Santa's come early this year, boys and girls. The tenderly topiarised Buxus sempervirens of the northwest have received the unwelcome of gift of... box blight.
What is box blight?
Box blight is a fungal disease affecting common box (Buxus sempervirens) and is caused by two similar types of fungi called Calonectria pseudonaviculata and C. henricotiae. Leaves of affected plants exhibit black and brown streaks. They then turn completely brown, die and fall, resulting in bare patches and die-back. The box blight fungi are most active in moist, warm conditions so its increasing prevalence may well be attributed to warmer, wetter summers due to climate change The fungal spores survive in the soil and spread in water, through rain-splash and potentially via animals.
How can outbreaks be controlled?
If plants are not too badly afflicted, cut out affected material. Clear away all fallen leaves at the centre and base of the plant and destroy. Don't compost as the fungal spores can survive for up to six years in leaf litter. Exercise good garden hygiene by cleaning and sterilising all cutting equiptment carefully. Plants are not affected at the roots, so if you have caught the outbreak early, there is no reason to dig out affected plants as new growth may well be healthy.
Do not trim or prune plants on wet or windy days, as this increases the chance of spores spreading. Do not trim plants too frequently, as this stimulates new growth and more compact foliage, which also facilitates the spread of spores. Prune branches out of the heart of the plant to open up its structure and promote air circulation; increased airflow reduces the humidity of the conditions within the plant.
During the growing season, feed plants after cutting back, to stimulate healthy new growth. Use a balanced or potassium rich feed. Avoid feeds high in nitrogen as this stimulates new soft growth, which is more susceptible to pest and diseases. Water plants at the base, not overhead, and water in the mornings, not at night, so periods of higher humidity are shorter. Clear old mulch at the base of the plant, and replace with new, to prevent reinfection from water splash on old spores.
In addition to blight, Buxus sempervirens is susceptible to a range of pests and diseases; Volutella blight, box rust, box red spider mite, and Box Caterpillar (below), to name a few.
If you are still intent on planting new box plants in your garden, there are a few ways in which you can reduce the risks of box blight and other infections;
Grow new plants from cuttings taken from healthy parent plants in your garden;
Buy plants from reputable nurseries and isolate them in quarantine for a month, to see if there is any evidence of disease that may have been surpressed by fungicides at the nursery;
Plant box in open, dry locations that are not susceptible to poor airflow or high humidity;
Space plants at least 40cm apart, to minimise spread between plants.
What are the alternatives to Box?
As box blight becomes more prevalent, selecting a different species altogether would seem to me to be the best management approach. There are a number of good alternatives to Box, which have compact, evergreen foliage and are suitable for topiarising but have less susceptibility to pests and diseases. If starting out from scratch, it’s well worth considering the various alternatives. Here are a few;
Ilex crenata 'Buxifolia' - a good, slow growing alternative that is perfect for kerbside locations as it has good pollution tolerance;
Lonicera nitida - fast-growing but extremely robust and easy to maintain. Suitable in partial shade.
Pittosporum tenuifolium 'Golf ball' - slow growing, and easy to clip to and keep in a ball shape.
Pittosporum tenuifolium