Calonectria pseudonaviculata
(Buxus blight)

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Identity

Preferred Scientific Name

• Calonectria pseudonaviculata (Crous, J.Z. Groenew. & C.F. Hill) L. Lombard, M.J. Wingf. & Crous, 2010

Preferred Common Name

• Buxus blight

Other Scientific Names

• Cylindrocladium buxicola Henricot 2002
• Cylindrocladium pseudonaviculatum Crous, J.Z. Groenew. & C.F. Hill, 2002

International Common Names

• English: box blight; boxwood blight; boxwood leaf drop; leaf and twig blight of box

EPPO code

• CYLDBU (Cylindrocladium buxicola)

Summary of Invasiveness

C. pseudonaviculata is an asexual species in a genus of common ascomycete plant pathogens. It was identified relatively recently in the UK, as an introduced species causing a devastating shoot blight of boxwood [Buxus spp.] plants that are commonly used in gardens and landscaping. The full extent of its host range is not known, but Buxus spp. from different continents were found to be susceptible (Henricot et al., 2008). It was placed on the EPPO Alert list in 2004, as it appeared to be spreading to the mainland (EPPO, 2009a), and removed from the list in 2008. This pathogen has been reported from additional European countries in recent years, and may have been transported in asymptomatic infected plants or propagating materials. It survives well in plant debris and probably also in soil.

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Fungi
•         Phylum: Ascomycota
•             Subphylum: Pezizomycotina
•                 Class: Sordariomycetes
•                     Subclass: Hypocreomycetidae
•                         Order: Hypocreales
•                             Family: Nectriaceae
•                                 Genus: Calonectria
•                                     Species: Calonectria pseudonaviculata

Notes on Taxonomy and Nomenclature

Cylindrocladium pseudonaviculatum was first described by Crous et al. (2002). The description of Cylindrocladium buxicola followed a few months later in the same year (Henricot and Culham, 2002), which later proved to be synonymous with Calonectria pseudonaviculata (Lombard et al., 2010). The taxon was determined to be a new species both by examination of morphology and comparison of sequences of several regions of nuclear DNA. No sexual morph could be obtained by mating of single spore cultures on carnation leaf agar (CLA), and the similarity of amplified fragment length polymorphism (AFLP) profiles of the different isolates indicated that all were probably descended from one clone.

Sexual morphs of Cylindrocladium species are members of Calonectria, in the Hypocreales (Rossman, 1993; Crous, 2002). The current scientific name for this species is Calonectria pseudonaviculata (Crous, J.Z. Groenew. & C.F. Hill) L. Lombard, M.J. Wingf. & Crous.

Description

Conidiophores comprised of a stipe, sterile stipe extension with a terminal vesicle, and penicillate arranged branches bearing phialides. Stipe septate, hyaline, 95-155 µm, the stipe extension terminating in a broadly ellipsoid vesicle, vesicle apex pointed to papillate, 6.5-11.0 µm diameter, the widest part above the middle. Primary branches 0-1-septate, (5-)15-41(-66) x 3-5 µm, secondary branches aseptate, (11-)13-25(-35) x 3-5 µm, tertiary branches rare. Terminal branches bearing two to five phialides. Phialides reniform, hyaline, aseptate, (10-)13-18(-21) x 2.5-5.0 µm. Conidia cylindrical, straight, hyaline, 1-septate, the ends rounded, 42-68 x 4-6 µm, in slimy clusters. Chlamydospores on carnation [Dianthus caryophyllus] leaves dark-brown, thick-walled, forming microsclerotia. Reverse of colony on malt extract agar (MEA) fuscous black at centre fading through sienna outwards. Mycelium at margin white. For additional details, see Henricot and Culham (2002).

Distribution

The new disease of boxwood (Buxus spp.) was first observed in 1994 in the UK, and a more severe outbreak occurred in 1997 (Henricot and Culham, 2002). Although the pathogen is considered to have been recently introduced (Brasier, 2008), it was already widespread in the UK by 2000 (Henricot et al., 2000). An isolate from New Zealand was determined by Henricot and Culham (2002) to be of the same species and closely related to the strains in the UK. Reports of C. pseudonaviculata have since come from Belgium, Ireland, Germany and the Netherlands (CABI/EPPO, 2012; Henricot et al., 2008), Italy (Saracchi et al., 2008; EPPO, 2013), Austria (EPPO, 2013) and Spain (Pintos Varela et al., 2009) suggesting that it spread from the UK to mainland Europe.

