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PlantsmanThe Photographs: Béatrice Henricot Béatrice Photographs: Box blight rampages onwards

N LATE 1994, a new disease Box blight causes leaf loss and severe dieback on Buxus was discovered in A new fungal disease on box plants. In formal plantings the disease has a devastating effect (above) Ia nursery in Hampshire. The causing severe dieback fungus was initially identified as similar symptoms and it was clear Cylindrocladium scoparium, a pathogen of box was first that the species causing the disease that causes disease on a wide range recorded 12 years ago was undescribed (Henricot et al of hosts distributed worldwide. This 2002). The new species status was species is also the most misidentified in the UK. As it confirmed using traditional cultural species in the genus, as its morpho- continues to spread, techniques and DNA sequencing logical characteristics resemble those and it was named Cylindrocladium of other species closely. BÉATRICE HENRICOT buxicola after its host (Henricot In 1998 symptoms of leaf and and Culham 2002). twig blight caused by a species of reviews the current Cylindrocladium were reported in situation and outlines Symptoms New Zealand. On this occasion Infection by the fungus produces the fungus was identified as future research leaf spots on the leaves that can be C. spathulatum or possibly C. ilicicola. either dark brown or lighter brown The same year the RHS received surrounded by a dark border. The the first samples of Buxus showing fungus can also infect the stems ➤

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producing characteristic black streaks. Eventually severe defoliation and dieback occur. The fungus has never been recovered from the root system. Death of plants, especially young seedlings, can occur.

Geographical distribution The RHS records reveal that the disease is now widespread in the UK. The number of cases recorded has Cases of box blight reported to the RHS declined after 2000 but rose last year. This may reflect publicity been on the increase since 1998 with a peak of records in 2000. Since Cylindrocladium are identified mainly ologically similar species such as then the number of cases has on the basis of the shape of their C. ilicicola, C. mexicanum, decreased but rose again in 2005. In vesicles (inflated cells arising from C. pauciramosum, C. scoparium, and 2000, the disease was also recorded a spore-bearing structure), and the C. spathulatum. In addition, though, on Box Hill in Surrey, the only exten- size and (septation) partitioning of the DNA sequences of three sive locality in the UK where Buxus the conidia (asexual spores). However, different genomic regions, including sempervirens is native. Outside the these characters are not always the ITS, ß-tubulin and mat2 genes, UK the disease has been recorded well differentiated among species were unique compared to other in New Zealand in 1998, Belgium in and can be influenced by cultural Cylindrocladium species. 1998, Ireland in 2001 (RHS conditions. Therefore DNA records), Germany in 2005 (Brand sequence data are often used to Origin of the pathogen 2005) and Holland in 2005 (Marcel support species identification. The geographic origin of C. buxicola van Raak, pers. comm.). There have Morphologically, C. buxicola is is unknown. Its aggressiveness on also been unconfirmed reports of characterized as having one-septate Buxus and its relatively limited the disease in Italy and France. conidia and ellipsoidal vesicles with geographic range in relation to the pointed or papillate apices. It is also distribution of its hosts suggests Species identification a low temperature fungus as it can it is an exotic species introduced The novel species status of C. buxicola grow below 10°C but its growth is recently. Indeed, the genus Buxus was confirmed using a series of inhibited at 30°C and it is killed at includes 91 species distributed over morphological characters and DNA 33°C. These characters alone would 4 continents with 10 species sequence data. Species of the genus differentiate C. buxicola from morph- occurring in Africa, 45 in America,

Box blight infection produces leaf spots that can be pale brown with a darker magin (left) or dark brown (right). Also, black streaks may appear on the stems

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to Europe from a geographically isolated region on apparently innocuous plant ‘carriers’ – a situation referred to as the Typhoid Mary syndrome (Brasier 2005).

