Preventing Soil Inoculum of Calonectria pseudonaviculata from Splashing onto Healthy Boxwood Foliage by Mulching

Multi-Year Field Plantings Evaluating Boxwood Cultivars for Susceptibility to the Blight Pathogens (Calonectria spp.) in Northern Germany

Two multiyear field trials were conducted to evaluate boxwood cultivars for their susceptibility to the blight pathogens Calonectria pseudonaviculata and C. henricotiae in northern Germany. Fifteen cultivars were included in the first trial from 2007 to 2012, and 46 cultivars were included in the second trial from 2014 to 2017. Both trials were done in a naturally infested field that was supplemented with infected plant tissue added to the soil before planting. Each cultivar had three replicate hedge sections with 10 plants per section, and they were assessed annually for blight severity expressed as proportion of leaves blighted and fallen. Blight severity varied significantly among years (P < 0.0001) and cultivars (P < 0.05) within each trial. In the first trial, mean severity ranged from 0.03 to 0.11 for the most resistant cultivars and 0.35 to 0.96 for the most susceptible ones. Similarly, in the second trial, mean severity ranged from 0.06 to 0.27 and 0.71 to 0.97 for the most resistant and susceptible cultivars, respectively. 'Suffruticosa' was consistently the most susceptible cultivar, followed by 'Marianne', 'Myosotidifolia', 'Raket', and 'Morris Midget'. 'Herrenhausen' was the most resistant cultivar, followed by B. microphylla var. japonica, B. microphylla var. koreana, 'Green Mound', 'Faulkner', and 'Winter Beauty'. This study provides field data showing the performance of boxwood cultivars under different levels of disease pressure in an area where C. henricotiae was dominant. This knowledge will help boxwood growers and gardeners to choose less susceptible cultivars and help plant breeders to select for disease resistance.

Rapid Decline of Calonectria pseudonaviculata Soil Population in Selected Gardens Across the United States

Calonectria pseudonaviculata (Cps) poses a serious threat to boxwood, an iconic landscape plant in American and European gardens. Under the mild climatic conditions of the United Kingdom, Cps remained recoverable in infected leaf debris after being left on the soil surface or buried for 5 years. The primary objective of this study was to determine how this fungus may be affected by the warmer summers and colder winters in the United States by sampling and baiting soil with boxwood cuttings and by on-site testing with sentinel plants. Soil sampling started in a Virginia garden in January 2016 and was extended to California, Illinois, New York, and South Carolina in early summer of 2017 through late fall of 2018. The Cps soil population as measured by the percentage of infected bait leaves declined sharply within the first year of blighted boxwood removal and fell to an almost undetectable level at the end of this study. To validate these baiting results, the Virginia garden was tested on site four times with container-grown boxwood plants while the South Carolina garden and three New York gardens were tested once. Each test began with sentinel plants set out for field exposure, followed by evaluation on site and then in the laboratory after plants were retrieved from these gardens and incubated under conducive environments for 2 weeks. Cps was not observed on any sentinel boxwood plant on site or in the laboratory with one exception. These observations indicate that Cps did not survive in the United States garden soil over time as well as it did in the United Kingdom. These results have important practical implications while challenging the notion that fungi producing microsclerotia will always survive in the soil for many years.

Impact of Calonectria Diseases on Ornamental Horticulture: Diagnosis and Control Strategies

Diseases caused by fungi in the genus Calonectria pose a significant threat to the ornamental horticulture industries in Europe and the United States. Calonectria spp. are particularly challenging pathogens to manage in ornamental production systems and the urban landscape for multiple reasons. A high level of species diversity and poorly resolved taxonomy in the genus makes proper pathogen identification and disease diagnosis a challenge, though recent molecular phylogenetic studies have made significant advances in species delimitation. From a disease management perspective, Calonectria spp. produce long-lived survival structures (microsclerotia) that contaminate nursery production systems and can survive multiple years in the absence of a susceptible plant host. Latent infection of plant material is poorly understood but likely contributes to long-distance dissemination of these fungal pathogens, including the clonal Calonectria spp. responsible for the global emergence of boxwood blight. Breeding for disease resistance represents a sustainable strategy for managing Calonectria diseases but is challenging due to the perennial nature of many ornamental plants and high levels of susceptibility in commercial cultivars. Ultimately, long-term sustainable management of Calonectria diseases will require an improved understanding of pathogen biology as well as integration of multiple disease management strategies.