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Introductions

Risk of Introduction

The origin of C. pseudonaviculata is not known; it is considered to be an introduced alien species in the UK, where it was first identified (Brasier, 2008). Transmission of the pathogen on asymptomatic Buxus plants, as suggested by Henricot and Culham (2002), may be responsible for its rapid spread in the UK and to various countries of Western Europe. It is also possible, given the wide host ranges of some Calonectria spp. and the possibility of their misidentification (see Henricot and Culham, 2002; Lombard et al. 2010), that an introduction on some other host went unchallenged because the fungus resembled a widespread species.

Habitat List

Hosts/Species Affected

The disease has only been found in some cultivars of three species out of the 91 in the genus Buxus worldwide: B. sempervirens,B. microphylla and B. sinica var. insularis (Henricot et al., 2008). When detached stems of other species, including plants native to four continents, were tested beside these, Henricot et al. (2008) found no immunity to the fungus. Differences between the species were not consistent in tests with different isolates of the pathogen. The lowest level of disease was observed in B. balearica, B. riparis and a Sarcococca sp.Sarcococca is a genus in the Buxaceae that includes some species imported for use as ground cover; Pachysandra species, also members of the family often used for ground cover, were not tested (Henricot et al., 2008).

Host Plants and Other Plants Affected

Growth Stages

Symptoms

The fungus causes dark-brown leaf spots, which may coalesce to cover whole leaves, black streaks on stems that appear to progress from the bottom of the plant to the top, and severe defoliation and dieback (Henricot et al., 2000, 2008; Henricot and Culham, 2002).

List of Symptoms/Signs

Biology and Ecology

Life Cycle

Conidia dispersed in water germinated on Buxus leaves beginning 3 hours after inoculation. Germ tubes penetrated through stomata or directly through the cuticle without forming an appressorium. Conidiophores were produced on the leaf surface after 7 days in a moist chamber at 20°C (Henricot et al., 2008). High humidity is required for infection of inoculated plants (Henricot et al., 2000).

In culture on PCA (potato-carrot agar), optimum growth occurs at 25°C. Growth is very slow at 27.5°C, and no growth was observed at 30°C. Incubation at 33°C for 7 days is lethal, and the low limit for growth is above 5°C (Henricot and Culham, 2002).

Mycelium of the fungus survived 5 years in decomposing leaves on or in soil in southern England, but microsclerotia were not seen (Henricot et al., 2008). Most Calonectria species are readily recovered from soil (Crous, 2002).

Reproductive Biology

No sexual reproductive structures have been observed in nature or in culture. The UK isolates examined by Henricot and Culham (2002), as well as one from New Zealand, exhibited little variation in AFLP patterns, thus appearing to be derived from one clone.

Climate

Means of Movement and Dispersal

Natural Dispersal

The 'slimy' conidia are easily splash-dispersed by water (Crous, 2002).

Vector Transmission

None is reported, but Crous (2002) hypothesizes a role for the stipe extension vesicle in attracting insects to the conidiophore.

Accidental Introduction

The fungus could be brought into gardens or nurseries on asymptomatic plants (Henricot et al., 2008).The means of introduction into the UK and mainland Europe could be the same, but is not known. In a nursery in Italy, potted plants that became infected had been without symptoms when imported from Belgium, where the pathogen was known to be present (Saracchi et al., 2008).

Pathway Causes

Pathway Vectors

Plant Trade

Impact Summary

Economic Impact

The disease is described as 'devastating' to boxwood (Buxus spp.) plants, which are widely used as landscape ornamentals (Henricot et al., 2008). Infected imported potted plants in an Italian nursery were destroyed due to the severity of disease (Saracchi et al., 2008).

Risk and Impact Factors

• Highly mobile locally
• Long lived
• Fast growing
• Has high reproductive potential
• Reproduces asexually

• Host damage
• Negatively impacts agriculture
• Negatively impacts livelihoods
• Reduced amenity values

• Pathogenic

• Highly likely to be transported internationally accidentally
• Difficult to identify/detect as a commodity contaminant

Diagnosis

Isolates may be grown on PCA (potato-carrot agar), but culture on carnation leaf agar at 25°C under near-UV light is required for production of conidiophores with diagnostic morphology (Crous and Wingfield, 1994).

