Camellia flower blight is a disease caused by the fungus Ciborinia camelliae. The fungus is capable of overwintering in the soil in the vicinity of affected plants.

It was first described in Japan in 1919 and has since spread to the USA, New Zealand and parts of mainland Europe. It was first found in the UK in 1999 and is now present quite widely through southern England, including at RHS Garden Wisley.

The disease is restricted to camellia, and affects no part of the plant other than the flowers. It is only found, therefore, while the plants are in flower. In the UK, spores of the fungus are produced most commonly in February to March, so flowers produced before this time are seldom affected.

You may see the following symptoms:

• Brown flecks on any part of the petal, rapidly spreading to form a brown blotch that engulfs the petals until the whole flower is dead
• Affected flowers often fall prematurely
• Hard black resting structures (sclerotia) of the fungus may be found embedded within the base of the petals
• The effects of flower blight are not easily distinguished from frost damage, or from infection of the flowers by the common fungal disease grey mould (Botrytis cinerea)
• Whereas frost damage is often confined to the edges of the petals, both flower blight and grey mould start as brown flecks at any point on the petal

Flower blight can be distinguished from grey mould by removing the calyx (the outer green part of the unopened bud, which then remains below the petals once it has opened) from the bottom of an affected flower and examining the bases of the petals. The flower blight fungus produces a characteristic white or greyish ring of fungal growth (mycelium) around the base of the petals. Occasionally, oily black droplets (one of the spore stages of the fungus) are also found towards the bases of the petals. By contrast, grey mould will often produce large numbers of powdery grey spores over the affected petals.

Non-chemical control

Cleaning up fallen flowers will help reduce infection in following years.

Putting down a deep mulch to bury the sclerotia might help, but the apothecia (see Biology, below) are produced on stalks that can grow up to 10cm (4in) in length. The mulch may have to be too deep to be practical.

Planting autumn-flowering camellia species and cultivars should avoid the problem, because these bloom before the spores are present. In the UK most spores are produced between February and March, so autumn-flowering camellia species (including C. sasanqua) will usually escape infection, despite being susceptible.

In some other Northern Hemisphere regions, the spore production period can be as long as December to April, so a wider range of camellias may be affected. For Southern Hemisphere regions, flowering and spore production periods will differ by approximately six months.

See our profile on autumn & winter-flowering camellias for more details of suitable species and cultivars.

Chemical control

Fungicide spraying is not a realistic option and there are no fungicides labelled for home use; nor are there any products available for soil treatment.

When camellias are infected by Ciborinia camelliae, hard, black resting structures called sclerotia develop within the base of the decaying petals. These survive as the rest of the fallen flower rots down, and remain dormant in the soil during winter. The sclerotia germinate the following year, producing small, brown, cup-shaped reproductive structures called apothecia. These can sometimes be found on the soil surface below an affected plant, but can be very difficult to spot amongst the old flower and leaf debris.

Huge numbers of spores are released from the surfaces of the apothecia and are carried upwards in air currents to reach camellia flowers. These spores require the petals to be wet to infect them, so infections are more common in wet weather. In the UK most of the spores are released between February and March, and it is species and cultivars of camellia flowering at this time of year that may be affected by the disease.

Sclerotia of Ciborinia camelliae can survive for up to five years in the soil. Not all of them will germinate in the first year, and an individual sclerotium can germinate for more than one year. The spores may travel up to 20km (12 miles) on the wind. Apart from these wind-blown spores, the fungus can be spread over long distances in soil contaminated with the sclerotia, for example on muddy boots or car tyres.