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Boxwood blight, caused by Calonectria pseudonaviculata, is a highly invasive emerging disease. Since the first US report in North Carolina and Connecticut in 2011, boxwood blight has spread to over 30 US states, risking more than 90% of boxwood production. Our study investigated the disease field epidemiology and evaluated the boxwood blight infection risk model's prediction by analysing weekly blight monitoring data collected on detector plants exposed to the prevailing environmental conditions at two different locations (western Virginia and North Carolina) from spring through fall of 2014 to 2017. Boxwood blight was recorded in 61 of 86 weeks, with the highest infected leaf counts recorded in late summer or early fall. Rainfall, high relative humidity outside rainy periods and optimal temperatures (13.6–22.7 °C) during prolonged leaf wetness (> 65 h per week) had a significant positive effect on boxwood blight development. Classification analyses showed that disease predictions from the model using leaf wetness estimated by leaf wetness sensor were more closely aligned with observations from the field than predictions based on algorithms. This study improved our understanding of disease field epidemiology, provided leads to improve the existing model, and generated essential knowledge for formulating effective strategies for blight mitigation. [ABSTRACT FROM AUTHOR]

Boxwood blight is a highly invasive disease, but studies on host–pathogen–environment interactions are rare because the initial research emphasis has been on developing resistant cultivars and chemical control of the disease. We used generalized additive models to investigate weather variables driving boxwood blight epidemics in field trials conducted between 2006 and 2020. Briefly, three or four replicate rows with 10 boxwood plants per row were planted in 0.75 m2 plots. Plants were artificially inoculated in 2006, while those in subsequent years were naturally infected with inoculum left over from previous trials. Disease severity was assessed by estimating the percentage of leaves blighted, including fallen leaves. There was a significant positive main effect of mean rainfall per rainy day, daily minimum temperatures and daily minimum relative humidity on disease severity observed over individual field trial periods. There was a significant negative interaction effect of mean rainfall per rainy day and daily maximum wind speed, and daily minimum relative humidity and daily minimum temperature on disease severity. Higher disease severity was associated with higher mean rainfall per rainy day and lower daily maximum wind speed. Likewise, an increase in daily minimum relative humidity at lower daily minimum temperatures was associated with a greater increase in disease severity than at higher temperatures, suggesting that higher temperatures resulted in lower humidity that led, in turn, to less disease severity. The implications of our findings for forecasting models and conservation of boxwood are discussed. [ABSTRACT FROM AUTHOR]

Fungicide protection is the current approach for management of boxwood blight caused by Calonectria pseudonaviculata (Cps) and C. henricotiae (Che). However, published studies evaluating fungicides under field conditions have been focused on Cps. The objective of this study was to evaluate fungicides in Northern Germany where both Cps and Che were present. Three trials were conducted between 2006 and 2016. In 2006, plants were artificially inoculated with a conidial suspension, while infested soil and plant debris were added to a different field as inoculum for the 2012 trial and this field was used again without further addition of inoculum in 2015. Fungicides were applied one to five times and assessments were done three to six times, depending upon the trial. The highest level of disease severity occurred in 2015 (0.91), while the lowest occurred in 2012 (0.01). Among the fungicides evaluated in 2006, preventive sprays of Cercobin FL, Switch, Harvesan, Pugil 75 WG, Dithane NeoTec and Euparen M WG were most effective, with blight control above 65%. In 2012, all treatments including Askon, Cabrio Top, Malvin WG, Dithane NeoTec and Osiris showed > 75% blight control. In 2015, Bayer Rosen-Pilzfrei Baymat and Switch were the most effective (> 82%). Extended in-season blight control was also observed with some fungicides. Additionally, a few fungicides that were evaluated in more than 1 year showed reduced effectiveness over time. This study filled several major knowledge gaps especially regarding fungicide efficacy against Che under field conditions and thus provides crucial information for developing chemical control strategies. [ABSTRACT FROM AUTHOR]

