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Catch that Witch!

Rosemary Newton Botany Dept UCT

Witches’ broom is a disease that occurs on many members of the genus Protea. The symptoms of the disease are a characteristic thick, bushy growth, consisting of poorly developed shoots with thin stems and minute, often distorted leaves. The "broom" is produced from normally dormant floral or axial buds. Single branches on plants or whole plants can be affected. The growth is formed from excessive bud proliferation. Diseased plants can only be identified by symptom observation, and the physical change of the plant is irreversible.

Infection may take place from the early seedling stage, and thereafter can occur at any time during the life of the mature plant. The cause, or agent, of the disease has not yet been identified, but is thought to be a microscopic mycoplasma-like organism, which remains in the plant throughout its life. Mycoplasmas are bacteria that do not possess cell walls, and are mostly pathogenic in animals and plants. A toxin from these mycoplasmas is believed to stimulate the bud to divide and subdivide to form the witches’ broom growth.

The mode of transmission of the disease to other parts of the plant and to other plants is unknown. However it has been suggested that, due to close association, transmission may be with an eriophyid mite (Aceria proteae), that is believed to show specificity for host plants. The mites are colourless, banana-shaped and microscopically small. These mites have been found to occur under bracts in dormant leaf buds, in flower buds (in unopened flowerheads) and in witches’ brooms. Other members of the Eriophyidae are known vectors for plant pathogens. The mite itself is probably dispersed by wind, man, and possibly birds.

Control of the disease in flower farms is most effective by prevention, by ensuring diseased plants are not sold from nurseries, and by mite control, through regular miticide application. Heavily infested adults and diseased seedlings must be destroyed as it is impossible to restore them to economic production. Light infestations on mature plants can be pruned. All infested material should be destroyed by burning, to stop further spread of the disease.

There is still a great deal about witches’ broom that remains a mystery. I have been struck by the patchiness of the disease in nature. Sometimes every plant is infected, sometimes only the rare few. Some Protea species carry heavy loads of the broom, others seem quite unaffected. Even within a plant, it is far from clear how the disease spreads. For example, do infected individuals produce infected seeds?

As part of an honours project, I am trying to determine the extent of the disease and patterns of infection and spread. I am hoping that results may prove useful to Protea growers in understanding the disease, and controlling it more effectively. This is where the assistance of avid amateurs will be invaluable. I am compiling a list of all the species infected by witches’ broom. Does the disease occur on genera other than Protea? Is it confined to only some sections of Protea? Have atlassers observed any interesting patterns which might help explain distribution or transmission of the disease? I’d be very grateful for comments or observations from atlassers, especially lists of species known to have witches’ broom, and their localities.

Please report all sightings of the "witch" to Rosemary Newton, Botany Dept, University of Cape Town, Rondebosch, 7700: FAX (021) 650-4041, or e-mail: RNEWTON@ BOTZOO.UCT.AC.ZA.

Useful References

If atlassers record the presence of witches' broom on the SRS in the additional remarks, we will forward the data to Rosemary. Please note: this project will have to be completed sometime around September, so exciting data are really required by June at the latest. Exciting data include witches' broom on genera other than the Sugarbushes. Less exciting, but just as important, are where witches' broom occurs on the known hosts - these data are of interest independent of Rosemary's work - so please let us have the data anytime.
We have unconfirmed reports of witches' broom-type infections on Ls tomentosum and Ld salignum. We will process existing data for Rosemary and report progress in the next newsletters.

Tony Rebelo

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