Calonectria pseudonaviculata
(Buxus blight)

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.

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.

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.

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).

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.

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).

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).

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).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

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).

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.

29/01/10 Original text by: