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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.

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.

Two commercial greenhouses producing potted plants in Pennsylvania using recycled irrigation water in an ebb-and-flood system have incurred significant crop losses due to Pythium aphanidermatum. In cooperation with the greenhouses, one or more of their water tanks was monitored continuously (128 tank samplings) for Pythium spp. by baiting. Nine species of Pythium and three species of Phytopythium were recovered, representing clades A, B, E, and K, but none was P. aphanidermatum. The recovered Pythium spp. were (i) P. rostratifingens, (ii) isolates identical to Pythium sp. nov. OOMYA1702-08 (clade B2), (iii) P. coloratum, (iv) P. middletonii, (v) and (vi) two new species in clade E2, (vii) a new species in clade B2, (viii) isolates very similar to Pythium sp. nov. OOMYA1646-08 (clade E2), and (ix) a new species in clade A. The Phytopythium spp. recovered were (i) Phytopythium litorale, (ii) P. helicoides, and (iii) P. chamaehyphon. This article illustrates the different communities of Pythium and Phytopythium spp. found in each greenhouse over 10 months. Some of the baited species display resistance to the oomycete fungicide active ingredient, mefenoxam. P. helicoides and the new species in clade B2 were pathogenic on seedlings in potting mix with fertilizer added.

Boxwood blight caused by Calonectria pseudonaviculata is typically expressed as a foliage disease with aboveground symptoms including defoliation, dieback and formation of dark narrow stem cankers. Whether this pathogen behaves like other Calonectria spp. and has a significant soil phase in the epidemiology of boxwood blight is not known. In this study we observed experimentally that (1) the boxwood blight pathogen consistently forms microsclerotia in artificially inoculated leaves and roots of Buxus spp., (2) soil artificially inoculated with conidia and microsclerotia of this pathogen can cause foliar blight, (3) conidia and microsclerotia can remain viable in soil for up to 3 and at least 40 weeks, respectively (4) and the pathogen can cause crown and root rot to plants only when roots and crowns are directly exposed to relatively high inoculum levels. Our results suggest that C. pseudonaviculata is primarily a foliar pathogen with a potentially epidemiologically significant soil phase.

From a survey for Phytophthora spp. in containment basins at one nursery each in Alabama and Mississippi, eight species and one taxon were recovered, with Phytophthora gonapodyides dominant in cooler months and P. hydropathica in warmer months, accounting for 39.6 and 46.6% overall recovery, respectively. Among the recoveries were P. macilentosa, P. mississippiae, and P. stricta, three new species recently described from a small lake (labeled M4) that serves as a primary water source for irrigation and to feed another irrigation pond (M5) at the Mississippi nursery. Neither of ponds M4 and M5 directly receives runoff from any production area. The three new species were tested for pathogenicity with Catharanthus roseus, Gardenia jasminoides 'August Beauty,' Hydrangea quercifolia 'Semmes Beauty,' Ilex magland 'Oakland,' Pieris japonica 'Mountain Snow,' and Rhododendron × 'Brandi Michele Raley.' None of the three species infected any of the test plants or became established in peat or pine bark growing media. Based on the result of pathogenicity trials as well as the field observation that none of the nine Phytophthora taxa recovered from irrigation reservoirs have caused episodic disease in the nurseries, they appear to not present a high risk to ornamental plants at those nurseries.

Boxwood blight caused by Calonectria pseudonaviculata results in severe defoliation and dieback to boxwood (Buxus spp.). The pathogen was first described in the United Kingdom and New Zealand in the mid to late 1990s and has since spread throughout Europe and most recently to the United States and Canada. While many Calonectria spp. have an epidemiologically significant soil phase, little is known of the role of the soil phase of C. pseudonaviculata in the epidemiology of boxwood blight. We optimized a leaf disc bioassay for detecting and quantifying this pathogen in soil and compared this bioassay with a standard soil plating assay originally developed for quantifying Calonectria using a Suffolk sandy-loam soil. Additionally, the sensitivity of both assays was compared among three distinct soil types (sand-loam, silt-loam, and sand-peat potting media). The optimal incubation time for baiting C. pseudonaviculata from soil using the leaf disc bioassay was 96 h. The optimal soil moisture for the bioassay was 1,000% of field capacity (flooded with 3 to 5 mm water). The leaf disc bioassay was able to detect C. pseudonaviculata at levels as low as 1 microsclerotium/ g soil while the soil plating bioassay was unable to detect the pathogen below inoculum levels of 10 microsclerotia/g soil in the Suffolk sandy-loam soil. Soil type had a significant impact on the sensitivity of both assays.

