Box blight

Box blight is a fungal disease of box resulting in bare patches and dieback of box, especially in topiary and parterres.

Box blight

Quick facts

Common name Box blight
Scientific name Cylindrocladium buxicola (syn. Calonectria pseudonaviculata) and Pseudonectria buxi
Plants affected Buxus spp. (Box)
Main symptoms Brown leaves and bare patches
Caused by Fungus
Timing All year round, most active in rainy seasons

What is box blight?

Box blight is a disease of box leaves and stems caused by two fungi, Cylindrocladium buxicola (syn. Calonectria pseudonaviculata) and Pseudonectria buxi. The two are often found together. Cylindrocladium buxicola is further subdivided into two genetic types which differ by their sensitivity to some fungicides (triazoles).

Cylindrocladium buxicola largely affects Buxus spp. (box) in the UK, but other plants in the Buxaceae family are also susceptible. Pseudonectria buxi only affects Buxus species.

Blight is just one of a number of problems box suffers from.

Symptoms

You may see the following symptoms:

  • Both fungi cause leaves to go brown and fall, leading to bare patches
  • C. buxicola, the more damaging of the two, also infects young stems causing black streaks and dieback
  • In wet conditions the spore masses of the fungi may be seen on the undersurfaces of infected leaves, white for C. buxicola and pink for P. buxi

    Cylindrocladium buxicola, the more damaging of the two box blights, infects young stems causing black streaks and dieback.In wet conditions the spore masses of the fungi may be seen on the undersurfaces of infected leaves, white for C. buxicola (as shown in this image) and pink for V. buxiBoth forms of box blight fungi cause leaves to go brown and fall, leading to bare patchesIt is not uncommon for box blight to affect whole side or tops of hedges, particularly in damp, shaded areas

    Control

    Non-chemical control

    Cultural control and hygiene

    Hold any commercially sourced plants in isolation for at least three weeks to confirm they are free of infection before planting out. Commercial nurseries may use fungicides which suppress but do not kill the fungus and this isolation technique will allow time for any suppressed disease to become visible.

    Inspect plants for early symptoms as Cylindrocladium spreads very rapidly in warm and humid conditions and is difficult to manage.

    Reduce the frequency of clipping to create more ventilation throughout the plants (regular pruning creates dense foliage and less air movement).

    Avoid overhead watering as Cylindrocladium thrives in humid conditions.

    As the disease is widespread, if possible, take cuttings from disease-free plants.

    If the disease does break out, remove and destroy affected plants. If they are mature and highly valued, cut out all affected parts, clean up fallen leaves (including stripping and replacing surface topsoil to ensure complete removal) and treat with a fungicide.

    Resistance

    Unfortunately there is not known to be a resistant Buxus at present.

    C. buxicola attacks Buxus balearica, B. bodinieri, B. glomerata, B. harlandii, B. microphylla, B. macowanii, B. riparia, B. sinica and B. sempervirens. There are differences of susceptibility among Buxus species and varieties. In general the varieties of B. microphylla are more tolerant than varieties of B. sempervirens. The dwarf form, B. sempervirens 'Suffruticosa', popular for parterres, is very susceptible. Based on work done in Belgium, B. microphylla 'Belvédère', 'Rococo', 'National' and 'Trompenburg' are tolerant to the disease. Other plants in the Buxaceae family are susceptible such as Pachysandra terminalis and Sarcococca species.

    Choose alternative hedging and topiary plants

    To be completely safe, choose an alternative hedge or topiary plant. The following all have small leaves and can be clipped into formal hedging styles:

    Chemical control

    There are no fungicides available to amateurs with specific recommendations for use against box blight. However, the fungicides myclobutanil (Bayer Garden Systhane Fungus Fighter and other products), tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra) are labelled for control of other diseases on ornamentals and could therefore be used legally on box (at owner's risk) to try and control box blight.

    Research undertaken by the RHS has investigated myclobutanil and found it to be ineffective at controlling Cylindrocladium blight. In Germany and Belgium, professional formulations of difenoconazole and tebuconazole have good protectant activity. Only tebuconazole is effective against both genetic types of Cylindrocladium.

    Download

    Fungicides for gardeners (Adobe Acrobat pdf document outlining fungicides available to gardeners)

    Biology

    The fungi survive as resting structures or mycelium on fallen leaves for up to 6 years and produce spores when conditions are suitable. The spores are dispersed in water and probably by animals and birds. They may be spread in wind-blown rain, but are unlikely to travel long distances on the wind. As well as natural means of dispersal, these fungi have been spread widely by human activity, especially on infected plants from nurseries.

    Pseudonectria buxi requires wounds for infection and is associated with clipping. Cylindrocladium buxicola can infect unwounded plants and causes more serious damage.

    P. buxi has long been known, but little studied. C. buxicola was first recognised in the UK in the mid 1990s and has also broken out in continental Europe, New Zealand and more recently in USA and Western Asia (Iran), but its origin is unknown.

    Further reading

    RHS research into box blight

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