Differential Tolerance to Calonectria pseudonaviculata of English Boxwood Plants Associated with the Complexity of Culturable Fungal and Bacterial Endophyte Communities

Isolated boxwood endophytes have been demonstrated to effectively protect boxwood plants from infection by Calonectria pseudonaviculata (Cps). However, the roles of endophytes as communities in plant defense are not clear. Here, we demonstrated differential tolerance to Cps of English boxwood (Buxus sempervirens 'Suffruticosa'), an iconic landscape plant and generally regarded as highly susceptible, and its link to endophyte complexity. Fifteen boxwood twig samples were collected in triplicates from three historic gardens-Colonial Williamsburg, George Washington's Mount Vernon and River Farm, and Virginia Tech's research farm in Virginia Beach in the summer and fall of 2019. A portion of individual samples was inoculated with Cps under controlled conditions. Significant differences in disease severity were observed among samples but not between the two seasons. Examining the endophyte cultures of the summer samples revealed that bacterial and fungal abundance was negatively and positively correlated with the disease severity. Nanopore metagenomics analysis on genomic DNA of the tolerant and susceptible group representatives confirmed the associations. Specifically, tolerant English boxwood plants had an endophyte community dominated by Bacilli and Betaproteobacteria, while susceptible ones had a distinct endophyte community dominated by Gammaproteobacteria, Actinobacteria, and diverse fungi. These findings may lead to boxwood health management innovations-devising and utilizing cultural practices to manipulate and increase the abundance and performance of beneficial endophytes for enhanced boxwood resistance to Cps.

Indications of Susceptibility to Calonectria pseudonaviculata in Some Common Groundcovers and Boxwood Companion Plants

Knowing the host range of a pathogen is critical to developing and implementing effective disease management programs. Calonectria pseudonaviculata (Cps) is known to attack a number of species, varieties, and cultivars in the genus Buxus as well as three Pachysandra species (Pachysandra terminalis, Pachysandra procumbens, and Pachysandra axillaris) and several Sarcococca species, all in the Buxaceae family. The objective of this study was to evaluate non-Buxaceae groundcovers and companion plants commonly associated with boxwood plantings for their susceptibility to Cps. Twenty-seven plant species belonging to 21 families were exposed to different levels of inoculum: 50 to 300 conidia per drop for detached leaf assays and 30,000 to 120,000 conidia per 1 ml for whole-plant assays. Inoculated plants were incubated in humid environments for at least 48 h to facilitate infection. Cps infection and sporulation were observed on 12 plant species: Alchemilla mollis, Arctostaphylos uva-ursi, Brunnera macrophylla, Epimedium × youngianum, Galium odoratum, Geranium sanguineum, Phlox subulata, Tiarella cordifolia, Callirhoe involucrata, Iberis sempervirens, Mazus reptans, and Vinca minor. These results suggest that there may be more hosts of Cps commonly grown in nurseries and landscapes. If corroborated by observations of natural infection, these findings have implications for the Boxwood Blight Cleanliness Program instituted by the National Plant Board and for planning disease mitigation at production and in the landscape.

A Potent Burkholderia Endophyte against Boxwood Blight Caused by Calonectria pseudonaviculata

Calonectria pseudonaviculata (Cps) poses an increasing threat to boxwood, a major nursery crop and iconic landscape plant worldwide. Here, we report on a potent biocontrol agent that produces small sage green (SSG) colonies on potato dextrose agar. SSG is a bacterial strain recovered from Justin Brouwers boxwood leaves with unusual response to Cps inoculation. Water-soaked symptoms developed on leaves 2 days after inoculation then disappeared a few days later. This endophyte affected several major steps of the boxwood blight disease cycle. SSG at 10⁷ cfu/mL lysed all conidia in mixed broth culture. SSG at 10⁸ cfu/mL reduced blight incidence by >98% when applied one day before or 3 h after boxwood were inoculated with Cps. Its control efficacy decreased with decreasing bacterial concentration to 10³ cfu/mL and increasing lead time up to 20 days. When applied on diseased leaf litter under boxwood plants, SSG reduced Cps sporulation and consequently mitigated blight incidence by 90%. SSG was identified as a new member of the Burkholderia cepacia complex with distinct characters from known clinical strains. With these protective, curative, and sanitizing properties, this Burkholderia endophyte offers great promise for sustainable blight management at production and in the landscape.

Boxwood Blight: Threat to Ornamentals

Boxwood blight, caused by Calonectria pseudonaviculata and Calonectria henricotiae, has had devastating effects in gardens since its first appearance in the United Kingdom in 1994. The disease affects two other plants in the Buxaceae: sweet box (Sarcococca spp.) and pachysandra (Pachysandra spp.). C. pseudonaviculata was likely introduced to Europe by nursery trade from East Asia on an ornamental species and then to western Asia and North America. Thus far, C. henricotiae has been seen only in Europe. Boxwood, valued at $126 million wholesale per year in the United States alone, is now besieged by an aggressive foliar blight active over a broad temperature range when there are long periods of leaf wetness. Research on inoculum, means of dissemination, cultivar susceptibility, environmental influences, fungicides, sanitizers, and detection methods has vastly improved knowledge of this new invasive disease in a short time. Boxwood with genetic resistance to the disease is critically needed.