The Facts about Box Blight: What is Box Blight?

Box Blight was first recorded in the UK in 1994 following an outbreak in a Hampshire nursery. It’s not certain where the disease originated, though Central America is at the top of many experts’ list of suspects. It is now widespread but not endemic. The disease is caused by two fungi that often attack in tandem: Cylindrocladium buxicola (syn. Calonectria pseudonaviculata) and Pseudonectria buxi. Cylindrocladium buxicola has two genetic types.
In the UK, C. buxicola mainly affects Buxus spp but can also attack other plants in the Buxaceae family. P. buxi only affects Buxus species. This means, of course, that Common Box (Buxus Sempervirens) is right in the firing line.
There are several symptoms to look out for:
Leaf Spotting
• Both fungi produce dark brown / black spots on the leaves that quickly cover the leaf. Within a couple of days, the leaves will drop
(Incidentally, the first stage of spotting is the most common cause of misdiagnosis and will be discussed in more detail in a future post)
Fungal Growth
• Before defoliation, fungal growth may be visible (whitish for C. buxicola and pinkish for P. buxi) on the undersides of infected leaves. This produces and then releases the asexual spores that further spread the disease and is particularly noticeable when it’s wet
• These spores can penetrate healthy leaves so while damaged foliage is more susceptible, it is not a prerequisite for infection – C.buxicola generally enters in moisture passing through the stomata (minute holes in the foliage through which plants transpire). This is why the need to water carefully will feature in several posts to come
• The more aggressive C. buxicola can also infect young stems resulting in black streaks and dieback
Some experts argue that there is, in fact, just one species of Blight and that it develops in two stages – perfect and imperfect – with differing virulence. This would help explain how (as we will see in future posts) Box Blight can be
PICTURES: Above: A scan from an electron micrograph showing Cylindrocladium buxicola efflorescence on the leaf surface of Buxus sempervirens. Clearly visible are the clusters of rod-like conidia (spores), conidiophores (the hypha that generate the spores) and club-shaped vescicle Below An infected Box