Boxwood (Buxus spp.) is one of the most common woody ornamentals on the landscape in New England. Unfortunately, they harbor and array of fungal pathogens with some capable of causing severe damage or even death. In addition to established pathogens of boxwood, emerging pathogens have been recently described and warrant mention as we advance towards the 2016 growing season. Boxwood is appealing to many as a landscape ornamental because of its evergreen nature, tolerance to pruning and compact size. However, because boxwoods are so regularly pruned, the resulting wounds are often colonized by stem cankering and foliar pathogens. In addition, the dense canopies and small foliage allows many pathogens to linger even when they are directly managed.
Volutella leaf and stem blight, caused by Volutella buxi (also known as Pseudonectria buxi), is perhaps the most common pathogen of ornamental boxwood. This fungus creates numerous foliar lesions that often coalesce, leading to a partial or complete defoliation. Volutella also attacks woody plant parts, causing destructive cankers that can girdle shoots. Salmon-colored pads of fungal tissue, that represent large masses of spores, can often be observed on the underside of infected leaves and on blighted shoots. The disease primarily affects shrubs predisposed to stress, but in certain cases can be found on otherwise healthy plants. Wet, humid weather is conducive to disease development and whenever weather conditions are favorable the pathogen will grow and reproduce. Pruning wounds are a common source of establishment for Volutella and the pathogen can be readily spread through contaminated pruning tools. A combination of chemical and cultural control is usually required to control this disease. Fungicides labeled for use against Volutella include: copper-based products*, propiconazole*, thiophanate-methyl*, chlorothalonil* and mancozeb*. Pruning and removal of dead plant material is very important to control this disease. Dead plant parts harbor the pathogen and allow it to disseminate spores in very close proximity to healthy tissues.
Boxwood blight, caused by Calonectria pseudonaviculata, is an aggressive disease of ornamental boxwood but is rare in the landscape in Massachusetts. Boxwood blight was first reported in southern New England in the fall of 2011. Prior to this time, the pathogen was first described in the U.K. in the mid-1990s and is present throughout Europe. Aside from New England, Calonectria is now widespread throughout the eastern United States. Several boxwood samples submitted to the UMass Plant Diagnostic Lab in 2014 tested positive for boxwood blight. All of these samples either came from recently transplanted (six months or less) boxwoods in suburban landscapes or from nursery stock that had not yet been sold. No positive samples were received in 2015. Boxwood blight has been reported on all species of Buxus, but the popular B. sempervirens and B. sempervirens ‘Suffruticosa’ are especially susceptible to infection. Recent research has found that Korean boxwood (Buxus sinica var. insularis ‘Wintergreen’) has some resistance to the pathogen. The fungus is able to attack all above-ground plant parts, but does not infect the roots. It can, however, produce a resting structure that allows it to survive in dead plant parts that have fallen to the ground and become incorporated into the organic layer. Spores are then produced from the dead material and blown or splashed onto nearby, healthy plants. Therefore, if diseased plants are removed, so must all fallen debris (leaves and stems) at the site. Symptoms first appear as scattered leaf and branch dieback and total collapse of the plant can occur rapidly after introduction. Black, circular to zonate leaf spots are the most distinguishing symptom, along with a grey-colored, fuzzy growth of fungal mycelium on leaf and stem tissue. Several fungicides have been proven effective against Calonectria and include: propiconazole*, myclobutanil*, thiophanate-methyl*, fludioxonil*, pyraclostrobin*, kresoxim-methyl*, and chlorothalonil*. However, the best course of action is removal and destruction of infected plants, as the pathogen spreads very quickly to nearby, susceptible boxwoods.
Two emerging pathogens of boxwood have been recently identified and while they have not yet been detected in Massachusetts, they may be readily spread through infected nursery stock. The first, Phytophthora occultans, causes root and stem lesions on infected plants. This species was first described from the Netherlands but has since been discovered at multiple nurseries in Oregon. Phytophthora species do not produce any signs that are visible to the naked eye. These pathogens thrive under flooded conditions and management of any Phytophthora species must address drainage issues if soils are heavy or if water pools at the planting site. The second, Colletotrichum theombromicola, causes scattered branch dieback and browning of the foliage. When the bark is removed from infected stems, blackened lesions are visible in the sapwood. These lesions are similar in appearance to those caused by Phytophthora, making diagnosis difficult based on symptoms alone. Colletotrichum species often produce pink-colored masses of spores on infected plant parts during periods of humid and wet weather. To date, the disease has been reported from several eastern states, including New York.
Spacing is critical when planting boxwoods. Too often, boxwoods are planted too close together and do not have adequate space to establish and thrive. When stressed, they can become disease and pathogens readily spread to adjacent plants. Only purchase nursery stock that is free of any symptoms of dieback. Avoid excessive pruning and sharpen and sanitize pruning equipment regularly. While supplemental water should be provided after shrubs are transplanted, avoid wetting the canopy with overhead irrigation, as this creates conditions conducive to disease development.
Nicholas J. Brazee
Extension Plant Pathologist
2/5/2016
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