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Camellia Flower Blight - Life History and Control

Life cycle. The fungus spends the summer and autumn (i.e. the non- flowering period) in the top layer of the soil, underneath camellia bushes that were infected in the previous season. The organism survives as sclerotia. These are hard black structures, which appear very much like small irregular pebbles. Even if seen, they are not easily recognised as being of fungal origin. Most will be buried 1 or 2 centimetres under the leaf litter. Exactly when the fungus activates itself is unknown in New Zealand, but if it behaves the same way it does overseas, then it synchronises well with the flowering period of C. japonica. Upon activation, sclerotia germinate to produce a stipe (i.e. a stalk) which grows upwards until it reaches the soil surface. It then expands to produce a saucer-shaped structure called an apothecium, These are usually a tan colour, and can range up to a centimetre or more in diameter. These structures are far more obvious than sclerotia, and may be spotted by a gardener on the lookout for them (Photos 1 and 2). However, small apothecia may go unnoticed.

These apothecia produce spores, which they forcefully eject into the air. Typically, just a few apothecia can release millions of microscopic spores over several days. As the old apothecia expend themselves, new ones form to replace them. Therefore an infested area may go on being a source of spores for months. The fact that the fungus can produce so many spores may seem alarming. However, there are many environmental factors that render them non-viable and so restrict the spread of most to a few hundred meters. Nevertheless viable spores may travel up to 2 km.

Once a viable spore lands on an open flower, it usually germinates to form a microscopic fungal thread. This penetrates the tissue then expands and grows in it. The result is the characteristic "blight" symptom. Only the flower tissue is attacked. Leaves and stems are unaffected. Interestingly, no spores are produced from the flower tissue itself! This is unusual for a fungal plant pathogen and helps to distinguish the disease from Botrytis, which will sporulate under conditions of very high humidity. The period from first infection to total blossom death can be as short as 5 days. Once invasion of the lower part of the plant occurs, a grey ring of fungal tissue forms around the base of the blossom, where it was attached to the plant. After 2-3 weeks, this area hardens and develops into a hard black sclerotium.

By this time of course, the blossom has fallen from the bush. The newly formed sclerotia lie in the leaf litter to be covered by falling petals and other organic material. They do not produce apothecia in the season of formation. A period of maturity is required which means they do not produce spores until the following season, whereby the cycle repeats itself. If therefore, apothecia are found under a camellia bush it means the disease must have attacked the flowers last year. Not all sclerotia will necessarily germinate the season after they are formed. Some remain viable for many years and may germinate and produce apothecia (hence spores) several years after their creation.

Control. No control program yet devised has been 100% effective on this fungus. It is crucial therefore, that preventive measures be taken to stop it ever establishing in the garden (or region). The control section is divided into two parts. One part deals with preventing the establishment of the disease, the other deals with managing it, once it has gained entry to the garden.

(a) Prevention. Do not move flowering camellia plants from known infected regions (e.g. Wellington) to non-infected regions. Be careful, even in the non-flowering season (or if the plants have been disbudded so there is no flower tissue). The following recommendations apply.
1. The plants should be small. Infected blossoms can lodge in the branches of large, bushy plants and sclerotia can form there unnoticed. These can then fall to the soil if the plant is moved.
2. The plants should be bare rooted, or at least growing in a sterile growth media that was not present in the previous season.
3. Nursery Managers in infected regions should take care. B1ossoms can fall into other plants or pots surrounding an infected camellia, thus providing a source of infection for someone's camellias at home.
4. Destroy all flowers at Camellia flower shows. As the fungus does not form secondary spores, showing flowers from infected regions is not as large a risk as it initially may seem. However, shattering varieties can drop infected petals without being noticed. Certainly, leaving the spent blooms there after the show is over, is very dangerous as these may be a source of spores in the following year.
5. Dipping cut blooms into a suspension of Bayleton fungicide will stop the disease, even if infection has occurred. If blooms must be sent out of an infected region (for shows and the like), then consider this procedure. 6. Watch your own camellias very carefully. If there is any sign of the disease, pull off the infected blossoms and burn them. Do not put them in the compost heap as this does not kill the sclerotia. Try to find the source - it may be a neighbour's plant. Chances are it will be within 200 meters or so of the property. If you do detect the disease, then switch to the management plan below for your own plants, and include the source of the infection too, if that's at all possible. Sclerotia can remain dormant for several seasons, hence a few dropped infected petals will provide spores for a number of years to come. If your plants are small, consider de-budding them for this season, until you have dealt with the source.

(b) Management. If the disease is present in your area, then there is a good chance your plants may have the disease or be challenged with it this season. The following measures will reduce its incidence.

1. Spray the soil under and around the bushes thoroughly with Terraclor, once the flowering season starts. This chemical affects the stipe (i.e. apothecial stalk) rather than the apothecium so it's important to do this as soon as possible once the flowers start opening. The chemical has no effect on dormant sclerotia.
2. Keep the area under the bushes clear of vegetation and cut back low branches so there is plenty of air circulating. The sclerotia in fallen blossoms form best on wet, sheltered, weedy ground. If feasible, collect fallen blossoms and burn them.
3. For small areas, consider using a black plastic as a ground cover under the trees. This will make it easier to collect fallen blossoms. Also, it will prevent apothecial development from any sclerotia that may already be present. Alternatively, use a smooth dry mulch, such as pine needles.
4. Spray the bushes regularly with fungicide. None of the fungicides tested overseas have provided complete control of this disease. As new fiowers appear every day, ideally a systemic (one that is absorbed into the plant and transported through it) rather than a protectant-only compound is needed. The best appear to be Bayleton (triadimefon), Cereous (triadimenol) or Folicur (terbuconazole) applied every 14 days or so. These are sold in NZ for field crops such as cereals, peas and onions but are not registered for ornamentals. None of the common garden systemic fungicides have been tested, but it might be worth trying either Bavistin (carbendazim) or Saprol (triforine). These have sarge systemic activity and are effective on related fungi. There is no easy answer to Camellia flower blight. Sanitation and exclusion are the best control measures yet available. Final note: Do not forget the mail-in survey detailed in the last issue of the Bulletin. We need to know if the pathogen has spread elsewhere, so send in suspicious blooms. Remember they must be accompanied by the form sent out with the last issue.

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