Camellia flower blight is a fungal disease of camellias, attacking the flowers causing them to rot.
What is camellia flower blight?
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.
Flower blight is restricted to species of camellia, and affects no part of the plant other than the flowers. It is only found, therefore, while the plants are in flower.
Camellia flower blight is a notifiable disease on planting material only, and any suspected cases must be reported to the Fera Plant Health and Seeds Inspectorate (PHSI) (see the Fera website for further information and details of PHSI offices).
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
- Fungal spores (sclerotia) that look like oily black droplets may, occasionally, be found towards 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 from the base of the flower. The flower blight fungus produces a characteristic white or greyish ring of fungal 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 material.
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 are produced on stalks that can grow up to 10cm (4in) in length. The mulch may have to be too deep to be practical.
Fungicide spraying is not a realistic option and there are no fungicides labelled for amateur 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, 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, so infections are more common in wet weather.
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.