Many of the pests and diseases that threaten box (Buxus Sempervirens) can be managed if identified and treated early.
Here are four common ones to look out for:
Cylindrocladium buxicola is the blight to be most concerned about. Signs of infection include blackening of the box leaves, defoliation and black streaks on the stems. White spore masses can be visible on the underside of the box leaves in damp conditions.
Fungicides approved for amateur use such as Provanto Fungus Fighter Plus will slow the spread of the blight. Commercial strength fungicides are far more effective but can only be applied by professionals with the correct certification.
Combine fungicide treatments with good gardening practices such as keeping clipping tools clean, clearing up fallen leaf litter and watering box plants at their base to avoid dispersing the fungal spores.
Mussel scale is a small insect about 3mm in length and protected under a blackish-brown, mussel shaped scale. They feed on the box sap, causing discolouration of the leaves and defoliation.
Eggs are laid under the scales in late summer and overwinter before hatching the following May or June. This is the best time to spray with an insecticide, when the young insects are active and before they become protected by their scales. I like to use sprays with natural pyrethrum, such as Neudorff BugFree Bug and Larvae Killer.
Two or three insecticide applications may be required. Spray carefully late in the evening to reduce the risk to beneficial, pollinating insects.
Box Tree Caterpillar
Box tree caterpillars can cause devasting damage in a very short time so check box regularly between March and October.
Look for patches of nibbled leaves. May need a closer inspection to see webbing and small frass balls (caterpillar poo). The box tree caterpillars are a yellowy green in colour with a black head and dark stripes down their sides. They will be found protecting themselves in the webbing.
An effective treatment is the combination of using box tree moth pheromone traps and spraying the plants two or three times in a season using a biological insecticide based on the Bacillus thuringiensis bacterium such as Xentari.
The caterpillar should stop eating treated box leaves within an hour and die in a couple of days. Another option is using nematodes but they need to be applied correctly in order to work.
The symptoms of box rust are often confused with box blight. Knowing the difference between the two is important as box rust is rarely severe enough to damage the plant and can be treated by trimming out the affected leaves. Fungicide treatments are not necessary.
Box rust appears as dark brown pustules that develop on the upper and lower surfaces of the box leaves, normally in early Autumn and remaining until the following Spring.
Improving the overall health of the box will help. Diluted seaweed extract applied as a foliar feed every 4-6 weeks during the growing season works well.