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Diagnostic Fact Sheet for Calonectria pseudonaviculata

Invasive and Emerging Fungal Pathogens - Diagnostic Fact Sheets

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Shoot blight of boxwood-Calonectria pseudonaviculata

Calonectria pseudonaviculata (synonym Cylindrocladium buxicola) is an asexually reproducing 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 plants that are commonly used in gardens and landscaping. The full extent of its host range is not known, but Buxus species 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, 2004), 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.

Calonectria pseudonaviculata Henricot 2002 (Ascomycetes, Hypocreales)

Conidiophores comprised of stipe, sterile extension with a terminal vesicle, and penicillate branches bearing phialides. Stipe septate, hyaline, 95-155 µm, the extension terminating in a broadly ellipsoid vesicle, vesicle apex pointed to papillate, 6.5-11 µm diam, the widest part above the middle. Primary branches 0-1-septate, (5-)15-41(-66) x 3-5 µm, secondary branches 0-septate, (11-)13-25(-35) x 3-5 µm, tertiary branches rare. Terminal branches bearing 2-5 phialides. Phialides reniform, hyaline, 0-septate, (10-)13-18(-21) x 2.5-5 µm. Conidia cylindrical, straight, hyaline, 1-septate, the ends rounded, 42-68 x 4-6 µm, in slimy clusters. Chlamydospores on carnation leaves dark brown, thick-walled, forming microsclerotia.

Reverse of colony on MEA fuscous black at center fading through sienna outwards. Mycelium at margin white.

Distribution:

The new disease of boxwood was first observed in 1994 in 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 Calonectria pseudonaviculata, often as Cylindrocladium buxicola, have since come from Belgium, Ireland, Germany, and the Netherlands (CABI/EPPO, 2007; Henricot et al., 2008), Italy (Saracchi et al., 2008; EPPO, 2009b), Austria (EPPO, 2009a) and Spain (Pintos Varela et al., 2009), suggesting that it spread from the UK to mainland Europe.

Host: 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).

For additional details, see Henricot and Culham, 2002, who used the name Cylindrocladium buxicola.

The species Calonectria pseudonaviculata was described by Crous et al. (2002) only a few months before Henricot and Culham (2002) who used the name Cylindrocladium buxicola. It was determined to be a new species both by examination of morphology and comparison of sequences of several regions of nuclear DNA. No teleomorph was obtained by mating of single spore cultures on CLA, and the similarity of AFLP profiles of the different isolates indicated that all were probably descended from one clone.

When the genera Calonectria 1867, formerly used for sexual states, and Cylindrocladium 1892, formerly used for the asexual states, are synonymized, Calonectria is the older name and thus has priority. The names in Calonectria for species previously known as Cylindrocladium are listed in Lombard et al. (2010a).

RISK OF INTRODUCTION

The origin of Calonectria 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 species and the possibility of their misidentification (see Henricot and Culham, 2002), that an introduction on some other host went unchallenged because the fungus resembled a widespread species.

SIMILARITIES TO OTHER SPECIES/CONDITIONS

The species Calonectria pseudonaviculata was distinguished from C. morganii, C. spathulata and C. pyrochroa by the shape of the terminal vesicle and the size and septation of its conidia (Henricot and Culham, 2002; Lombard et al. 2010b). The vesicles of C. morganii are not pointed, and those of C. spathulata and C. pyrochroa are spathulate to clavate. In addition, C. morganii and C. pyrochroa grow at 30 C and above (Crous and Wingfield, 1994), while 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 or above (Henricot and Culham, 2002; Lombard et al. 2010b).

Another new species isolated from leaf spots on Buxus sempervirens, but in New Zealand, was observed to be similar to C. naviculata in vesicle shape as well as in the partial sequence of its beta-tubulin gene, and was therefore named C. pseudonaviculatum by Crous et al. (2002). Although these two species have not been compared directly, the greatest distinction from C. pseudonaviculata appears to lie in the vesicle morphology and size, which Crous et al. (2002) confirm as crucial to species separation in this genus. Also, the maximum growth temperature for the New Zealand species is apparently higher than that of C. pseudonaviculata (Crous et al., 2002; Henricot and Culham, 2002).

DETECTION AND INSPECTION METHODS

The 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 extension are produced on shoots incubated in a moist chamber at 20° C (Henricot and Culham, 2002).

DIAGNOSTIC METHOD

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; Lombard et al. 2010b). 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 (Henricot and Culham, 2002; Crous et al., 2002; Crous et al., 2006; Lombard et al. 2010b). Most of these sequences, including at least eight for C. pseudonaviculata, are available in the GenBank database for comparison (NCBI, 2010).

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; Henricot and Culham, 2002; Henricot et al., 2008).

BIOLOGY AND ECOLOGY

Life cycle

Conidia dispersed in water germinated on Buxus leaves beginning three 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 seven 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, 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 seven 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.

MOVEMENT AND DISPERSAL

Natural dispersal: The "slimy" Calonectria 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).

IMPACTS

Economic impact: The disease is described as "devastating" to boxwood 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).

MANAGEMENT

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 the trade of boxwood 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.

Cultural control and sanitary measures

Because C. pseudonaviculata can persist for years in plant debris on or in the ground, 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 favor 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 species of Calonectria species could help to prevent further introductions.

References

Nomenclature

Specimens in BPI

Additional distribution data

 

Suggested citation: Chalkley, D. Systematic Mycology and Microbiology Laboratory, ARS, USDA. . Invasive Fungi. Shoot blight of boxwood-Calonectria pseudonaviculata. Retrieved November 24, 2017, from /sbmlweb/fungi/index.cfm .




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Calonectria pseudonaviculata
Calonectria pseudonaviculata - Conidiophores (view from above), 100x. CBS 114417: on cornmeal-dextrose agar.
Calonectria pseudonaviculata - Conidiophores (view from above), 150x. CBS 114417: on cornmeal-dextrose agar.
Calonectria pseudonaviculata - Conidiophore, 200x. CBS 114417: from cornmeal-dextrose agar.
Calonectria pseudonaviculata - Conidiophore extension with vesicle, 400x. CBS 114417:  from cornmeal-dextrose agar.
Calonectria pseudonaviculata - Conidia and phialides, 400x. CBS 114417:  from cornmeal-dextrose agar.