Calonectria pseudonaviculata (Cps) infects Buxus (boxwood), Pachysandra (pachysandra), and Sarcococca spp. (sweet box); yet, how it adapts to its hosts has been unclear. Here, we performed serial passage experiments with the three hosts and measured Cps changes in three aggressiveness components: infectibility, lesion size, and conidial production. The detached leaves of individual hosts were inoculated with isolates (P0) from the originating host, followed by nine serial inoculations of new leaves of the same host with conidia from the infected leaves of the previous inoculation. All boxwood isolates maintained their capability of infection and lesion expansion through the 10 passages, whereas most non-boxwood isolates lost these abilities during the passages. Isolates from plants of origin (*-P0) and their descendants isolated from passages 5 (*-P5) and 10 (*-P10) were used to evaluate aggressiveness changes on all three hosts with cross-inoculation. While post-passage boxwood isolates gave enlarged lesions on pachysandra, sweet box P5 and pachysandra P10 isolates showed reduced aggressiveness on all hosts. Cps appears to be most adapted to boxwood and less adapted to sweet box and pachysandra. These results suggest speciation of Cps, with its coevolutionary pace with the hosts the fastest with boxwood, intermediate with sweet box, and the slowest with pachysandra. [ABSTRACT FROM AUTHOR]

In a recent study, we observed a rapid decline of the boxwood blight pathogen Calonectria pseudonaviculata (Cps) soil population in all surveyed gardens across the United States, and we speculated that these garden soils might be suppressive to Cps. This study aimed to characterize the soil bacterial community in these boxwood gardens. Soil samples were taken from one garden in California, Illinois, South Carolina, and Virginia and two in New York in early summer and late fall of 2017 and 2018. Soil DNA was extracted and its 16S rRNA amplicons were sequenced using the Nanopore MinION® platform. These garden soils were consistently dominated by Rhizobiales and Burkholderiales, regardless of garden location and sampling time. These two orders contain many species or strains capable of pathogen suppression and plant fitness improvement. Overall, 66 bacterial taxa were identified in this study that are known to have strains with biological control activity (BCA) against plant pathogens. Among the most abundant were Pseudomonas spp. and Bacillus spp., which may have contributed to the Cps decline in these garden soils. This study highlights the importance of soil microorganisms in plant health and provides a new perspective on garden disease management using the soil microbiome. [ABSTRACT FROM AUTHOR]

Simple Summary: Knowledge of where invasive species could potentially establish (potential distribution) is critical to prioritizing and addressing biological invasion threats. In this study, we predicted the potential distribution of Calonectriapseudonaviculata (Cps), an invasive fungal pathogen that blights boxwood, an iconic landscape plant, major evergreen nursery crop, and keystone forest species. We used climate data, presence records from Europe and western Asia, and multiple modeling approaches to predict the potential distribution of Cps at regional and global scales and to explore the roles of temperature and moisture in shaping its distribution. Model predictions were validated using an independent presence record dataset. A consensus map of model predictions revealed that Cps could potentially spread and establish well beyond its currently invaded range in Europe, western Asia, New Zealand, United States and Canada. These include a number of not-yet-invaded areas in eastern and southern Europe, North America, and many regions of the world where boxwood is native. This knowledge informs policymakers and other stakeholders in these areas on the need for implementing a strict phytosanitary protocol for risk mitigation of accidental introduction, having an effective surveillance for early detection, and developing a recovery plan for the pathogen when accidental introductions occur. Boxwood blight caused by Cps is an emerging disease that has had devastating impacts on Buxus spp. in the horticultural sector, landscapes, and native ecosystems. In this study, we produced a process-based climatic suitability model in the CLIMEX program and combined outputs of four different correlative modeling algorithms to generate an ensemble correlative model. All models were fit and validated using a presence record dataset comprised of Cps detections across its entire known invaded range. Evaluations of model performance provided validation of good model fit for all models. A consensus map of CLIMEX and ensemble correlative model predictions indicated that not-yet-invaded areas in eastern and southern Europe and in the southeastern, midwestern, and Pacific coast regions of North America are climatically suitable for Cps establishment. Most regions of the world where Buxus and its congeners are native are also at risk of establishment. These findings provide the first insights into Cps global invasion threat, suggesting that this invasive pathogen has the potential to significantly expand its range. [ABSTRACT FROM AUTHOR]

