This disease is now widespread in the UK, having been first recognised in the mid 1990s. It affects Buxus sempervirens and its cultivars. These include B. suffruticosa, B. variegata and B. elegantissima among others. Plants can defoliate and show dieback on stems, leading to large, brown dead sections.
The disease favours humid and dry weather - infections can establish within 48 hours and can peak in the autumn months. Spores can overwinter and are not affected by frost but in the summer, spores can be affected by temperatures higher than 30 degsC.
How to recognise it
There are two species of box blight - Cylindrocladium buxicola and Pseudonectria (Volutella) buxi. Both can occur together and identification in the early stages can be difficult because the leaf symptoms can be similar to other cultural problems. Spotting and streaking can occur along the leaves that turn grey for the Cylindrocladium species and pinkish leaves on the Pseudonectria species.
The distinguishing morphological feature separating P. buxi from C. buxicola is the shape of their conidia (asexual spores), which can only be identified in a laboratory. C. buxicola has multicellular cylindrical conidia in contrast to those of P. buxi, which are single-celled with rounded ends. The shape of the conidiophores of C. buxicola is also unique compared with P. buxi.
The pathogen has a rapid disease cycle that can be completed within one week. Spores can be spread from plant to plant, on soils, animals and insects and through water splash.
P. buxi infects wound sites and can be more of an issue after trimming. C. buxicola infects through leaf stomata.
The disease survives as resting spores or mycelium on leaf debris, in some cases for up to six years. They may be spread in windblown rain but are unlikely to travel long distances on the wind. As well as natural means of dispersal, these fungi have been widely spread by human activity, especially on infected plants from nurseries.
Germination of spores can occur three hours after inoculation, with penetration occurring in as little as five hours. Hyphae have the ability to actively penetrate plant cuticles or they can enter passively through leaf stomata. Fungal growth can occur in the plant, producing conidia on the underside of the leaf after seven days.
Both fungi cause leaves to go brown and fall, leading to bare patches. C. buxicola, the more damaging of the two, also causes black streaks and dieback on young stems. Sporulation can be seen in high humidity on the underside of leaves revealing white spore masses called sporodochia.
This fungus infects all above-ground portions of boxwood and black lesions can be found along stems from shoot tips to the soil line. However, it does not infect the roots like other Cylindrocladium species. This characteristic of C. buxicola allows time for roots to regenerate after an attack and to support growth after a severe infection.
Treatment: cultural control
Pathogen-free material should be obtained from reputable suppliers or nurseries. Once plants and cuttings arrive they should immediately be inspected for symptoms. Newly received plants and cuttings should be isolated from existing boxwood growing and planting areas for at least a month to check for disease.
Overhead watering or irrigation should be avoided because water and humidity are essential for the spread of box blight. It is advisable not to work with the plants when they are wet because this creates conditions suitable for the pathogen to spread. In commercial production, increasing the space between plants can also help to reduce the humidity and create sufficient air circulation in a production area. Cut out infected twigs and remove and destroy any fallen leaves, crop debris and topsoil to reduce the amount of inoculum.
Sanitation of pruning equipment is essential between blocks of host plant material with an effective disinfectant such as Jet 5. Carry out weekly inspections on crops. If host plants do become infected, plants should be removed and destroyed immediately. Composting affected leaves may not destroy the fungus - spores can remain viable on decomposing material for up to 11 months. If plants require irrigation then a directed spray is less likely to increase the risk of the spread of this disease than overhead irrigation.
Treatment: biological control
Foliar applications of Serenade ASO (EAMUs required) may help to encourage plants' natural defences against a range of fungal pathogens.
Treatment: chemical control
Active ingredient Azoxystrobin
FRAC code 11
Formulations Various including Amistar* (Syngenta)
Action(s) Systemic, translaminar, protectant broad-spectrum fungicide - compatible with biological controls.
Active ingredient Bacillus subtilis strain QST713
Formulation Serenade ASO* (Bayer)
Action(s) Protectant biofungicide compatible with biological controls.
Active ingredients Boscalid + pyraclostrobin
FRAC code 7 + 11
Formulation Signum* (BASF)
Action(s) Systemic, protectant and curative fungicide - compatible with some biological controls.
Active ingredient Chlorothalonil
FRAC code M5
Formulations Bravo 500* (Syngenta)
Action(s) Protectant fungicide for selected outdoor crops - compatible with some biological controls.
Active ingredients Cyprodinil + fludioxinil
FRAC codes 9+12
Formulation Switch (Syngenta)
Action(s) Systemic, translaminar fungicide with long residual activity - incompatible with biological controls.
Active ingredient Prochloraz
FRAC code 3
Formulation Octave (Everris)
Action(s) Broad-spectrum protectant and eradicant fungicide - compatible with some biological controls.
Fully updated by Dove Associates
Use plant protection products safely. Always read the label and product information before use.
* Extension of Authorisation for Minor Use (EAMU) required for use in ornamental plant production outdoors and/or under protection. Signum has an EAMU for amenity vegetation.
Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.