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Background: Anti-desiccant is a class of agrochemicals widely used to protect plants from water stresses, rapid temperature variations, heat and sunburn, frost and freeze damages, transplant shock, and pathogen and pest attack. Although anti-desiccants are generally considered non-toxic to organisms, it is unclear whether they may impact the phyllosphere microbial communities. In this study, three film-forming anti-desiccant products, TransFilm, Vapor Gard, and Wilt-Pruf were applied to the canopy of two boxwood cultivars 'Vardar Valley' and 'Justin Brouwers' on April 13 and August 26, 2021. Shoot samples were collected from boxwood plants treated with each of the three products, as well as nontreated control on June 16, August 26 (before the second treatment), and October 18. Microbial and plant genomic DNA was isolated together and 16S rRNA gene and the extended internal transcribed spacer regions were amplified with PCR and sequenced on a Nanopore MinION platform for bacterial and fungal identification., Results: Bacterial communities were more diverse than fungal communities. At the phylum level, the boxwood phyllosphere was dominated by Proteobacteria and Ascomycota; at the genus level, Methylobacterium and Shiraia were the most abundant bacteria and fungi, respectively. Among the three film-forming anti-desiccants, Vapor Gard and Wilt-Pruf had more impact than TransFilm on the microbial communities. Specifically, broader impacts were observed on fungal than bacterial community composition and structure, with most affected fungi being suppressed while bacteria promoted., Conclusion: This study addressed several major knowledge gaps regarding boxwood phyllosphere microbiota and the impact of anti-desiccants on plant microbiome. We identified diverse microbial communities of boxwood, a major evergreen woody crop and an iconic landscape plant. We also found differential effects of three film-forming anti-desiccants on the composition and structure of bacterial and fungal communities. These findings advanced our understanding of the associated microbiome of this landmark plant, enabling growers to fully utilize the potentials of microbiome and three anti-desiccants in improving boxwood health and productivity., (© 2023. BioMed Central Ltd., part of Springer Nature.)

Boxwood blight (BB) caused by Calonectria pseudonaviculata ( Cps ), an economically devastating disease affecting everyone in the supply chain from growers to gardeners, was first officially documented in the United States in 2011. This disease has taken a heavy toll on boxwood, an iconic landscape plant and the number one evergreen nursery crop. Instead of abandoning boxwood production and switching to other evergreens, growers in the United States, informed by the latest research, have elected to combat this disease. One of the strategies employed has been to shift boxwood production from highly susceptible to less-susceptible cultivars. The objective of this study was to investigate the ongoing shift by comparing boxwood sales of 17 selected nurseries from seven states across the country in 2011, 2016, and 2021. Results revealed an additional 21.24% (from 38.58 to 60.04%) in sales of less-susceptible boxwood cultivars in 2021 compared with 2016. The less-susceptible cultivars responsible for increased boxwood sales are 'Winter Gem', 'Wintergreen', 'SB 300' (Freedom), 'SB 108' (Independence), and 'Little Missy'. The potential for long-term positive impact on sustainable boxwood production and plantings in the United States through the use of less-susceptible cultivars is discussed. This shift in boxwood choices builds crop health into new plantings of this landmark plant and sustains growth in demand for boxwood. This sets a new example of sustainable protection for a crop that is under serious pressure from an invasive pathogen., Competing Interests: The author(s) declare no conflict of interest.

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.

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.

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.

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.

Boxwood blight caused by Calonectria pseudonaviculata (Cps) is an emerging disease in the United States. Understanding the biology and survival of microsclerotia, the resting propagules of Cps, is important to managing the disease and restoring Cps-infested sites. In this study, an image-analyzing software (OpenCFU) was adapted to enumerate microsclerotia produced in culture plates. Microsclerotial counts increased with time up to 42 days while their size continued to grow for 70 days. Additionally, younger microsclerotia were less sensitive to extreme temperatures of -10 and 40°C than older ones. For instance, 21-day-old microsclerotia survived at almost twice the rate of 70-day-old ones after a 24-h exposure to -10°C. Likewise, the 21-day-old microsclerotia survived at much greater rates than 70-day-old ones (50 to 70% versus <10%) after a 24-h exposure to 40°C. The practical implications of these results are discussed.

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]