Life cycle of the fungus The infection by the fungus is rapid in warm (18–25°C) and humid conditions. The spores of the fungus are sticky so primary infection is unlikely to occur from wind-borne spores. Water-splashed spores or spores carried by a vector such as insects, birds or plants are the most likely source of primary inoculum. Spore germination starts three hours post-inoculation. Penetration can be observed as quickly as five hours after inoculation. It occurs through the stomata on the lower surface of the leaves or directly through the cuticle on the upper surface. No specific penetration or feeding structures are formed. The fungus can grow intercellularly in the mesophyll layers. Re-emergence of the fungus through the stomata County records of box blight in the UK compiled by the RHS show a steady expansion of range since 1998 occurs two or three days after infection. After seven days the lower 34 in Asia and 2 in Europe (Batford when closely related but previously leaf surface is covered with a layer of 2004). So far the disease is present geographically isolated taxa come conidiophores producing conidia only in Europe and in Australasia, into contact (Brasier et al 1999), for (asexual spores). The leaves are which is the only continent where instance, in nurseries or gardens. The eventually killed, probably by general Buxus is not native. In addition, DNA threat has largely increased as a result stress resulting from colonisation of evidence from amplified fragment of present trade practices, which the intercellular spaces and length polymorphisms (AFLPs) have led to more physical contact deregulation of the stomata. indicates that C. buxicola has a clonal between exotic species (Brasier In comparison with other species population structure. All the isolates 2005). However, the phylogenetic of Cylindrocladium, the fungus can seem to belong to one mating type as analysis combining the sequences of form resting structures called micro- suggested by the mating experiments the ITS and ß-tubulin showed that sclerotia, which can survive in the which failed to produce any fertile C. buxicola forms a distinct clade (a soil in the absence of a susceptible perithecia (sexual fruiting structures). group consisting of a common host. Our experiments so far have These data support the hypothesis ancestor and all its descendants) in shown the fungus can survive on of an exotic origin. Another the genus. This seems to rule out the decomposing leaf material for at hypothesis is that this new species possibility of a hybrid, as a more least five years but no microsclerotia has arisen through interspecific intermediate grouping would have development has been observed. hybridisation. Cases of new been expected if it was a hybrid pathogens emerging through (Henricot & Culham 2002). There- Host range hybridization are rare, but might fore the most likely explanation is To date the disease has been have been overlooked. It can occur that C. buxicola has been introduced detected on three different Buxus ➤

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species and many of their cultivars. produced by the fungus once it re- ornamentals may be used to control These are B. sempervirens, B. emerged from the stomata, it was box blight at the owner’s risk. These microphylla and B. sinica var. insularis. found that B. balearica was the most products include myclobutanil and These three species are resistant and B. sempervirens penconazole. Similarly, there are predominantly grown in the UK ‘Suffruticosa’ was the most approved commercial products that and other European countries. The susceptible. The apparent resistance can be used on ornamental crops in full list of species and cultivars of B. balearica could be due to the nurseries but no data is available on infected naturally since 1998 is: texture of its leaves which are more their efficacy, or the dosage and rate B. sempervirens leathery than some of the other to control the disease on box plants. B. sempervirens ‘Angustifolia’ species; this could disadvantage a Preliminary in vitro experiments B. sempervirens ‘Blauer Heinz’ fungus that penetrates directly carried out at the RHS showed that B. sempervirens ‘Compacta’ through the cuticle. penconazole was a more effective B. sempervirens ‘Elegantissima’ product than myclobutanil in B. sempervirens ‘Latifolia Maculata’ Control measures inhibiting mycelium growth and B. sempervirens ‘Memorial’ Control measures include cutting spore germination. They were, B. sempervirens ‘Suffruticosa’ back infected twigs to healthy tissue however, much less effective than B. sempervirens ‘Suffruticosa and removing fallen material and some of the commercial products Variegata’ topsoil. Badly affected plants should available. Of 10 fungicides tested, B. sempervirens ‘Variegata’ be disposed of. Making sure the carbendazim, prochloraz and B. microphylla ‘Faulkner’ foliage is kept dry to create a kresoxim-methyl completely B. microphylla var. japonica ‘Morris microclimate unfavourable to the inhibited the mycelium growth while Midget’ growth of the fungus limits the kresoxim-methyl was the most B. microphylla var. japonica ‘National’ spread of the disease. effective at inhibiting spore B. sinica var. insularis ‘Justin Brouwers’ There are no fungicides germination. However none of the It is not yet clear whether these specifically labelled to control fungicides killed the fungus. This species and cultivars are particularly this disease available to amateur finding has important consequences susceptible to the fungus or whether gardeners. However, products in understanding how the disease has they are more likely to be affected labelled to control any disease on and continues to spread in gardens. because of their widespread use as ornamentals. BOX BLIGHT ON BOX HILL Pathogenicity assays carried out at In 2001, Cylindrocladium buxicola was RHS Garden Wisley on detached confirmed on native Buxus sempervirens stems showed that the host range in growing wild (right) on Box Hill, a site the genus was in fact quite extensive, of special scientific interest in Surrey. including B. balearica, B. bodinieri, At the time the disease was mainly confined to the north side of the hill, B. glomerata, B. harlandii, B. macowanii affecting young seedlings and mature and B. riparia. All these species occur plants. Since then the disease has been naturally over four continents. In left unmanaged and no scientific survey addition, Sarcococca (in the same on its impact has been carried out. family, Buxaceae) was found to be a During a visit in 2006, it did not host for the disease but it has never appear that the fungus had spread been found naturally infected. This further. The damage seen in 2001 was still obvious as dead branches on the means that potentially other lower parts of the older trees. However, members of the Buxaceae such as regeneration seemed to have been Pachysandra could be host for the reduced as not many seedlings were disease, although this was not tested. seen on the affected site and a high Although no Buxus species was proportion of those present had fresh immune to the disease, there was lesions on the stems. If these infections some variation in the susceptibility spread throughout the plants then the seedlings will die, posing a threat to the pathogen. Based on leaf symp- to this unique population and habitat. toms and the number of conidia

