthy overly dense foliage will improve air circulation through the plant, providing excellent results in both containing and eliminating the fungus. The fungus prefers a moist, cool, dark area, which can be found in the center of boxwood with overly dense foliage. The real secret of control is not to let the Macrophoma leaf spot get started. Preventative measures are always far easier to perform than attempting to heal an infected plant. Annual thinning of the dense foliage in fall will permit both air and sunlight to circulate freely through the plant, providing an unfavorable site for the fungus to establish itself. Boxwood grown in sunny locations are more likely to develop overly thick foliage. Sanitation is an additional preventative measure. Collecting and removing infected foliage from the site will help avoid a re-infection of Macrophoma.
Nematodes are small eel-like worms that cause plant disease. Nematodes live as an obligate parasite that can only grow and multiply when on a living host. However, they can live as an egg or cyst for several years without the benefit of a host. Nematodes are an important pathogen of boxwood, especially in the southeastern regions of the U.S. There are thousands of nematode species, but only two are important to boxwood.
How do I know if the boxwood have nematodes?
Nematodes feed on the roots of boxwood. The first visible symptoms, which occur on the leaves, are: wilting, stunting, yellowing or bronzing, and reduced vigor. Symptoms appear fairly uniformly throughout the plant, not branch-by-branch like Volutella.
If boxwood are growing in proper cultural conditions and still exhibit these symptoms, then nematodes are suspect. The only reliable method of determining the presence of nematodes is to collect soil and root samples from a suspect boxwood and have the sample examined by a plant nematologist.
Which are important to boxwood?
Lesion or Meadow nematodes, Pratylenchus vulnus, are most destructive on B. sempervirens cultivars. These nematodes prefer sandy and loam soils, feeding on the smallest roots, which has a root-pruning effect. This may be followed by root rotting from a secondary infestation of pathogens. Above-ground damage is slow to appear, but results in smaller leaves that are yellow or, more often, reddish. The branches may keep only one or two years of foliage.
Root knot nematodes, Meloidogyne species, prefer B. microphylla var. japonica (Japanese boxwood). These nematodes cause swelling of the roots. Their gall formation stops root growth, which reduces the availability of nutrients and water. The plant will be stunted with reduced shoot growth and small yellowish or bronze-colored leaves. Root symptoms include swollen galls throughout. The galls are typically white, firm, and round to irregularly elongated. Older galls deteriorate, leaving discolored areas on the root.
How are nematodes controlled?
Nematodes cannot be totally eliminated from the landscape. Solarization of the soil prior to planting is effective. The goal is to keep the population low enough to prevent damaging symptoms that weaken the plant. Boxwood should not be grown in soils heavily infested with nematodes. In the long term, growing plants such as grasses that are not affected by nematodes will reduce nematode populations.
For Pratylenchus vulnus, the best control option is strict sanitation. That is, nematode-free potting soil and placing containers on a ground cloth or gravel. Nematicides are ineffective.
For Meloidogyne species, maintaining clean tools and potting soils as well as avoiding reuse of pots is effective. A biological control, Bacillus penetrans, has been effective when treating the soil for root knot nematodes.
This fungus, Volutella buxi, is a stem blight or canker. It primarily affects B. sempervirens. It is the most serious disease to affect mature and overly thick B. sempervirens ‘Suffruticosa’. It is particularly severe in periods of high humidity. A heavy infection results in defoliation and the death of entire branches.
What does it look like?
In moist weather, a mass of creamy or light pink dust or fuzz like growth is visible on the underside of the leaf. These colorful spores are very distinctive. This stage is often overlooked as it forms in the densest portions of the plant. The pink spore masses may not be visible under dry conditions. As the infection continues, the outer green leaves will become quickly and progressively discolored, changing to a dark brown and then tan color. The branch appears and smells almost as if diesel fuel had been poured on it. As the disease progresses, the entire branch will drop all of its leaves. The Volutella will usually cause the soft tissue of the current years growth to discolor black. If the plant is weak, the discoloration will extend well into the previous years growth, producing a stem canker. A canker is formed from a wound or a dead, discolored area that often sinks beneath the bark on the stem. On the trunk and large branches, the healthy tissue immediately next to the canker may slightly increase in thickness and appear higher than the normal surface. Cankers will produce a crack on the surface of the bark that may be several inches (5 cm to 13 cm) in length. These cracks cause wilting and death to the parts of the branch beyond the canker.
How can I correct this problem?
First, be sure that the problem is Volutella buxi. If the pink masses of pycnidia are not visible, they easily can be cultured. To do this, prune a small twig or take several leaves from a portion of the plant that appears to have Volutella symptoms. Wet the sample with ordinary water. Place it in a zip-lock plastic bag and keep the sample in a cool area. After a week or two, fuzzy pink colonies will develop if it is Volutella..
Avoid the initial infection
Do not water boxwood with turf-type spray heads, oscillators, or other methods that wet the foliage. Frequent rain or overhead watering encourages establishment of Volutella. Maintaining good air circulation through the interior portions allows the plant to dry quickly. Keep the boxwood in vigorous health by following good cultural practices.
Control established infections
Prune the diseased branch 6" to 12" (15 cm to 30 cm) below the affected tissue. This is best done during dry weather to prevent spreading the pycnidia. Removing the diseased branch from the site will provide nearly total control.
While the pathogen species is a topic of much debate, Paecilomyces buxi (formerly Verticillium buxi) is presumed responsible for boxwood decline. Decline is thought to be the result of fungi and perhaps nematodes that invade the root and crown portions of boxwood that are culturally weak. There does appear to be a complex of several fungi, parasitic nematodes, and environmental and cultural factors associated with decline. Their specific interaction with each other and their association with the decline is not clearly understood. Decline is limited to B. sempervirens ‘Suffruticosa’.
The distribution of decline is limited to North America, extending from New York to North Carolina and from the east coast shore to the Appalachian Mountains. While numerous cases of decline were reported in the 1970s, there have been few since.
What does decline look like?
The deepest roots are affected first. As the disease progresses, the stem below the ground begins to turn brown. This will extend upward into branches, often in a random pattern. The above-ground symptoms take the form of small leaves that are brittle and yellow or red, as well as characteristic browning of one branch or several branches in a random fashion. In severe cases, there is a sudden wilting of foliage or dieback of entire branches, resulting in the death of the boxwood.
How can I correct the problem?
Boxwood in poor health are susceptible to decline. There is no tested, proven treatment for boxwood infected with decline. Initial infections can be avoided by keeping B. sempervirens ‘Suffruticosa’ culturally healthy. Improving cultural conditions, including soil drainage and thinning, have been suggested controls. Removal of the plant has often been necessary.