Research

Camellia petal blight

Occurrence

Camellia petal blight, caused by the fungus Ciborinia camelliae Kohn, was first described in Japan in 1919 and has spread to the USA, New Zealand and parts of mainland Europe. It was first detected in the UK in 1999 and until recently was known only in Cornwall and Dorset, where it had probably been present for some years before it was noticed. In recent years it has spread more widely through southern England and is now present at sites in Sussex and Surrey including RHS Garden Wisley.

Symptoms

The effect of the disease is not easily distinguished from the normal damage by wind, rain and frost. The latter is usually confined to the outer edges of the petals. Blight infection starts as brown flecks, not usually at the edge, and spreads rapidly to form a brown blotch that engulfs the petals right back to the base. Infected flowers may or may not fall prematurely.

A distinguishing feature of the disease is seen when an infected corolla is separated from the calyx: there is a white or grey ring of fungus mycelium around the base of the infected petals.

Life cycle

The host range is restricted to some species of Camellia (C. japonica, C. reticulata and C. sasanqua). Infected flowers may fall early and a hard, black resting structure (sclerotium) forms at the base of the petals. This remains dormant in the soil below the tree until just before the flowering season, when it germinates to produce small, cup-shaped reproductive structures (apothecia). These release spores (ascospores) which are dispersed in air currents and land on petals. Infection follows, leading to brown patches on the flowers that rapidly spread to the base. The flower falls and so the cycle continues. The fungus is commonly said to infect only flowers. There is a suggestion from New Zealand that infection can spread back into stems, but this is not yet proven. Sclerotia can survive at least five years. Not all of them germinate in the first year and they can germinate in more than one year.

The ascospores may travel up to 20km on the wind. There is no spread from flower to flower, which limits the damage done by the disease. Apart from the wind-blown spores, the fungus can be spread over any distance in soil contaminated with the sclerotia, in car tyre treads or on muddy boots, for example.

Control measures

Cleaning up fallen flowers will help reduce infection in following years. Putting down a deep mulch to bury the sclerotia might help, but the apothecial stalks can grow several centimetres, so it might 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 suitable products for soil treatment.