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Future work gardens in the country. This way of After a primary screen of fungicides transmitting disease is well in vitro, those that showed documented by Brasier (2005), significant levels of inhibition of where he calls it Trojan horse spore germination and mycelium syndrome, and is not limited to the growth need to be tested on whole spread of Cylindrocladium. Still, the plants. The RHS will carry out a nursery trade (suffering from what I two-year trial evaluating fungicides would refer to as ostrich syndrome) for the control of Cylindrocladium has made the economic choice of blight at the ADAS Arthur spraying to suppress symptoms. The Rickwood site (Cambridge) that is failure to eradicate the disease has partially funded by English Heritage. perpetuated the problem for future The work will assess the protective The vesicles of box blight fungus are ellipsoidal generations of box lovers. The and eradicative action of seven with a mucilaginous surface and acute apices disease is now too extensively different fungicides on Buxus These include azoxystrobin distributed to be eliminated by sempervirens ‘Suffruticosa’. These (Amistar), chlorothalonil (Repulse) cultural measures alone and will include the three professional and epoxiconazole, kresoxim- continue to wreak devastation to fungicides which were the most methyl, pyraclostrobin (Opponent). new and established box plantings effective in vitro: carbendazim, The fungistatic action (ability to wherever it is introduced. ■ prochloraz and kresoxim-methyl. inhibit growth and reproduction Also included is penconazole which without killing the fungus) of the DR BÉATRICE HENRICOT is was the most effective fungicide fungicides will also be assessed. a Principal Plant Pathologist available to amateur gardeners. The at RHS Garden Wisley three remaining fungicides that will Conclusions be tested were reported by box The use of fungistatic chemicals REFERENCES growers as being effective at in nurseries has allowed the controlling this disease (Moran introduction of the disease through Batdorf, LR (2004) Boxwood An 2005, Karel Goossens, pers. comm.). apparently healthy material to many Illustrated Encyclopedia. The American Boxwood Society, Virginia ALTERNATIVES TO BOX Brand, T (2005) Auftreten von Cylindrocladium buxicola B. Henricot List 1 includes a traditional selection of plants suitable for hedge planting and an Buchsbaum in Nordwest- trimming. List 2 includes less commonly used plants for hedges, but planted Deutschland. Nachrichtenbl. Deut. in the right conditions they should be suitable for that purpose. Pflanzenschutzd. 57 (12): 237–240 Plants indicated as * can withstand temperatures down to -5°C but may still be Brasier, C (2005) Preventing damaged by less severe frost, while plants indicated as † require an acid soil. invasive pathogens: deficiencies in the system. The Plantsman n.s. 4 (1): 54–57 List 1 List 2 Brasier, CM, Cooke, DEL & Berberis buxifolia ‘Pygmaea’ Azara microphylla * Duncan, JM (1999) Origin of a new B. x stenophylla ‘Corallina Compacta’ Camellia x vernalis ‘Yuletide’ † Phytophthora pathogen through B. x stenophylla ‘Irwinii’ Corokia x virgata * interspecific hybridization. Proc. Natl. B. x stenophylla ‘Nana’ Erica erigena ‘W.T. Rackliff’ Acad. Sci. USA 96: 5878–5883 B. thunbergii ‘Atropurpurea Nana’ Escallonia ‘Compacta Coccinea’ * Henricot, B, Pérez Sierra, A & (deciduous) Hebe rakaiensis, H. topiaria, Prior, C (2000) A new blight Euonymus fortunei cultivars H. ‘Emerald Gem’ disease on Buxus in the UK caused E. japonicus ‘Microphyllus’ * Hedera helix ‘Glymii’ by the fungus Cylindrocladium. Hebe cupressoides ‘Boughton Dome’ H. helix ‘Walthamensis’ Pl. Pathol. 49: 805 Ilex crenata Ligustrum delavayanum * Henricot, B & Culham, A (2002) Lavandula angustifolia cultivars Olearia x haastii Cylindrocladium buxicola, Lonicera nitida Pieris japonica ‘Little Heath Green’ † a new species affecting Buxus spp., Osmanthus delavayi Pittosporum tenuifolium * and its phylogenetic status. Rosmarinus officinalis Rhododendron dwarf cultivars † Mycologia 94 (6): 980–997 Santolina chamaecyparissus * Teucrium x lucidrys Morgan, S (2005) Buxus. Hort. Week Taxus baccata ‘Semperaurea’ Vaccinium delavayi * † 26 May: 16–17

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