Phytophthora cinnamomi is a destructive root pathogen of numerous woody plant species in the ornamental plant nursery. Sixty-five isolates of P. cinnamomi were evaluated for mefenoxam sensitivity on 20% clarified V8 agar amended with mefenoxam at 0 or 100 μg/ml. In the presence of mefenoxam at 100 μg/ml, eight isolates were intermediately sensitive, with mycelium growth ranging between 11 and 18% of the nonamended control, and 57 isolates were highly sensitive, with little or no mycelium growth. Five intermediately sensitive and five sensitive isolates were chosen to characterize their responses to mefenoxam at 0, 0.1, 1, 10, and 100 μg/ml. For intermediately sensitive isolates, the mefenoxam concentration causing 50% inhibition of mycelium growth (EC 50 values) ranged between 0.03 and 0.08 μg/ml; EC 50 values for sensitive isolates varied from 0.01 to 0.02 μg/ml. Five intermediately sensitive and seven sensitive isolates were selected further to assess in vivo sensitivity to mefenoxam using Lupinus angustifolius 'Russell Hybrids'. Lupine seedlings were treated with distilled water or mefenoxam at label rate (Subdue MAXX, 1 fl. oz. of product per 100 gal.) and then, 2 days later, inoculated with a 5-mm-diameter mycelial plug of P. cinnamomi on each cotyledon. Mefenoxam-treated plants averaged more than 96% less disease than water-treated plants. Mefenoxam provided adequate protection of lupines from infection by all 12 isolates regardless of their in vitro levels of sensitivity to mefenoxam. The ability to develop mefenoxam resistance was assessed in P. cinnamomi isolates with different mefenoxam sensitivity by UV mutagenesis and adapting mycelium to increasing concentrations of mefenoxam. Both UV mutagenesis and mycelium adaptation generated isolates with reduced sensitivity to mefenoxam. These isolates, however, did not grow as quickly as their corresponding parent. This study suggests that P. cinnamomi populations from ornamental nurseries in Virginia are sensitive to mefenoxam.

Eighteen isolates from 12 species of Phytophthora, including several new taxa, were tested for pathogenicity to six ornamental and four vegetable species. The following three inoculation methods were used depending on infection court targeted: vermiculite culture inoculation for roots, agar block inoculation for fruit, and zoospore inoculation for foliage. All six new taxa (P. irrigata, P. hydropathica, Dre III, Cil I, Cip-like, and Gon I) are pathogenic to one or more test plants. Specifically, taxon Cil I was identified as a growing threat to horticultural crops, particularly ornamental crops in container production nurseries. The potential host list of P. tropicalis was expanded to four new families (Apocynaceae, Asteraceae, Begoniaceae, and Fabaceae) and one additional genus within each of three existing families (Ericaceae, Cucurbitaceae, and Solanaceae). New potential hosts were also identified for other existing species of Phytophthora. The practical implications of these results in crop health management programs for both ornamental and vegetable crops locally, and for development and implementation of agricultural biosecurity programs globally, are discussed.