Calonectria pseudonaviculata causes lesions on boxwood leaves and twigs. Controlled‐environment experiments were conducted to determine the effects of temperature and leaf wetness period on C. pseudonaviculata sporulation on diseased (cv. Suffruticosa) leaves and of dryness periods and high temperature on conidial survival. Infected leaves were incubated in moist chambers and subjected to six temperatures (9, 13, 17, 21, 25, and 29°C) and six leaf wetness periods (0, 12, 24, 40, 48, and 72 h). Spore production was influenced significantly by wetness period, temperature, and their interaction. Increasing duration of leaf wetness and increasing temperature generally increased sporulation, with no sporulation occurring at 29°C or 9 and 13°C, except at 72 h of wetness exposure, while it was optimal at 21°C. Detached leaves with profuse conidia were subjected to a range of drying (relative humidity at 65%) times (0, 2, 4, 6, and 8 h) at two temperatures of 21 and 29°C. Conidia were then harvested and plated on water agar. Germinating conidia were counted to measure the spore viability. Spore mortality increased with increasing dryness duration at both temperatures but occurred more quickly and severely at 29 than 21°C. Overall, this study extended biological knowledge of conditions required for crucial stages of the C. pseudonaviculata disease cycle and the obtained results will be vital for developing boxwood blight forecasting and management tools. [ABSTRACT FROM AUTHOR]

Main conclusion: Boxwood leaves are more susceptible to Calonectria pseudonaviculata (Cps) and better suited for Cps reproduction than those of pachysandra and sweet box. Passages through a non-boxwood host may alter Cps ability to sporulate.Calonectria pseudonaviculata (Cps) infects boxwood and its two common companion plants—pachysandra and sweet box. This study investigated how boxwood, pachysandra, and sweet box respond to Cps isolates of different host origin. Detached leaves were inoculated with nine isolates, three from each host, and evaluated for colonization, infection rate, lesion size, and production of conidia and microsclerotia. Cps colonized boxwood leaf tissue within 12 h of inoculation, and 60 h ahead of pachysandra and sweet box. Cps also produced significantly larger lesions and more conidia on boxwood than on pachysandra and sweet box. Isolates originating from different host plants did not differ in all the components evaluated except for conidia production. Isolates from boxwood and sweet box produced significantly more conidia than those from pachysandra. Overall, boxwood leaves are more susceptible to the disease and are better suited for Cps reproduction than those of pachysandra and sweet box. Passages through a non-boxwood host may alter Cps ability to sporulate. These results advance the understanding of Cps biology and affirm the importance of taking pachysandra and sweet box into consideration in disease management planning. [ABSTRACT FROM AUTHOR]

Boxwood blight caused by Calonectria pseudonaviculata is an emerging destructive disease of great concern to horticulture and forest industries, public garden managers and homeowners across the globe. Current management strategies include use of less susceptible species and cultivars, chemical protection and sanitation practices. Here we report on isolation and identification of Trichoderma koningiopsis Mb2 from collapsing wild mushrooms for biocontrol of boxwood blight. The Trichoderma suppressed C. pseudonaviculata culture growth and controlled boxwood blight in a pre-treatment period dependent manner. Infection was reduced by 85% when Buxus sempervirens ‘Suffruticosa’ cuttings were challenged with the pathogen nine days after pre-treatment with Mb2. An extended interval of 18 or 36 days between pre-treatment and pathogen challenge was required to reduce disease incidence in containerized B. sinica var. insularis ‘Justin Brouwers’ plants by 54%–63%, respectively. Modes of action of Mb2 and the potential implications of these results are discussed. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]