Research

Box diseases

Box (Buxus spp.) is one of the oldest garden ornamental plants, which in the last two decades, has been rediscovered and become one of the most fashionable plant in the UK. Box is mainly used for parterres, knot gardens, hedging and topiary work. The genus Buxus contains more than 70 species found in Europe, the West Indies, East Asia and Central America. In the UK, the most commonly planted species is the native Buxus sempervirens.

Until recently, the main diseases known in the UK on Buxus were:

Root rot: caused by Phytophthora 

Rust: caused by Puccinia buxi

Twig and leaf blight: caused by Pseudonectria rousseliana

In late 1994, a new blight disease of box was discovered in a nursery in Hampshire. Until 1997, no new cases were reported when a sudden outbreak of the disease was noticed (Henricot et al ., 2000).

Volutella: The new disease is distinct from the previously described Volutella blight of box, which causes very similar symptoms but is caused by the fungus Volutella buxi. The incidence of Volutella is relatively constant. It seems to be associated with wounds, particularly those caused by clipping.

Symptoms of Cylindrocladium box blight

The symptoms of the disease are dark brown spots on the leaves and black streaks on the stems. As a result of infection, the plants defoliate and dieback ensues.

A species of the fungus Cylindrocladium was isolated from the diseased material. To confirm that Cylindrocladium was the cause of the leaf blight symptoms, infection assays were carried out on 2.5-year-old Buxus sempervirens 'Suffruticosa' with a spore suspension of the fungus. Typical symptoms of leaf blight were recorded and Cylindrocladium was reisolated from the infected leaves and stems fulfilling Koch's postulates (Henricot et al., 2000).

Note: To ensure that an organism associated with a disease is actually causing it, pathologists are guided by Koch's postulates, first elaborated in the late 19th century by the pioneering medical pathologist Robert Koch. To be proven as the cause of the disease, the organism must be constantly associated with the problem, then cultured in a pure state from the affected tissue, reinoculated into a healthy organism which will then show the symptoms, and be reisolated again. Pathologists will not carry out all these tests for routine diagnoses, but Koch's postulates remain an essential tool to determine the true pathogenic status of suspected pathogens.

Species affected

To date, the disease has only been recorded on Buxus but the host range of this fungal species has not been fully established. The species on which the disease has been recorded are

• B. sempervirens', and its cultivars
• 'Suffruticosa',
• 'Variegata',
• 'Suffruticosa Variegata',
• 'Elegantissima',
• 'Latifolia Maculata',
• B. sinica var insularis 'Justin Brouwers',
• B. microphylla var japonica 'Morris Midget',
• B. microphylla var japonica 'National'.

Distribution and origin of the disease

The number of cases recorded through the RHS advisory service has been increasing since 1998 and the disease is now widespread throughout the UK. The number of cases usually peaks during the autumn months from September to November. The disease has also been recorded in Belgium, Italy, France, Holland and New Zealand.

Comparison of isolates collected in the UK and one isolate from New Zealand revealed that they are genetically homogenous (see image). The disease was first reported in the UK in the mid-90s and was widespread in 1998, but in New Zealand it was first reported in 1998, suggesting that the fungus was subsequently introduced there. No close relationship has been found with other described Cylindrocladium species. It may be that this species has been introduced recently to Europe, where Buxus plants are widely grown, from a geographically isolated area where it has evolved on one or more Buxus species.

Identification of the fungal species of Cylindrocladium

The conventional morphological characteristics and the DNA sequence data from three different regions of the genome demonstrated that the species isolated from diseased box plants is a new species of Cylindrocladium, given the name C. buxicola. Morphologically, C. buxicola is characterized by having one-septate conidia (spores with two cells) and an ellipsoidal vesicle with a pointed or papillate apex (see image). It is a low temperature species and its growth is inhibited at 30 °C. These morphological and cultural properties alone are unique to this fungus. Mating of the isolates collected in the UK and New Zealand was carried out but no fertile perithecia (bodies containing the sexual spores) were obtained suggesting that C. buxicola is heterothallic and all isolates belonged to one mating type. The teleomorph (perithecial) stage of this fungus is therefore unknown. The sequencing of three parts of the genome of the fungus confirmed it is a new species.

Disease cycle

Preliminary work on the infection process of C. buxicola suggests that this fungus penetrates directly the cuticle of box plants without forming any penetration structures. Germination of the spores occurs as soon as 3 hours after infection and penetration follows 5 hours post-infection. The mycelium resurges through the stomata 48 hours after infection and conidia are produced shortly afterwards. High humidity favours infection and in dry conditions, the spores will abort quickly. Studies looking at the survival of C. buxicola in the soil are underway. So far, the fungus has been able to survive on decomposing leaf material for 11 months.

Pathogenicity assays on whole plants and cuttings give rise to the same symptoms, indicating that wounding is not necessary for the development of this disease.

Control

There is currently no fungicidal treatment for the disease in gardens, although professional nurseries can treat it in the nursery. Gardeners should take particular care to buy uninfected plants. All infections should be pruned out and all the fallen leaves destroyed. If the fungus is similar to other Cylindrocladium species it may produce resting spores and survive in fallen leaves. These resting spores could easily be spread in soil, for example on muddy boots. They could also be spread by water splash over short distances, or carried by animals. The same control precautions apply for Cylindrocladium and Volutella . The two fungi also often occur together.

Publications from this research

Henricot, B., Perez-Sierra, A. and Prior, C., 2000, A new blight disease on Buxus in the UK caused by the fungus Cylindrocladium , Plant Pathology , 49(6) , 805 - (PDF file)

Henricot, B. & Culham A., 2002, Cylindrocladium buxicola , a new species affecting Buxus spp., and its phylogenetic status, Mycologia, 94(6) , 980-997

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