Propamocarb hydrochloride is a systemic fungicide commonly used for control of Phytophthora diseases of nursery crops. Here we report on the effect of this compound on different growth stages of Phytophthora nicotianae, a major pathogen of numerous herbaceous and some woody ornamental plants. A total of 71 isolates were assayed for sensitivity to propamocarb at two concentrations of 1.8 mg/ml (label rate) and 10 mg/ml using clarified V8 agar as a base medium. All isolates grew at 10 mg/ml with the most sensitive isolate having 34.8% relative growth compared with growth on nonamended medium. Nine isolates were selected and further tested for mycelial growth at 0, 1, 10, 25, 50, and 100 mg/ml, and for sporangium production, zoospore motility, and germination at 0, 5, 50, 500, 5,000, and 50,000 μg/ml. EC 50 values ranged from 2.2 to 90.1 mg/ml for mycelial growth, 133.8 to 481.3 μg/ml for sporangium production, 88.1 to 249.8 μg/ml for zoospore motility, and 1.9 to 184.6 μg/ml for zoospore germination, respectively. In addition, 17 selected isolates were evaluated for propamocarb sensitivity on Pelargonium × hortorum cv. White Orbit. Two days after seedlings were treated with propamocarb at 3.6 mg/ml, they were inoculated by either inverting one 5-mm-diameter plug of a 3-day-old culture or applying a 10-μl drop containing 20 zoospores onto each cotyledon. Propamocarb hydrochloride provided good protection of geranium seedlings from zoospore infections but not from mycelial infections. These results suggest that this fungicide must be used preventively for Phytophthora disease management and that mycelial growth may not be the most reliable measurement to determine the development of fungicide resistance to this compound in Phytophthora species at production facilities and in the landscape.

Stringent standards of water quality have prompted many horticultural enterprises to limit pollutant discharge associated with nutrient and pesticide applications. Collecting and recycling effluent is a method that has been implemented by many operations to contain pollutants; however, plant pathogens may be spread through recycled effluent. In this study, Phytophthora and Pythium spp. present in a water-recycling irrigation system at a perennial container nursery in southwestern Virginia were characterized using filtering and baiting techniques with two selective media. Members of Phytophthora were identified to species, whereas Pythium spp. were identified to genus only. Pythium spp. were recovered more frequently and in greater numbers than Phytophthora spp. Phytophthora capsici, P. citricola, P. citrophthora, P. cryptogea, P. drechsleri, and P. nicotianae were recovered in filtering assays. Only P. cryptogea and P. drechsleri were identified from baits placed on the surface of the irrigation reservoir, whereas P. cactorum, P. capsici, P. citricola, P. citrophthora, P. cryptogea, and P. drechsleri were recovered at depths, specifically at 1 and 1.5 m. This research provides data for development of detection technology and management practices for plant pathogens in irrigation water and may lead to improvements in conventional assay protocols.

Apothecia were produced in the orchard, lath house, and laboratory from peach and nectarine fruit infected and stromatized by Monilinia fructicola. Fully stromatized "mummies" and nonstromatized infected fruit were placed in the orchard either on the soil surface or buried to a depth of 2 to 3 cm. Mummies were placed in the orchard at monthly intervals from August to February in 1993-94 and 1994-95. Nonstromatized infected fruit, which were fleshy and decomposed rapidly, were soon unavailable and were only placed in the orchard in August and September. Apothecia developed in February and early March only from mummies that were placed in the orchard in either October, November, or December. Mummies placed in the field in August, September, January, and February did not produce apothecia. Leaving mummies on the soil surface versus burying them 2 to 3 cm did not affect the development of apothecia. Apothecia were never produced from nonstromatized or recently-infected (fleshy) fruit. In the laboratory, apothecia were only produced from mummies that were partially buried in moist sand and stored without light at 2°C and >97% relative humidity (RH) for more than 8 weeks prior to incubation for 2 weeks (12, 15, or 20°C) with a 12-h photoperiod. Mummies that were incubated at >97% RH for less than 8 weeks or incubated at <90% RH never produced apothe-cia when stored at 2°C and then transferred to warmer temperatures with light. In orchard experiments, apothecia were only observed in plots with nondisturbed orchard floor vegetation; whereas no apothecia were found in either herbicide-treated or rototilled plots. Apothecia in the San Joaquin Valley were only produced from mummies that were subject to an 8-week or greater cold-temperature incubation while in contact with soil.