Comparison of sequences of the ITS region is insufficient to distinguish all the known species of Calonectria (Crous et al., 1999; Henricot and Culham, 2002). Instead, sequences of the 5' end of the beta-tubulin gene, the HMG box of the MAT2 gene, and partial sequences of the histone H3 gene have been used (Crous et al., 1999, 2002, 2006; Henricot and Culham, 2002). Most of these sequences, including at least eight for C. pseudonaviculata, are available in the GenBank database for comparison (Lombard et al., 2010).

Detection and Inspection

This pathogen causes dark leaf spots, dark streaks on the stems, and eventual defoliation of Buxus species. Conidiophores bearing clusters of distinctive large cylindrical conidia and a vesicle-tipped sterile stipe extension are produced on shoots incubated in a moist chamber at 20°C (Henricot and Culham, 2002).

Similarities to Other Species/Conditions

Calonectria pseudonaviculata is distinguished from C. morganii and C. pyrochroa by the shape of the terminal vesicle and the size and septation of its conidia (Henricot and Culham, 2002). The vesicles of C. morganii are not pointed, and those of C. pyrochroa are spathulate to clavate. In addition, C. morganii and C. pyrochroa grow at 30°C and above (Crous and Wingfield, 1994) whereas 30°C is the maximum growth temperature for C. pseudonaviculata (Henricot and Culham, 2002). Other species, C. pauciramosa and C. mexicana, that have somewhat similar vesicles, are also warm-temperature species, with growth maxima of 30°C or above (Henricot and Culham, 2002).

Prevention and Control

Prevention

SPS Measures

The likelihood that the pathogen is transported across borders in asymptomatic plants (Henricot et al., 2008; Saracchi et al., 2008) would require restriction of trade of boxwood (Buxus spp.) plants and vegetative propagating material, such as cuttings, including either quarantine or certification procedures to prevent further spread of the fungus. If there is a possibility that it was, or can be, introduced on other hosts upon which it has been misidentified, stricter attention to identifications of Calonectria pathogens, with the aid of molecular methods, may be necessary.

Control

Cultural Control and Sanitary Measures

C. pseudonviculata can persist for years in plant debris on or in the ground, therefore removing infected twigs, fallen leaves and the topsoil under affected plants are reasonable efforts to reduce inoculum (Henricot et al., 2008). Batdorf (2004) asserts that regular fall pruning to thin the branches of Buxus sempervirens "Suffruticosa" will control fungal foliage diseases such as Cylindrocladium blight.

Chemical Control

Henricot et al. (2008) tested a number of fungicide products available in the UK. Those most effective at inhibiting mycelial growth of C. pseudonaviculata in vitro were carbendazim, prochloraz, kresoxim-methyl, penconazole, the combination of epiconazole and pyraclostrobin, and the combination of epiconazole+kresoxim-methyl+pyraclostrobin. Those most effective at inhibiting conidial germination in vitro were azoxystrobin, chlorothalonil, kresoxim-methyl, mancozeb, boscalid+pyraclostrobin, epiconazole+pyraclostrobin, and the combination of epiconazole+kresoxim-methyl+pyraclostrobin. Carbendazim, myclobutanil, penconazole and prochloraz had little or no effect on germination.

Chlorothalonil, which was the most inhibiting to spore germination in vitro, has also been reported as effective in the field (Henricot et al., 2008).

Host Resistance

Differences in isolate-species interactions observed by Henricot et al. (2008) suggest a potential for use of varieties and species in landscaping of areas where the pathogen is known to occur. Unfortunately, the growth habit that contributes to the ornamental desirability of the susceptible species and varieties may favour the development of disease (Henricot et al., 2008).

Gaps in Knowledge/Research Needs

Many Calonectria species infect roots (Crous, 2002); whether C. pseudonaviculata can infect those of Buxus or other plants, should be investigated. The possible role of chlamydospores and microsclerotia in survival in plant tissues and in soil should be examined further. Efforts to discover the origin of the fungus and to explore the possibility of insect vectors for Calonectria species could help to prevent further introductions.

References

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Contributors

29/01/10 Original text by:

Distribution Maps