The effects of wounding, inoculum density, and three isolates (New, Ta291, and 23-E-6) of Trichoderma spp. and one isolate (BI-54) of Rhodotorula sp. on postharvest brown rot of stone fruits were determined at 20°C and 95% relative humidity (RH). Brown rot was observed frequently on wounded nectarine, peach, and plum fruits inoculated with two spores of Monilinia fructicola per wound, and occasionally on unwounded nectarine and peach fruits inoculated with the same spore load. Brown rot was observed on wounded plums only. A substantial increase in lesion diameter of brown rot was also recorded on wounded nectarines and peaches inoculated with suspensions of ≤20 spores and ≤200 spores per wound, respectively, compared with unwounded fruit. At concentrations of 10 7 and 10 8 spores per ml, all Trichoderma isolates substantially reduced brown rot on peaches (63 to 98%) and plums (67 to 100%) when fruits were inoculated with M. fructicola following the application of a biological control agent. Similarly, at 10 8 spores per ml, the yeast BI-54 also suppressed brown rot on peaches completely and on plums by 54%. Significant brown rot reduction was also achieved with the isolate New at a concentration of 10 8 spores per ml, even when the biocontrol agent was applied 12 h after inoculation with M. fructicola and under continuous conditions of 95% RH. The isolates Ta291 and 23-E-6 also reduced brown rot significantly under drier (50% RH) incubation conditions. These isolates provided the best control of brown rot on plums when they were applied 12 h earlier than inoculation with M. fructicola. Satisfactory control of brown rot on plums inoculated with M. fructicola at 8 × 10 4 spores per ml was achieved with New at 10 6 spores per ml and with Ta291 at 10 7 spores per ml. Measures taken to avoid injuring fruit will greatly reduce brown rot of stone fruit at any spore load for plum, but only at ≤50 spores per mm 2 for peach, and at ≤5 spores per mm 2 for nectarine. This study identifies two isolates (Ta291 and New) of Trichoderma atroviride, one isolate (23-E-6) of T. viride, and one of Rhodotorula sp. that show potential for further development as biocontrol agents of postharvest brown rot of stone fruits.

Naturally growing apothecia of Monilinia fructicola were collected from two commercial plum orchards near Reedley and Sanger, both in Fresno County, California. Ascospore discharges from 90 (1996) and 86 (1997) apothecia were monitored individually using spore traps at four constant temperatures. The period of discharge decreased as temperature increased from 10 to 25°C. However, daily discharge increased as temperature increased from 10 to 15°C and remained high at 20 and 25°C. The greatest discharge occurred with apothecia at 15°C, followed by those incubated at 20, 10, and 25°C. The germination of ascospores of M. fructicola and the length of germ tubes increased as temperature increased from 7 to 15°C; however, increasing temperatures above 15°C did not increase either ascospore germination or length of germ tubes. This information may help in the development of warning systems and management strategies for brown rot blossom blight of stone fruits.

The significance of thinned fruit as a source of secondary inoculum in the spread of brown rot, caused by Monilinia fructicola, under semi-arid weather conditions of the San Joaquin Valley in California, was investigated in seven nectarine orchards in 1995 and 1996. Between 6 and 60% (depending on the orchard) of thinned fruit showed sporulation by M. fructicola. Brown rot was significantly less severe at preharvest (five orchards) and postharvest (one orchard) on fruit harvested from trees in plots from which thinned fruit were completely removed than on those in plots from which thinned fruit were not removed. M. fructicola sporulated more frequently on thinned fruit placed into irrigation trenches than on those left on the dry berms in tree rows. The incidence of preharvest fruit brown rot increased exponentially as the density of thinned fruit increased on the orchard floor. These results suggest that thinned fruit left on the floor of nectarine orchards can be a significant inoculum source of secondary infections. Removal or destruction of thinned fruit should reduce brown rot in nectarine and possibly other stone fruit orchards under semi-arid California conditions.